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32X
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SCIENCE ?RIMERS, 'diud ij^
Professors Huxley, Roscoe, afui
Balfour Stewart.
III.
PHYSICS.
'tttrae primers.
PHYSICS
BY
BALFOUR STEWART,
Professor of Natural Philosophy, the Owens College, Mamhatcr,
Author of ** Elementary Lessons in Physics.**
WITH ILLUSTRATIONS.
IK0tottt0:
JAMES CAMPBELL & SON.
- - 7
Entered accordiuj; to the Act of Parliament of Canada, in the year one
thousantt eight hundred and eighty -one, by Jamrs Gampbbll
AND Son, in the office of the Minister of Asjiculture.
PREFACE,
In publishing the Science Primers on Physics and
Chemistry, the object of the Authors has been to state
the fundamental principles of their respective sciences
in a manner suited to pupils of an early age. They
feel that the thing to be aimed at is not so much to
give information, as to endeavour to discipline the
mind in a way which has not hitherto been cus-
tomary, by bringing it into immediate contact with
Nature herself. For this purpose a series of simple
experiments has been devised, leading up to the chief
truths of each science. These experiments must be
performed by the teacher in regular order before the
class. The power of observation in the pupils will
thus be awakened and strengthened; and the amount
and accuracy of the knowledge gained must be tested
and increased by a thorough system of questioning.
The study of the Introductory Primer will, in most
cases, naturally precede that of either of the above-
named subjects; and then it will probably be found
best to take Chemistry as the second and Physics as
the third stage.
The whole of the apparatus needed for all the ex-
periments (except a few marked in the text with an
asterisk) will be supplied by Messrs. J. J. Griffin and
Sons, 22,-Garrick Street, Covent Garden, London,
W.C., (or j£ic) 3 J. 8//. exclusive of packing.
If prompt payment be made, Messrs. Griffin are
willing to supply the set for jCi'j exclusive of packing
cases, which may be either 9^. or 21s, according to
quality. Those who have already purchased the ap-
paratus for the Chemical Piimer need not purchase
another Grove's battery, and in this case they must
make their own arrangement with Messrs, Griffin,
TABLE OF CONrENTS.
Introduction.
ART. FAG*
Definition of Physics 1 i i
„ of Motion . 2 2
„ of Fercc 3 4
The Chief Forces of Nature.
Definition of Gravity 4 7
,, of Cohesion ....... . 5 8
„ of Chemical Attraction 6 9
Uses of these three Forces . . . , 7 10
How Gravity acts.
Centre of Gravity 8 1 1
'Xi\z Balance 9 13
The Three States of Matter.
General Remarks 10 14
Dehnition of Solids 1 1 16
„ of Liquids 12 16
,, of Gases . 13 16
Properties of Solids.
General Remarks on Cohesion ....... 14 16
Bending 15 19
Strength of Materials . • '6 20
Friction. 17 20
Properties of Liquids.
Liquids keep their size though not their sliape . . 18 21
They communicate pressure 19 21
Water-press explained 20 23
Liquids find their level .,.,., ^ .,., 21 24
TABLE OF CONTENTS.
^\
ART.
V/ater- level and Spirit,-level ........ 22
Pressure ot deep Water . . . . • 23
Buoyancy of Water . 24
Flotation in Water 25
Comparative Density or Specific Gravity .... 26
Buoyancy of other Liquids 27
Capillarity 28
, . . Properties of Gases.
Pressure of Air 29
Weight of Air .............. 30
Barometer explained^— Mercurial t^olunin . ... 31
Uses of the Barometer .... ^ ..... 32
Air-pump explained . . 33
Water-pump explained — limits of working; . . ,34
Syphon described ....'. 35
Moving Bodies.
Definition of Energy . . . . . ... . .36
,, of Work ...... •. •."... 37
Work done by a moving body 38
Energy in repose 59
Vibrating BooFis.
Sound explained 40
What is Noise and what Music 41
Sound can do work 42
It requires a medium (Air) to carry k ..... 43
Its mode of moticn through the Air ..... .44
Its rate of motion , . . 45
Echoes or reflection of Sound 46
How to find the number of vibrations in one second
coi responding to any rvote ....... ^ 47
Heated Bodies.
Nature of I Icat (first notice) ........ 48
Expansion of bodies generally when heated ... 49
Thermometer described . , .50
How to make a Centigfad6 Thermometer . . . . 51
Expansion of Solids 52
„ of Liquidi .,*.,- ^ . - , 53
PACK
25
26
28
30
31
32
33
34
35
38
40
41
43
46
47
48
49
50
52
53
54
54
54
56
57
59
61
63
65
66
69
70
via
TABLE OF CONTENTS.
AKT. PAGB
Expansion of Gases 54
Remarks on Expansion • 55
S.pecific Heat • . 5^
Change of state, with table of melting-points ... 57
Latent heat of Water 5^
,, of Steam . . ....••• 59
Ebullition and Evaporation ..*,.••. 60
Boiling-point depends on pressure 61
Other effects of Heat , .> 62
Freezing mixtures 63
Distribution of Heat — General statement .... 64
Conduction of Heat . • • • 65
Convection of Heat 66
Radiant Light and Heat — General statement ... 67
Velocity of Light, how discovered 68
Reflection of Light, laws of 69
Refraction or bending of Light 70
Lenses and Images given by them 71
Magnifying-glasses and Telescopes 72
Different l^ending of different rays 73
Recapitulation 74
Nature of Heat (second notice) ...,...* 75
Electrified Bodies.
Conductors and Non-conductors 76
■f wo kinds of Electricity — their mutual action . . 77
They exist combined in unexcited bodies .... 78
Action of excited on unexcited bodies . . . . > . 79
The Electric Spark 80
Sundry Experiments — Electroscope 81
Action of roints 82
Electrical Machine described 83
Leyden Jar described 84
Energetic nature of Electrified Bodies ..... 85
Electric Currents , 86
Grove's Battery described 87
Properties of Current j heating, chemical, and mag-
netic effects 88
Electric Telegraph .......'» 89
Concluding Remarks 90
Things to be remembered ........
Instructions regarding Apparatus . . . . .
Ll3T OF Apparatus , ,
70
71
72
72
75
76
78
79
81
82
83
85
86
89
89
91
95
97
99
100
102
103
106
107
109
no
III
112
113
115
117
119
119
121
122
125
126
12S
130
13?
SCIENCE PRIMERS.
PHYSICS.
INTRODUCTION.
1. Definition of Physics. — You have been told
in the Chemistry Primer what sort of things we have
around us. You have seen what the chemist does j
how he weighs things and finds their quantity, and
also how he finds that certain things are compound,
and may be split up into two or more new things;
while again other things are simple or elementary and
cannot be so split up.
In fact you have been told about the various kinds
of things we have in the world, but you have not
yet learned much about the affections or moods
of these things. You are yourself subject to change
of moods; sometimes you appear with a smile on
yoiT face, and sometimes, perhaps, with a face full of
frowns or tears ; sometimes, again, you feel vigorous
and active ; sometimes dull and listless.
Now if you think a little you will see that the things
around you are subject to moods very much like yours.
To-day the face of nature looks bright and happy, and
SCIENCE PRIMERS, ^[introduction.
full of smiles ; to-morrow the same face is dark and
lowering ; the rain falls, the thunder roars, and the
sea is tossed with waves and very stormy. Or again :
let us take an iron ball which lies upon the floor ; it
is cold and heavy to the touch, but let us put it into
the fire, and when it comes out the same substance is
there, but the state of it is very different ; if you now
attempt to touch it, you will be sure to burn your fingers.
Or again : if, instead of putting it into the fire, we put
it into a cannon and discharge the cannon, it will
come out with tremendous velocity, and will knock to
pieces anything it touches.
Thus you see that a cold cannon-ball is a very dif-
ferent thing from a hot cannon-ball ; and also that a
cannon-ball at rest is a very different thing from a
cannon-ball in motion.
Now if we see you crying and unhappy, we ask
what is the cause of this mood, and we always find
there is a cause ; or if we find you listless and sleepy,
and wanting energy, we inquire what is the meaning of
c.U this, and we find that it has a meaning and a cause. .
So likewise when we find changes in the moods or
qualities of dead matter we inquire what is the cause
of these changes, and we always find they have a cause.
This inquiry we shall make in the following pages, and
you must attend well to the answer we get. You have
already been told that this mode of questioning nature
is called experiment.
2. Definition of Motion. — You must in the
first place get a clear idea of motion. Motion means
change of place. Some of you may have heard that
this solid earth on which we dwell is in truth moving
very fast round the sun, but we may, in the meantime,
INTRODUCTION.]
PHYSICS.
put away this thought altogether from our minds, be-
cause although the earth is moving very fast it carries
us all along with it, and everything goes on as smoothly
and quietly as if the earth were at rest.
Well then, if I sit on a chair in a room I may say
that I am at rest, but if I walk up and down the
room I am in motion. Now in order to understand
my movements, you must know something more than
the mere fact that I am moving about ; you must know
the direction or line in which I am moving, and
you must also know the rate or velocity with
which I am moving. You must try clearly to under-
stand the meaning of this word " velocity ; " and to
make you do so, let us suppose that I go out of doors
and walk along a straight road for two or three hours,
and always at the same pace. Well, I find that in
one hour I have got four miles beyond my starting
point, and that in two hours I have got eight miles
beyond it, and I therefore say that I am walking on at
the rate or with the velocity (for both words mean the
same thing) of four miles an hour.
But what if the rate be not always the same ? Sup-
pose a railway train to be coming near a station, and
just beginning to slacken its speed. The train is
first of all moving, let us say, at the rate of forty
miles an hour, but presently its velocity gets less and
less, until when it arrives at the station it comes quite
to a standstill. Now, how can we find its rate when
ihis is always changing ? or what do we mean when
we say that the train, before it began to slacken its
speed, was moving at forty miles an hour ? We simply
mean, that if the train had been allowed to move
for ^ whole hoiur at the same rate it had before it
SCIENCE PRIMERS. [iNTRonucTiON.
began to slacken its speed it would have moved over
forty miles. In fact, if instead of coming to rest at
the station it had been an express train, and gone on,
it would have been forty miles away an hour after we
began to notice it
There are different ways of expressing velocity:
sometimes we speak of so many miles an hour, as we
have done here, but sometimes it is better to use feet
and seconds ; thus if I drop a stone down a well
I Should say that it fell sixteen feet during the first
second after it was dropped. Sixty seconds, you all
know, make a minute, and sixty minutes make an hour.
In this little book, when speaking of velocity or rate,
we shall use feet and seconds more frequently than
miles and hours, and speak of a body as moving at the
rate of ten, or twenty, or thirty feet a second, as the
case ma^ be.
3. Definition of Force.—Now what is it that sets
in motion anything that was previously at rest ? Or
what is it that brings to rest a thing that was pre-
viously in motion ? It is force that does this. It
is force that sets a body in motion, and it is force (only
applied in an opposite direction) that brings it again
to rest Nay, more, if it requires a strong force to set
a body in motion, it requires also a strong force to
bring it to rest You can set a cricket-ball in motion
by the blow of your hand, and you can also stop it by
a blow, but a massive body like a railway train needs
a strong force to set it iu motion, and a strong force
to stop it. That which is easy to start is easy to stop ;
that which is difficult to start is difficult to stop. You
see now that force acts not only when it sets a body
in motiv D, but as tnily when it brings a body to rest
VM^n^-^St^ti^^
_fil£.:£^.^&
INTRODUCTION.]
PHYSICS, V
X 1.
In fact that which changes th# state of a body
is called force, whether that state be one of
rest or of motion.
Experiment i. — To prove this, take a tin pan
with some peas in the bottom of it, and hold the pan
in your right hand. Now quickly raise your right hand,
with the pan in it, until your right arm is brought to
a stop by a fixed bar of wood, which you tiave placed
a little above it (your other arm held stiffly will do as
w^U as the wood). Now what you have done is to
make the pan with the peas rise quickly upi and then
m
Fig. 1.
suddenly come to a dead stop. You have first, by the
force of your arm, given an upward motion to the pan,
and the pan has forced the peas to mount with it,
since clearly they could not remain behind. Then,
again, when your right arm holding the pan was mount-
ing quickly, you allowed it to be stopped all at once
by the bar of wood ; that is to say, the bar of wood
forced your hand to stf;p, and your hand in its turn
forced the pan, which you held tightly, to stop also.
But this stopping force does not affect the peas which
lie loosely at the bottom of the pan, so that they will
SCIENCE PRIMERS.
[forces
t)F
continue to mount up after the pan has been stopped,
and many of them will fall over the edge and be
scattered about upon the floor.
Experiment 2, — Now put some more peas into
the pan, having spilt the last ones; but iftstead of
raising the pan quickly upwards, lower it as quickly as
you possibly can. Here, the force of your arm makes
the pan move down very quickly, but does not affect
the peas which lie loosely on the bottom of the pan ;
the result will be, that the peas will not follow the
quick motion .of the pan, but will lag behind until at
last they are all scattered about upon the floor.
Let us now pause for a moment, and see what we
really learn from these two experiments. We learn from
the first, that after we have once set the peas in
motion upwards, since the stopping force of the bar
of wood does not affect them, they continue to move
upwards after the pan has been stopped. It requires
force to stop their upward motion, and this force we
could not apply by means of the bar of wood, so that
they continue to mount upwards until the force of
the ea'-.h at last brings them downwards to the floor.
You see, therefore, that it needs force to stop a
moving body.
Again, in the second experiment, we communicate
a downward motion to the pan, but the force of our
arm which does so, does not affect the peas which lie
loosely on the bottom of the pan. They, therefore,
keep their state of rest, and lag behind the pan until
at last the force of the earth brings them downwards
to the floor. You see, therefore, that it needs forcjft
to start a body at rest.
Force, therefore, may do two things j it may either
t>F NATURE.]
PHYSICS,
5f
stop a body in motion, or it may set in motion a body
at rest. But very often we find that a force, although
present, does not appear to act. Now, why is this ?
We reply, because it is prevented from doing so by
another equal and opposite force. Thus, I hold a
heavy weight in my hand ; if I open my fingeiK, the
force of tke earth which acts upon the weight will bring
it very soon to the floor ] but as long as I keep my
fingers shut I prevent this force from acting. Or,
imagine the same weight to lie on the table; if there,
were no table, it would fall to the floor ; but the force
of the earth which gives it a tendency to fall, is pre-
vented from acting or is resisted by the table. The
weight presses against the table, but the table with-
stands this pressure. So that you have here two forces
resisting or withstanding each other, the one being the
weight, and the other the resisting force of the table.
From all this we learn that force is that which
changes the state of rest or of motion ot a body, but
that very often force is resisted or prevented by an
equal and opposite force, so that it is not able to do
anything or to produce any eff"ect
THE CHIEF FORCES OF NATURE.
4. Definition of Gravity. — I have thus told
you what is the meaning of the word force, and
now let us loc>k about us in order to see what are
the chief forces with which we have to do, and to
see also what part each plays, and what is its use*
The most prqminent force is the attraction of the
earth. If we let go a heavy thing out of oui handsj
8
SCIENCE PRIMERS,
[forces
we know where to look for it; we know that it
will not mount towards the sky, nor will it move off
sideways in some direction, but it will fall to the
ground or earth. It falls dov/D, we say, and the very
words up and down depend upon the earth's force ;
so that if the earth had no force, we should not use
such words at all. The word " up " denotes a difficult
motion against the earth's force ; the word "down " an
easy motion, by help of the earth's force. It is difficult
,to walk up a hill, but it is very easy to walk down.
Now when we say that the earth attracts things, you
must not think that all, or nearly all of the things
which we see are moving towards the earth. You and
I are not so falling, nor should we wish to be in such
a very dangerous condition. Why are we not falling?
Because we stand upon the floor; but if there were
no floor, we should fall through to the ground, and
the floor must be strong enough to support our
weight, otherwise it would give way and we should
fall. Sometimes a wooden floor or platform has been
so filled with people that it has given way, and they
have fallen to the ground, and many of the people
have been killed or very much hurt.
Thus you see that the earth attracts everything, but
yet most of the things which we see around us are
not moving towards the earth, because they are sup-
ported by something else that is able to i-iist their
weight. In fact, this property of things called weight
is really caused by the attraction of the earth.
This force which the earth exerts is called gravity*
5* Definition of Cohesion. — But there are
oth^r forces besides that which the earth exerts. If we
take a piece of string or of wire, and try to break it
OF NATURE.]
PHYSICS,
')
into two parts, it exerts a force to prevent our doing
so, and it is only when the force we exert is greater
than the force with which it resists us that we succeed
in breaking it. In fact the different parts or particles of
the string or of the wire are held together by a force
which resists any attempt to pull them asunder. And
30 are the various parts or particles of all solid bodies,
such as w^ood, stone, metals, and so on. It is often
very difficult to break a substance to pieces, or bend
it, or powder it, or alter its shape or size in any way.
Now that force which binds together the various
particles of a body is called cohesion.
You see now the difference between gravity and
cohesion : gravity is that force which the earth exerts
to pull bodies to itself, and which acts at a great
distance ; so that, for instance, the moon, which is 240
thousand miles away, is attracted by the earth. Cohe-
sion again is that force which the neighbouring particles
of a body exert to keep each other together, but this
force does not act except when the particles are very
near each other ; for if once a thing is broken or
ground to powder, its particle j cannot come easily
together again.
6. Definition of Chemical Attraction. — Be-
sides these two forces there is the force of chemical
attraction or affinity. You are told in the Chemistry
Primer (Art. 4) that the two things coal and oxygen gas
unite chemically together, and that carbonic acid gas
is the result of their union. The coal and the oxygen
gas are pulled together by a force which thev exert
on each other as truly as ai stone is pulled towards
the earth. In virtue of this force they rush together
and unite, and the result is something quite different
lO
SCIENCE PRIMERS,
[forces
from either. Thii, then, is the force which we call
chemical attraction, and which has this peculiarit|^
that it can only be exerted by different bodies ; for
in chemistry it is only bodies of different kinds that
rush together and unite after this fashion.
7. Use of these Forces. — Having now told you
something about the chief forces of nature, let us try
to find what part they play, and why they are there at
all \ and I think we shall soon see that we should get
on very badly without them. Let us begin by supposing
that there was no such thing as gravity, and that the
earth did not attract things to it. Now sometimes
when we climb a steep hill we are tempted to think
how pleasant it would be if we could go up as easily as
we go down. How we wish there was no gravity!
But it would be a terrible misfortune if on^^ of
those spirits we read of were at once to grant us
our request. There being no gravity there would of
course be no weight, and we should then get up a
hill easily enough, but if we jumped into the air
we should remain there ; and possibly we might be
able to leave this world altogether. The furniture of
our houses would be found some on the floor, some
on the roof, some floating about, and we ourselves
could walk on the roof as easily as on the floor. The
moon meanwhile, not being bound to the earth, would
leave us for ever ; and in like manner the earth, being
no longer bound to the sun, would leave it far behind
and wander off among the stars.
So much for gravity. Let us now see what would
happen if there were no cohesion. If this force were
absent, the particles of solid bodies would not adhere
to one another, but they would all fall to pieces or
OF ITATURE.]
PHYSICS.
II
call
for
that
rather to powder. The wood of our tables and chairs
would fall to powder, and we should have no furniture ;
and the bricks of our houses would do the same, so
that we should have no houses. We should do the
same ourselves, and in fine all things would resolve
themselves into a huge mass of dust
Finally, let us think what would happen if there
were no such thing as chemical attraction. In the
first place the fire would cease to burn because the
carbon of the coals would no longer care to unite
with the oxygen of the air.
In the next place no two simple or elementary
substances would unite together to forrfi a compound
substance, but we should have nothing but about sixty
simple substances consisting of a great number of
metals and a small number of gases. There would be
no variety in such a world, and indeed there would be
no living in it, for our own bodies are compound ; and
if chemical affinity were destroyed part of them would
go up into the air and mix with it, while another part,
consisting of a quantity of carbon, a little phosphorus,
and some one or two metals, would fall to the ground,
and thus we shoaid come to an end.
HOW GRAVITY ACTSt
8. Centre oi Gravity. Experiment 3.— Let us
now endeavour to find out what sort of a force gravity
is, and for this purpose let us take this irregular sheet
of iron and hang it up by a thread. You see i\
hangs in a particular way, and you also see that the
line already drawn with paint on tl^^ sheet is in
12
SCIENCE PRIMERS.
[now
the same direction as the line of the thread. Next
let us hang the sheet freely from some other point,*
here again you have another white line in prolonga-
tion of the thread, and you further see that these two
white lines cut each other in a point marked g.
Fi::. 0.
Now let us hang up the sheet by some third point
in its rim. As before, you have a white hne in pro-
longation of the thread. Now you will easily see that
these three white lines all cut one another at the same
point Gj in fact, if you suspend the sheet from
any point freely by. a thread, and draw a white line
in prolongation of the thread, all sucli lines will cut
one another in the same point g, so that this point
will always be directly under the point from which
the sheet is hung, and if you push the sheet to one
side it will return agaih to its old position. Now
what is this peculiar point g ? To find out let me
«
■i^^iW'^
GRAVITY ACTS.]
PHYSICS.
n
attach a string to G, and hang the sheet by the string;
you see that the sheet will balance round g in all
directions just as well as if its whole weigh were
ccmdensed into the point G. Now G is what we
call the centre of -gravity of the sheet; and if
,1
Fig, 3.
I hang up the sheet freely by a siring, it will put
itself in such a position that its centre of gravity G
shall be as low as possible. If instead of hanging the
sheet by a string I suspend it loosely upon a peg, it
will still try to place the point G as low as it possibly
can, and it will not hang as in fig. 3, but the point G
will be immediately under the peg.
9. The Balance. — Every substance has a point
G of this kind, which we call its centre of gravity.
The balance which you see on page 28 has, like every-
thing else, its point g — its centre of gravity. And it
will endeavour, just like the sheet of iron, to place
this point as low down as it possibly can.
Now when there are equal weights in both scale-
pans, this point g is somewhere directly under the
point upon which the balance is swung ; and hence,
if by pushing it I try to tilt it to a side, when freed
it will ultimately return to its old position. In fact,
when the weights in each pan are equal, it will always
t4
SCIENCE PRIMERS.
tnikEE
keep this position, with the pointer pointing exactly in
the middle ; so that if I am weighing a substance, and
place this substance in the one scale-pan, and the
weights in the other, and if the pointer points exactly
in the middle, I am then quit» sure that the weights
in the one scale-pan are exactly equal to *^ //eight
of the substance in the other. But if the weights are
not heavy enough, the beam of the balance will be
tilted over by the substance in one direction ; while
if the weights are too heavy, they in turn will tilt over
the beam in the other direction.
Experiment 4. — Suppose that I put this piece of
metal into one of the scale-pans, and put weights
equal to 150 grains into the other, the scale-pan with
the metal in it sinks down, thereby showing that the
metal is heavier than the weights. Next let me put
weights equal to 250 grains into the other scale-pan.
Now again it is these 250 grains that are too heavy,
and you see that the scale-pan containing them sinks
down, whereas before it was the other that sank.
Thus the weight of the metal is somewhere between
150 and 250 grains. Let us therefore \xy a 200-grain
weight, and you see that now the pointer points
exactly in the middle, and the beam of the balance
is exactly horizontal, showing that the weight of the
metal is exactly 200 grains.
THE THREE STATES OF MATTER.
10, You have seen that we cannot do without the
various forces of nature, and that if one piece of
matter were not drawn or attracted to another piece,
STATES.]
PHYSICS.
IS
there would be no such thing as a world at all. You
have seen, too, that if there were no cohesion, there
would be nothing but powder. I may now proceed
to tell you that if everything possessed cohesion to a
great extent, we should be nearly as badly off, for we
should in such a case have neither liquids nor gases,
neither water nor air.
The particles of a bar of iron or steel possess
very great cohesion, and it is very difficult to force
them apart. But water and mercury have hardly any
cohesion whatever, and the very slightest touch will
scatter in all directions a quantity of water or of mer-
cury. Yet these two liquids have still a little cohesion
left, as you may see by the following experiments.
Experiment 5. — Take a very small quantity of
mercury from the bottle containing it, and put it on a
flat glass surface. By pressing it you may split it up
into small globules. Now these globules are a proof
that the particles of mercury cling together. For, put
another plate of glass above them, and you may by
this means squeeze them flat ; but if you take away
the glass, the mercury will resume its previous glo-
bular shape.
Experiment 6. — Sprinkle a few drops of water
on an oily or greasy surface, and these will be found
to have a rounded form, not unlike drops of mercury,
showing that the particles cling to one another.
On the other hand, the particles of gases, such as
the air we breathe, have no tendency to keep together,
but rather the reverse. Indeed they will separate
from one another unless there is some force which
keeps them from doing so.
So that, you see, we have three very different states
i6
SCIENCE PRIMERS.
[rROrCRTIEfc
It
of matter, the solid, the liquid, and the gaseous ;
and each of these states has certain properties which
serve to distinguish it.
11. Definition of Solids. — A sclid body, such
as a piece of iron or wood, resists any attempt to alter
its shape or its size, always keeping the same size or
volume and the same shape, unless it be violently
destroyed.
12. Definition of Liquids. — A liquid like water,
when kept in a bottle or other vessel, always spreads
itself out, so as to make its surface level, but yet it
will always keep its proper size or volume. You
cannot by any means force g, pint of water into a
half-pint measure; it will insist upon having its full
volume, but it is not particular as to shape.
13. Definition of Gases.— A gas again has no
surface; for if you put a quantity of any gas into a
perfectly empty vessel, the <ras will fill the whole
vessel. Nor does a gas insist so violently as a liquid
upon occupying a certain space ; for by means of a
proper amount of force I can compress the gas which
now fills a pint bottle into half a pint, or even into
less~ space, if I use sufficient force. In fact, a gas will
be persuaded to go into less space, but a liquid will
not be persuaded.
PROPERTIES OF SOLIDS.
14. Ihe peculiar distinction of a solid is that it
insists upon keeping not only a certain space or size
for itself, but also a certain figure or shape.
* Experiment 7. — In fig. 4 you have two vessels of
different shapes, but of the same size. And if
OP SOLIDS.]
PNYSJCS,
i a
it
»ize
t7
you exactly fill the one with water and pour it into the
ether, you will find that the water exactly fills it also.
Here, again, you see two pieces of wood that have
both the same shape or figure, but the one is much
larger than the other— their size is diff/'Tent.
rig. 4.
^ou see now what is meant by space or size or
volume (for the three words mean the same thing),
and what by figure or shape. Now, you cannot take
a solid which has the shape of the one bottle and
force it into the shape of the other, although the size
or volume of both is the same ; nor can you take a
solid of the size or volume of the first wooden block
and squeeze it into that of the second, although the
shape of both blocks is the same. A perfect solid will
keep its figure, and it will also keep its size.
Bear in mind, however, that when we say we can-
not do a thing, we really mean we cannot do it
without very great difficulty, and then not completely,
but only to a very small extent; in fact, what we
' in. c
iS
SCIENCE PRIMERS. [properties
really mean is best explained by making a series of
simple experiments.
* Experiment 8. — Let me take a bar of iron ; I
will first of all try to break it in pieces by means
of a blow, but it won't be broken.
I will next try to stretch it out by hanging it up
tightly by, one end, and then applying to the other
end a heavy weight, but it won't be stretched.
I will now, by means of two rods, fitting in to the
bar at its ends, as you see in the figure, try to twist
Fig 5.
round the one end, while I hold the other still, but
it won't be twisted.
I will now set the bar endwise upon the table, and
put a heavy weight above it, to try and squeeze it
together, but it won't be squeezed.
And finally I will hang it up horizontally by both
ends, and attach a weight to the centre, and I find it
won't be bent.
Now the bar of iron which I can neither break by
a blow, nor stretch, nor twist, nor squeeze together,
nor bend, is a very good example of a solid body; and
yet, if I applied an exceedingly great force, this bar
might be stretched, or twisted, or squeezed, or bent
And in uuth I did actually stretch, and twist, and
.„.^«^.
OF SOLIDS. 1
Pl/VS/CS.
t^
squeeze down, and bend it, in the experiments I have
just described, but not enough to make it visible to
you. In fact the amount by which I stretch, or twist,
or squeeze down, or bend the bar, depends upon the
amount of force I use ; and in Physics we try to find
out the relation betv/een the force which we use and
the effects which we produce. I cannot tell you all
about this subject, because it would take up a great
deal of time, but we may take one operation, such as
bending, and endeavour to find in what way its effects
depend upon the force which we employ.
15. Bending. Experiment 9. — For this purpose
let us *ipport a wooden beam in a horizontal position
by both ends, and let us hang a somewhat hea\7
weight from its middle or centre. Then let us mea-
sure upon a scale how far the centre has been bent
down by the weight. Let us now double the weight
that hangs from the centre, and mark the new position
of the centre of the beam under the increase of weight,
and we shall find that the centre of the beSim has
been lowered about twice as nnich by the double
weight as by Ihe single weight, or
in fact the bending is nearly
proportional to the weight
applied.
Experiment 10. — Let us now
take the very same beam of wood,
and place it in edgewise, so as to
give it a great depth, rather than
a great flat surface, and let us
apply the same force as before.
We shall find that the beam is not bent nearly so much
as it was before. ^2
Fig. 6.
20
SCIENCE PRIMERS, tPKOPEkTlB^
i6. Strength of Materials. — Now if an archi-
tect or an engineer were using great wooden beams in
the construction of a building, it would evidently be
most advantageous to strength were he to place them
in such a way that their depth might be as great as
possible, for in such a position they would give way
much less under any heavy weight.
An architect or engineer ought therefore to know
all about the strength of things, and how to place
them so as to get the greatest possible strength out of
the least possible amount of material ; in fact he ought
to know how to use his wood or his iron in the best
possible way.
Another point that the architect or engineer should
bear in mind is to make his house or his bridge five or
six times strong enough to bear the greatest load that
will ever be put upon it. For sometimes a building
may be strong enough to stand a heavy weight on the
floor, or a bridge may be strong enough to stand the
piissage of a long train, without absolutely breaking
down, and yet the floor of the building may be so
much bent that it won't quite recover itself when the
weight is taken off", or the bridge may in like manner
be so much bent that it won't recover itself when the
train has passed. In such a case the floor will be less
strong each time the weight is put on it, and the
bridge will be less strong each time the train passes.
They will in fact go on bending more and more, until
at last they give way. The architect or engineer must
therefore take great care that his structure is never
bent beyond the limits of perfect recovery.
17. Friction. — Before leaving solids, let us say a
few words about friction. If I put a very heavy
OF LIQUIDS.]
PHYSICS,
21
weight on the table, it will require a very strong force
to move it along. But if the table were of marble
and not wood, then a much less force would make
the weight slide along, while if the weight were on a
sheet of ice it would move with a still smaller force.
Now the force which makes it difficult for me to push
along a heavy weight, is called the force of friction.
We should fare almost as badly without friction as
we should without the other forces : for if there were
no friction, we should be always walking, as it were,
on ice ; and if there were the slightest slope, nothing
would be able to stand upon it, but everything would
slide down to the bottom.
PROPERTIES OF LIQUIDS.
i8. They keep their Size. — In a liquid such
as water, we can move the particles about very easily,
but we cannot by any means force a quantity of water
into smaller size, or make a quart content itself with
a pint bottle.
* Experiment ii.— ^Let us, however, try to do so,
and see what result we get, because we ought always
to make an experiment when we can. Let us take
a quantity of water shut in at one end, while at the
other there is a water-tight piston or plug. Now
let us try to drive this piston down in order to force
the water into smaller volume, and to do so let us
put a large weight upon the piston ; but notwithstand-
ing all this we cannot compress the water.
19. They communicate Pressure. * Experi-
ment 1 2. — Let us now take a quantity of water shut
IS
SCIENCE PRIMERS, [rp.oPERTiES
Fig. 7.
in by two plugs or pistons. If we push the one
piston down, we cause the other to mount up. Now
if we put a ten-pound weight on the
one piston, and an equal weight on the
other piston, the one will exactly balance
the other, and neither will be moved.
* Experiment 13. — In the last expe-
ment both pistons were vertical, as in
fig. 7 ; but now let the one piston be
vertical and the other horizontal, and
by means of a simple arrangement
apply a ten-pound weight to the hori-
zontal piston. If now we apply a ten-
pound weight to the vertical piston, we shall exactly
balance the ten-pound weight attached to the hori-
zontal piston. , If, however, we apply a twelve-pound
weight to the vertical piston, we drive along the
horizontal piston ; and in like manner if we apply a
twelve-pound weight to the horizontal piston, we drive
up the vertical piston. Thus, by means of the water
we can convert the downward push of the ten pounds
on a vertical piston into an equal push, only horizontal
and outwards against the other piston. And thus you
see a liquid such as water communicates pressure in
all directions. This faci' was found out by Pascal.
* Experiment 14. — In this experiment we have two
vertical pistons, but the surface of the one piston is
double that of the other. Now if we put ten pounds
on the smaller piston, it will no longer be balanced
by the ten pounds on the larger piston, but we shall
require to put twenty pounds on the larger piston,
in Older to balance the ten pounds on the smaller
piston. In like manner, if the large piston has three
OF LIQUIDS.]
PHYSICS,
23
times the surface or area of the small one, we shall
find that ten pounds on the small one will balance
thirty pounds on the large one. Not only, there-
fore, does the downward pressure on the one piston
communicate an upward pressure to the other, but
the whole upward pressure is proportional to the
surface of the piston ; so that if the one piston has
three times the surface of the other, it will be driven
up with a pressure three times as great, and so on.
20. Water Press. — Now this is a very valuable
property of water, and it has been made use of in the
construction of a very powerful machine, called the
Bramah Press, from the name of its inventor. We have
here a figure of it You see ci couple of bales of wool
■V
Fig. 81
which we wish to squeeze as much together as pos-
sible, in order that they may occupy little space when
carried about from one place or country to another.
You see also two pistons— a large and a small piston
— the large piston having one hundred times the area
24
SCIENCE FRIMERS.
[PROPfeRTIES
or surface of the small one. Now if I put a ton on the
small piston, I must put a much greater weight on the
large piston to keep it down, for the large piston is
one hundred times the area of the small one. I must
therefore put one hundred tons on the large piston
in order to balance the ton on the small piston, so that
this large' piston will rise with the enormous force of
one hundred tons, and press with this force against
the bales of wool, which will therefore be squeezed
very tightly together. It is necessary, of course, in a
machine of this kind, that every part of it should be
very strong and very tight, otherwise the water would
burst out with immense force through any crevice or
weak part. ■ y
21. Liquids find their level.— The next pro-
perty of liquids is that they always place themselves
so as to have a level surface. You will see at once
that this surface could not be slanting, for then the
part which is high up, having no friction, would slide
down towards the lowest part. A geometrician would
tell us that if we hang a plumbline above tl surface
of water, this plumbline will be perpendicular to
the surface ; that is to say, it will not slant towards
the surface in any one direction, but will stand
straight up, and we may show this by a very simple
experiment.
Experiment 15. — Take all the mercury in the bottle
and pour it into a flat vessel, and get it to cover all the
bottom of the vessel by making the vessel level. Now
hang a plumbline over the vessel, and you will see
that the reflection of the plumbline and the plumb-
line itself are in one direction, so that tl^e one appear*
to be a continuation of the other. This shows that
OF LIQUIDS.]
PNYsrcs,
25
the plmnbline does not slant towards the surface;
for if it did, the reflection and the plumbline itself
would not foriii one line, but would appear as two
lines bent towards one another.
Experiment 16. — Even when the liqu'd is con-
tained in bent tubes, that in the left-hand tube will
always be at the same level as that in the right, and
this will take place whatever be the shape of the tube.
Fig. 9. ' -::;
Indeed, I have only to fijl some of these curiously
shaped tubes with water in order, to convince you that
this is the case. You see the water is at the same
level in all the tubes.
22. Water-level. — And this leads me to speak
of the water-level which you see in the figure. If I
place my eye in a line with the top of the water in
both the ends of the tube, I know that I am looking
along a level line, and that all the points near me
which I see along this line are precisely at the same
level, so that if a flood were to come it would reach
them all precisely at the same moment.
It is often very important to know what points are
on the same level : a man who constructs a csuiai or
26
SCIENCE PRIMERS,
tPROPBRTlES
a railway, must know this ; and in order to do so, he
must use a level of some kind. The kind which is
Fig. TOW
most often used is called the spirit-level ; that which
we have described is called the water-level.
' 23. Pressure of deep Water. — L'^t us now take
a somewhat deep vessel filled with water. You will
see at once that the layers of water near the bottom
are pressed upon by the weight of all the water above
them, so that the pressure upon these layers will be
greater the further they are below the surface. In
fact, the layers two feet below the surface will be
pressed upon with twice as much water as those only
one foot below ; in other words, the pressure will
be proportional to the depth.
Experiment 17.— This pressure will act in all direc-
tions, upwards and sideways, as well as downwards.
To show this let me nearly fill a vessel with water
and withdraw a plug from the side near the top. You
'see the water is pushed out by the pressure upon it,
but not very forcibly ; let me now withdraw a plug
near the bottom, and you see that, owing to the
great weight of water above, the pressure is now much
stronger, and the water rushes out with great foim
So much for a pressure sideways. I shall now try t%
\
5f liquids.]
PHYSICS.
27
^«
\
show you that there is also an upward pressure. To
do so I take what is called a cylinder or wide tube of
glass without either top or bottom. But here you see
I have a separate closely fitting bottom which I attach
to it, and you see, too, that I have a string coming up
through the cylinder, by which I can hold it tightly^
Holding it on by the string, I will now plunge
the cylinder below the surface of water in the vessel,
and you see that I may now
let go the string, but yet the
bottom does not fall off be-
cause it is kept on by the
upward pressure of the water
against it. I will now pour a
quantity of water coloured blue
by indigo into the cylinder, and
yet the bottom is held on, and
it will only drop off when the
water in the inside of the cy-
linder has reached to nearly
the level of the water on the
outside, because then the up-
ward pressure against the outside of the loose bottom
is balanced by an equal downward pressure of the
coloured water against the inside of the same.
If any of you should ever be in a boat on deep
water, you may easily prove to yourselves the great
pressure of water at a great depth. Take an ordinary
quart bottle and fill it three-fourths full of water ; then
cork it tightly, and attaching it to a long String, let
it down into the deep water. If it be allowed to
descend su(aciently far, the pressure of the outside
water will be so great as to force the cort into the
Fig ir.
28
SCIENCE PklkEIi:^, [pRoPi^RtiBA
bottle, and when you pull it up you will find the
bottle full of water with the cork inside.
24. Buoyancy of Water. — Let us now try to get
precise ideas about the buoyancy, or floating power
of watery and, to do this, let us make one or two
experiments.
Experiment 18. — Let us take our balance, which
we have previously spoken about (page 13), and get it
into order for weighing. Now here we have a sub-
stance which weighs 1,000 grains, as you see, when
I
^'
Fig. X3.
we make the weighing in air. Let us now attach ^
substance to the right-hand scale-pan, and make t^^
weighing in waten What is the result? Wefindtilit
the
get
wer
;wo
ich
tit
ub-
len
I
OP LIQUIDS.]
PHYSICS.
^
At
actually it appears to have no weight at alli and I
require to put on the right-hand scale-pan i,ooo grains,
or the whole weight of the substance, in order to
make it equal to the other scale-pan in weight
Experiment 19. — Are we to imagine that this sub-
stance, when in water, loses its weight altogether?
Let us try, by experiment, whether or not this is the
case. First of all I shall place a vessel with some
water in it on one scale-pan, and balance it by
weights in the other, I now drop the substance
weighing 1,000 grains into the water, and you see the
result. The scale-pan with the water having the sub-
stance in it is now much too heavy, and I have to put
1,000 grains into the other in order to restore the
balance. But this is precisely the weight of the sub-
stance, and therefore you see the substance does not
really lose its weight. The weight is still there ; that
is to say, the vessel with the substance in it is 1,000
grains heavier than if the substance were not there,
but the substance itself has its weight apparently
taken away by the buoyancy of the water, which acts
as an upward pressure.
Experiment 20. — Here we have (fig. 12) a brass
cylinder which fits, a^ you see, exactly into a hollow
socket Let us now take it out of the socket and attach
it, as well as the socket, to the hook at the bottom of the
right-hand scale^pan (see figure), and let us counterpoise
thtrn both so that they are exactly balanced. Let us
now weigh the cylinder, not in air, but water, by
{Placing a vessel containing water below the right-hand
scale-pan so that the cylinder is wholly immersed in
tiie water. The right-hand scale-pan is now too light
'itie brass cylinder has, in fact, lost part, though no^
30
SCIENCE PRIMERS,
Xproperties
all, of its weight by being weighed in water. To
see how much, we will pour some water into the empty
socket which is hung below the scale-pan. Now we
have exactly filled it with water, and we have, at the
same time, restored the weight which the brass cylinder
lost through being weighed in water, for now you see
the two scale-pans are balanced once more. But the
brass cylinder exactly fitted into the socket, so that
we have added water exactly equal in bulk to the brass
cylinder (that is to say, a socketful) in order to
restore the loss of weight. We gati 'Vi from this that
the brass cylinder, when weighed in water, appeared
to suffer a loss of weight exactly equal to the weight
of its own bulk of water, and we may extend this to
any other .substance, and say that when anything
is weighed in water it will suffer a loss of
weight exactly equal to the weight of its
own bulk of water.
25. Flotation in Water. — Let us now see what
this means. It means that if a substance immersed
in water be heavier, bulk for bulk, than water, such
as the cylinder, it will suffer a loss of weight ^^ual to
the weight of its own bulk of water, but yet it will not
appear to lose all its weight, because it is heavier,,
bulk for bulk, than water is. It will therefore fall to
the bottom because it will still have weight.
Experiment 21. — If, however, the substance be
of the same weight, bulk for bulk, as water, such as
that of Experiment 18, then it will lose all its weight
when in water, and will not sink. If I therefore put
this substance into water, yon see it neither sinks nor
swims, but moves 9.bout anywhere, just as if it had nq
weight,
I
.
IBS
OF LIQUIDS.]
PHYSICS,
I
3«
'
Now what will happen if the substance be h'ghter,
bulk for bulk, than water? How can it lose more
than its own weight ? you may ask. Let us learn, by
means of experiment, what will take place in such a
case as this.
Experiment 22.— Here I have a piece of wood
which is lighter, bulk for bulk, than water, and I force
it beneath the surface of the water ; but I find that
the upward pressure caused by the buoyancy of the
water is now greater than the weight of the substance,
so that it is forced up to the top of the water and
swims upon the surface.
Well, as the result of all these experiments, we may
conclude, firstly, that any substance immersed in water
appears to become lighter by the weight of its own
bulk or volume of water. And secondly, that in con-
sequence of this, if the substance be heavier, bulk for
bulk, than water, it will sink; if of the same weight,
bulk for bulk, as water, it will neither sink nor swim ;
but if lighter, bulk for bulk, than water, then it will
swim. '
26. Comparative Density. — Now I wish to
show you that we have here got a method by which
we can tell how much any substance is heavier, bulk
for bulk, than water.
* Experiment 23. — Let us imagine that we have
a small piece of gold that weighs in air exactly 19
grains — this is its weight. Let us next weigh it in
water, and we find that it now weighs only 18 grains,
showing a loss of weight equal to i grain. Now this
loss is equal to the weight of its own bulk of water,
which is therefore i grain. But the gold in itself weighs
19 grains, so that it weighs 19 times as much as it?
52
SCIENCE PRIMERS. [properties
own bulk of water. This is what we mean when we
say that the specific gravity of gold is 19. Now
we shall get the same result whatever be the size or
shape of the piece of gold we use. But on the other
handy if a person put something into our hand that
was not really gold, but only like it, we should no
doubt find by weighing it in water that the sub-
stance was not so much as 19 times heavier than
its own bulk of water. This method of finding out
the specific gravity or relative density of bodies was
discovered more than 2,000 years ago by a philosopher
calkd Archimedes. Hiero, King of Syracuse, had a
crown of gold, and he had reason to believe that the
goldsmith had mixed a quantity of silver with the gold,
but he could not think of any way of finding this out —
so in his difficulty he applied to Archimedes. The true
way of finding it out occurred to Archimedes one day
when he had gone to take a bath, and the tradition is
that he immediately ran out of the bath quite naked,
shouting out "Eureka! Eureka!" which means "I have
found it out ! I have found it out I " He then went
home and got a piece of gold which he knew was
pure, and found that when weighed in water it lost
one-nineteenth part of its whole weight, from which
he argued as we have done, that pr^e gold is 19,
times as heavy as water, bulk for t ^Ik. He next
took Hiero's crown, but he found that when weighed
in water it lost more than one-nineteenth part of its
whole weight, from which he argued that it was not
made of pure gold, and doubtless the goldsmith was
properly punished for his theft.
27. Buoyancy of other Liquids. — Other
liquids besides water have buoyancy. Indeed, e^H
.
OF LIQUIDS.]
PHYSICS.
33
-:
liquid has its own peculiar amount of buoyancy. A
very light liquid, such as alcohol or ether, has com-
paratively little; while a very heavy liquid, such as
mercury, has a great deal To convince you of this, I
have only to pour some of this mercury into a vessel,
and put on its surface a bit of iron — ^the iron, as you
see, floats ; showing that it is lighter, bulk for bulk,
than mercury. Gold, on the other hand, is heavier
than mercury; in fact, mercury is 13^ times as heavy
as water, bulk for bulk ; wh* ^ gold, you have already
seen, is about 19 times as 1 .avy, bulk for bulk.
Salt water is somewhat heavier than fresh ; and
there is in Palestine an inland lake called the Dead
Sea, so salt, and consequently so heavy, that a man
immersed in it could not possibly sink.
28. Capillarity. — Before leaving liquids, let me
just mention a well-known case in which water will
rise above its own level.
Experiment 24. — If we hold a lump of sugar above
the surface of water in a vessel, and allow its lower
end to touch the surface, we shall soon find the whole
lump wet In like manner, if we dip a strip of blotting-
paper or cotton-wick in water, we may convey it above
its level by these means.
But if we hold the sugar or strip of blotting-paper
with its lower end touching a surface of mercury, the
mercury will not rise into the sugar or the blotting-
paper ; so that these two liquids, water and mercury,
behave differently as regards the lump of sugar or the
strip of blotting-paper. In the first place, we see the
water rise mto them, and not only rise into them, but
remain there; the mercury, on the other hand, will
l^p( rise into them, and wiH not wet them ; in fact,
III. . D »
3*
SCIENCE PRIMERS.
[PROPEKTIES
mercury has not a sufficient attraction for sugar to rise
into it, nevertheless mercury may be made to adhere
to a surface of silver or of gold, because it has a
great attraction for these metals.
PROPERTIES OF GASES.
OF
29. Pressure of Air.— Gases have many points
of likeness to liquids, but in other respects the two
are very different. A liquid has a surface, so that you
may fill a bottle half full with a liquid and shake the
liquid against the sides of the bottle. But you cannot
do this with a gas. Here, for instance, I have a bladder
which contains gas, but the gas fills the whole bladder,
and not a part of it In fact, a gas has an intense
desire to fill any vacant space that is not already filled,
and will strongly exert itself to do so.
Experiment 25. — I can easily prove this by a very
simple experiment. I have here an air-pump which
I will afterwards describe to you ; meanwhile let me
tell you that by means of this air-pump, we can take
out of this bell-jar the atmospheric air which it now
contains. You see the india-rubber ball full of air
which I will put under the bell-jar. Now I will ex-
haust the bell-jar, that is to say, take its air out, and
what is the result ? There is air in the india-rubber
ball, but there is now none round about h, and in
consequence the air in the ball tries to fill the empty
space, but it can only do this by enlarging the ball,
arwi you see the ball grow bigger and bigger as I con-
tinue the exhaustion. I shall now let the air in, and
you se^ th^ b^U once more resumes its former siaj^,
OF GASES.]
FHYSICS.
35
5'ig. T3.
Experiment 26.— We may vary the experiment
this way. I shall now place on ,
the bed-plate of the air-pump a
jar which is covered at its top
by a piece of india-rubber tied
tightly round the rim. I now
exhaust the jar as before, and
find that as I withdraw the air
from the inside of the jar, the
outside air trying to force itself
into the void space presses down
the india-rubber cover, and per-
haps, before the experiment is
over, the pressure may be great enough to burst the
india-rubber.
30. Weight of Air. — You thus see that air will
force itself into any space that is empty, if it possibly
can, and indeed we have the greatest
difficulty in emptying all the air out of any
vessel. We can, however, take out the
greater part of the air which fills a vessel.
In fig. 14, for instance, is a vessel which
we can attach to the air-pump, and by this
means deprive it of air, and it will be found
that the vessel full of air weighs heavier than
the vessel empty, or, in other words, air has
weight.
Experiment 27. — Let us now attach a
light box bottom downwards to one of the
arms of the balance, and ascertain its weight. ^^^' **•
This weight may be said to be that of the box filled
with atmospheric air.
^xpi^RjMENT ?8,— While this light box remains
3<»
SCIENCE PRIMERS.
[properties
counterpoised let us t ext fill it by displacement (see
Instructions) with a heavy gas called carbonic acid gas,
which you are told how to make in the Chemistry
Primer, Art. 33. You see that the pointer is dis-
placed showing that the vessel now weighs heavier
than when it was filled with ordinary air, so that
some gases are heavier than others.
Experiment 29. — Hydrogen is the lightest of all
gases, and accordingly let us now attach the box bottom
upwards to the arm and when it is counterpoised fill
it by (^^cement (sec Instructions) with hydrogen,
which you are told how to make in the Chemistry
iVimer, Art. 1 7 ; the pointer will now be displaced in
the opposite direction showing that the vessel weighs
much lighter than wher^ filled with air, although not
so light as if it had nothing in it. We learn from
this, that although the particles of gases appear to
repel each other, trying to get as far from one another
as they possibly can, and always filling the vessel that
holds them, yet they are attracted by the earth and have
weight, so that there is no danger of our atmosphere
rushing away from the earth. Instead of this the at-
mosphere clings to the earth as a sort of ocean, and at
the bottom of this ocean of air we all live and move.
Now as far as regards pressure and weight an ocean
of air is similar to one of water, and you may remem-
ber you v/ere told, page 26, . - the pressure of water
against the bottom of a vessel depends upon its depth,
so that at a great depth you have a great pressure,
and this pressure is exerted in all directions.
Now if you are told that we have a great pressure of
air upon us, you will naturally ask, — How is It then
that we do not feel this pressure ? We reply — simply
because the pressure is exerted in all directions, up*
%
dt? CASES.]
pHysics,
at
(see
gas,
istry
dis-
wards, downwards, and sideways. Take a sheet of
paper — the pressure of the air not only acts on the
top of the sheet pressing it down, but it acts just as
strongly on the bottom of the sheet pressing it up, and
in consequence the sheet of paper can move about
freely just as if there was no pressure of the atmo-
spheric ocean upon it at all. And for the very same
reason you and 1 move about freely and do not feel
the pressure. Notwithstanding, I hope to convince
you by a simple experiment that we can make the
pressure of the air very perceptible.
Fig. IS.
Experiment 30. — Here are two hollow half-spheres
which exactly fit on to one another. Now let us press
them together and shut the stopcock, and you will
naturally ask why does not the pressure of the air
3S
SCIENCE PRIMERS, tPROPERTiES
01?
hold them finnly together ? The reason is that there
is also air within them, and this air presses out-
wards just as much as the air without them presses
inwards. But now let us fit on these two half-spheres
to the air-pump and take the air out of them, and
having done so let us shut the stopcock, and detach
them from the pump ; you will now find it very dif-
ficult to pull the two half-spheres asunder, because
while the air from without presses them together there
is no air from within to counteract this pressure, and
they are in consequence held very firmly together.
Now, since air is a fluid, and has weight, it will
have a certain amount of buoyancy, although not
nearly so much as water. If, therefore, a large bag
be filled with coal gas, or, better still, with hydrogen,
it will be lighter, bulk for bu?k, than the air, and will
therefore rise in it Such a bag is called a balloon,
and if sufficiently large it may also support a sm^l
car containing several people.
31. Barometer. Experiment 31. — Let us now
take a hollow tube of glass, open at one end and closed
at the other, fill it with mercury, and keeping the
finger tightly against the open end invert it into a
glass vessel also containing mercury, taking care not
to withdraw the finger from the open end until this
end is below the surface of the mercury in the glass
vessel. Here you see (fig. 16) we have the tube so
inverted standing upright in the vessel of mercury.
Now mark what happens. You see a blank space left
at the top of the upright tube of mercury, and your
first idea is that we must have let some air in, but
this is not the case. There is absolutely notHinsf ia
this blank space. You are next inclined i^ ask. Why
dol
pn
th(
m<
<w
UliliDI III II'--
Ol^ GASES.]
PHYSrCS.
39
does not the atmospheric air, which is no doubt
pressing in all directions, and therefore pressing upon
the surface of the mercury in the vessel, drive up the
mercury so as to fill this empty space ? The reply is
Fig. 16.
that it would if it could ; as it is it presses upwards
a^inst the surface of the mercury in the vessel with
ftifce sufficient to keep up in the tube a column of
¥>
SCIENCE PRIMERS.
[PllOPERTlES
hea^ mercury thirty inches high ; but it can do no more
— the weight of this mercury pressing downwards ex-
actly counterbalances the pressure of the air forcing it
upwards, and hence on the one hand the column of
mercury cannot push itself downwards, and on the
other the pressure of air cannot push the column
upwards, and we have therefore a blank space above
the column. This experiment was devised by an
Italian called Torricelli— the tube is called a Baro-
meter, and the empty space at the top is called tfec
Torricellian Vacuum. Most barometers are pro-
vided with a scale of inches by which the height of
the top of the column above the surface of mercury
in the cistern may be accurately measured.
32. Uses of the Barometer. — The barometer
is useful in many ways j for instance, we may by its
means tell the height of a mountain. You were told
(page 26) that the pressure is greater at the bottom of a
deep vessel of water than near the top, and the sane
thing takes place in this ocean of air in which wc
live — the pressure is greater near the bottom of this
aerial ocean than it is far up near the top. If there-
fore we go to the top of a high mountain, we have a
smaller weight of air above us than we had when down
below, and in consequence the pressure of the air will
be smaller at the top of the mountain than at the
bottom. The air will not now be able to balance the
same column of mercury as at the bottom, so that, in
the barometer, instead of a column of mercury thirty
inches high, we shall only have one of twenty-five inches
or possibly of twenty inches, depending upoyi the
height of the mountain. In fact the mercury will sink
lower and lower down in the tube of the barometer
EITIES
more
s ex-
ingit
nn of
n the
)lumn
above
by an
3aro-
sd the
repro-
ight of
lercury
ometer
by its
re told
)mof a
le sane
^ich wc
of this
there-
have a
down
air will
at the
ce the
that, in
thirty
inches
op the
ill sink
ometer
OF GASES.]
PHYSICS,
4t
the higher up you rise in the air, and thus by means
of the barometer you can tell to what height you have
gone. The barometer is also useful in telling us when
bad weal her is at hand. When the barometer falls,
that is to say, when the top of the column of mercury
gets lower in the tube and especially when it falls
quickly, we may expect bad weather. On the other
hand, if the mercury remains steady and high we may
expect a continuance of fine weather.
33, Air-pump. — We have abready spoken about
taking the air out of a jar, now this is done by the
air-pump. You wi see how this instrument acts
by means of the figure. But first of all I must tell
Fig. 17. : .
you what is meant by a valve. A valve is just a
tightly fitting trap-door that closes a hole, and that can
only open in one way — upwards, for instance. You
have, most of you, seen trap-doors in floors that open
upwards. Now in the figure you see to the left a bell-
jar full of air, which fits tightly upon a plate. You see
too coming out from the middle of the plate a tube
which opens into the bell-jar on the left side, and into
the cylinder or barrel oil the right, and thus connects
the two together. You see also a piston or plug that
can move up and down in the cylinder or banel.
J—
4»
SCIENCE PRIMERS, [properties
Finally you see two valves or small and tightly
fitting trap-doors, one of which is placed where the
tube enters the bottom of the cylinder, while the other
is in the piston itself. Both of these valves open
upwards and not downwards.
Now suppose we start with the piston at the bottom
of the cylinder, and the valves shut, and begin to pull
the piston up. In doing so we make an empty space
which the air on all sides will try to fill up if it possibly
can (Art 29). The air from above will try to press into
this space, but it will not be able to get in, and all it
can do will be to press against the outside of the
upper valve and keep it tightly shut, since the valve
does not open downwards. The air from the bell-jar
will succeed better, for it will rush through the tube
and press open the lower valve which opens upwards,
and then get into the empty space. Let us now
suppose that we have got the piston to the top of the
cylinder, and that we are beginning to press it down.
The push that we give to the piston, the piston gives
to the air ; and the air in its turn communicates this
push to the lower valve, which is kept shut. But the
air within is more successful with the upper valve, for
it pushes this open ; and so, as we continue to push
down the piston, all the air that was in the cylinder
below it is pushed out through the upper valve or trap-
door. But this air which we have pushed out was part
of the air that was originally in the bell-jar, so you
see that in the first double or up-and-down stroke of
the piston we have succeeded in squeezing out part of
the air of the jar. Let us now repeat the same process,
that is to say, raise the piston again, and the air from
above will shut the upper valve, while the air from the
SITIES
ghtly
i the
other
open
jttom
pull
space
>ssibly
yS into
1 all it
of the
valve
jell'jar
e tube
iwards,
is now
of the
down.
gives
es this
Jut the
ve, for
push
ylinder
or trap-
raspart
50 you
roke of
part of
process,
ir from
om the
OP GASES.]
PHYSICS.
43
bell-jar will rush along the tube, push open the lower
valve and fill the empty space which we make when
raising the piston. And when the piston descends
once more, the lower valve is kept shut, while the air
within pushes open the upper valve and gets out,
and thus in every double stroke we get rid of part
of the air in the bell-jar. Of course it is quite
necessary in working the pump that the piston shall
fit quite tightly into the cylinder ; for, if not, the air
will get in from without, and therefore we shall not
succeed in getting the air out from within. I have
now told you the way in which the air-pump works,
but you must not expect every air-pump to be precisely
like the figure I have given you; the principle,
however, of all air-pumps is the same, although the
appearance may be very different in each.
34. Water-pump. — Having now told you about
the air-pump, let us return for a moment to the baro-
meter. You have seen how the pressure of air is just
strong enough to hold up a column of mercury about
thirty inches high. But water is much lighter, bulk for
bulk, than mercury, and we might therefore expect the
pressure of the air to hold up a much longer column
of water than one of thirty inches. In truth, the pres-
sure of the air will hold up a column of water very
nearly thirty feet in height.
This will enable you to understand the mode of ac-
tion of the common pump. In the figure on the next
page you have a sketch revealing the interior of such a
pump. Below we have the reservoir from which we
wish to pump the water up, and we have a tube leading
from this reservoir up into the barrel of the pump. In
thi9 barrel ^ou s^e a piston which fits tightly into the
44
SCIENCE PRIMERS, [PROPERTIES
barrel, and in this piston there is a valve opening up*
wards, while at the bottom of the barrel there is another
valve also opening upwards. In fact, the barrel of the
lifting pump is quite similar to that of the air-pump,
and we may begin by supposing that the piston is at
the bottom of the cylinder. Let us now raise up the
piston, and just as in the air-pump, the air above will
press down the upper valve and keep it shut. The
air in the tube will on the other hand, rush up through
the lower valve in order to fill up
the empty space made by raising
up the piston. When we lower the
piston again, just as in the air-
pump, the lower valve will be shut,
and the valve in the piston will
open and let out some air. In
fact, we are now pumping out the
air from the barrel and the tube.
But meanwhile, what is the water in
the reservoir doing ? The air from
without continues pressing on the
surface of the water in the reservoir ;
but as we have been taking away
the air in the tube, this pressure of
outer air is no longer counter-
balanced by that of the air in the
tube; the outer air will therefore find itself unopposed,
and will drive up the water into the tube, until at last,
when all the air is taken away, the whole tube will
be filled with water. This water will then enter the
pump barrel through the lower, valve.
But all this will not take place if the distance be-
^wc^p the surface of water in the reservoir and the
Fig. xS.
OP GASES.]
PHYSICS.
4S
the
lower valve be more than thirty feet. For you have
just been told that the pressure of the air will support
a column of water thirty feet high, but if the column be
higher than this it will not support it. So that if there
be a greater distance than thirty feet between the sur-
face of the reservoir and the pump barrel, the wrier
will refuse to enter into the barrel, and do what, you
can you will not be able to entice the water quite up
into the barrel. If. however, the distance be not more
than about twenty-six or twenty-seven feet, \aq pump
will work well, and you will get the water to enter the
barrel. Suppose now that you have got the barrel
filled with water, and that you 'are pressing down the
piston. As you do this the pressure you give the
piston will be communicated > by the water to the
lower valve, which ./ill be kept closed. On the
other hand, the pressure of the water will force open
the upper valve which opens upwards, and the water
will get above the piston. Next time when you
pull up the piston, you will pull up this water with i.,
and it will empty itself through the spout of the pump,
and the water will now come out of the spout at every
stroke.
* Experiment 32. — To enable you to see witl.
your own eyes what goes pn in a common pump,
take a model in which the pump barrel is made ol
glass, so that you can see into it. You will thus see
that when we raise the piston, the upper valve shuts
and the under one opens, while, as the piston descends,
the under valve shuts and the upper valve opens. You
quite understand that the piston of the pump must fit
tightly on to the barrel, because otherwise the air will
get ia from above and prevent the action. SometimeS|
4«
!^CiENCE pRIMERt
(MOVlNd
however, if a pump is not much used, the leather or
Other packing around the piston gets dry, and the
pump will not act In that case, if a little water is
thrown upon the piston, it wets the packing and
serves to make it tight.
35* Syphon* — Before leaving this subject, let me
describe to you an inr*rument called a syphon, of
which the action depends, like the pump, upon the
Fig. Z9.
pressure of the air. I shall not, however, explain its
principle. You see the syphon before you in the
figure ; it is used for conveying liquids from a vessel
at a higher to one at a lower level In the first place,
you must invert the syphon tube, and completely fill
it with water, keeping your finger at the end of the
ihorter tube. Now place the shorter end beneath
INti
•or
the
r is
and
me
, of
the
BODIES.]
PHYSICS,
47
in its
n the
vessel
)lace,
ilyfiU
rf the
sneath
the surface of the water in the higher vessel as in the
figure, and remove your finger. Once you have done
this, the water will, thereupon, flow in a continuous
stream from the end of the longer tube into the lower
vessel, and you may by this means remove the water
completely from the upper into the lower vessel, pro-
vided the short tube of the syphon be long enough
to reach to the bottom of the upper vessel
MOVING BODIES.
36. Energy. — You have been told (page i) about
the moods or affections of things, and how a cannon-
ball in motion is a very different thing from one
at rest, or a hot cannon-ball from a cold one ; and
you have also been told that one of our great ob-
jects in this Primer is to find out something about
these varying moods or affections of matter. We
could not begin with this, for we had first of all to tell
you about the things themselves, and you ought now
to have a tolerably good acquaintance with solids,
liquids, and gases ; it is time, therefore, that you
learned something about the varying moods or affec-
tions of things. You were told that bodies were
sometimes full of energy, such as a cannon-ball in
motion, and sometimes utterly listless and devoid of
energy, such as a cannon-ball at rest, and in what follows
we cannot do better than study the most conspicuous
cases in which a body is full of energy. Now this
happens when a body is in actual motion, or when
it is in rapid vibration, or when it is heated, or
when it is electrified, and we shall therefore class
4^
SCIENCE P/^/M£/^S.
tMOVlNG
energetic bodies under these four divisions. We shall
first of all speak of bodies in actual motion, and uitder
this head give you some idea of the way of acting of
such bodies ; we shall then speak of bodies in vibration,
such as a sounding drum or bell, and under this head
we shall tell you something about sound. We shall
next speak about heated bodies, and under this head
tell you something about light and heat ; and lastly,
when speaking about electrified bodies, you will hear
about that mysterious thing called electricity. We
cannot in this little Primer give you anything like a
complete account of the various moods of bodies, or
the various kinds of energy which they sometimes
possess. This must be reserved for a more advanced
stage; we can only give you a mere outline of the
subject, telling you at the same time that it is one of
very great importance.
37: l>efinition of Work.— When we say that a
man is full of energy, we mean that he is full of the
power of doing work ; and when we say that a thing is
full of energy, we mean in like manner that it is full
of the power of doing work. In fact, we measure the
energy of anything by the amount of work which it
can do before it is utterly spent. Now if we raise a
pound weight one foot high, we do a certain amount
of work, but if we raise it two feet high we do twice as
much work, if three feet high three times as much
work, and so on. If therefore we call the work of
raising a pound weight one foot high one, we should
call the work of raising it three feet high three.
Again, the work of raising two pounds to any height
is double the work of raising one pound to the same
height, so that the work of raising two pounds threi^
' fMT' "
BODIES.]
PHYSICS,
49
he
it
a
mt
as
feet high would be six. In fact, multiply the
number of pounds you raise by the number
of feet you raise them, and the product will
give you the work done.
Let us now suppose that we point a cannon straight
up into the air, and discharge a ball weighing i«^o lbs.
with velocity just enough to make it mount up i,ooo
feet before it turns ; we can tell at once from this how
much energy the ball had when it was discharged. It
had energy enough to carry loo lbs. (that is to say,
itself) up I, coo feet, and consequently ^ergy enough
to do work equal to i oo x i ,000 or 1 00,000. If we now
put a larger charge of powder into the cannon, we shall
make the ball come out with greater velocity. Sup-
pose that now it can mount up 1,500 feet before it
turns ; it has herefore energy capable of doing work
equal to 100x1,500=150,000. In fact, you see at
once that the greater the velocity or quickness with
which the ball is shot out, the higher will it go, the
more work will it do, and hence the greater energy
will it have.
38. Work done by amoving body. — I cannot
enter very fully here into the subject, but I will tell
you that a body shot upwards with a double velocity
will mount not twice but four times as high—
a body with a triple velocity not thrice but thrice
three times or nine times as high — and so on.
You see therefore that a cannon-ball of double the
velocity will do four times the jrk. But there are
other ways of measuring the work of a cannon-ball than
by seeing how high it can lift itself into the air, for we
may fire it into wooden planks placed one behind the
oAeft and we shall then find that a ball with a double
UL 1
Sd
SCIENCE PklMERS,
[moving
velocity will go through nearly four times as many
planks, a ball with a triple velocity through nearly
nine times as many, and so on. You thus see that a
ball with a double velocity will have four times the
destructive effect of one with a single velocity, and
indeed in whatever way we measure its energy it will
have four times as much energy as the other,
39, Energy in repose. — It is very easy to see
that a body moving very fast has the power Ca doing
a great deal of work, but besides this we have often
energy in a quiet state, just as a man may be quiet,
and yet able to do a great amount of work when he
sets about it. Suppose there are two equally strong
men fighting together, each with a heap of stones which
fchey are throwing at each other, only the one with his
pile of stones is standing on the top of a house, while
the other man is standing at the bottom with his pile.
I need not ask you which of the two is likely to win
the day ; you will tell me at once the man at the top
of the house. Now why has he the advantage ? He
is not stronger or more energetic than the other — his
advantage is therefore due to the stones ; it is clearly
because his pile of stones is high up. He himself
has not more energy than the man at the bottom, but
his pile of stones has more energy than the pile of
stones of the man at the bottom, and thus you see
that the stones have an energy arising from the high
position in which they are placed ; they are, in fact,
capable of doing work, whether this be the very use-
less work of knocking down a man or the very useful
work of driving in a pile. Or let us ^mppose two
water mills — one having a large tank or pond of water
at a high level near it, while the other has a pond ot
HODIBS.]
Pb^s/cs.
Si
tank of water, but at a level lower down than that of
the mill ; which mill is likely to work ? You will at .
once tell me, the one with the pond of water at a high
level, because the fall of water will drive round the
wheel. You see, therefore, that there is a great deal
of work to be got from a pond of water high up, or
a head of vrater, as this is called — real substantial
work, such as grinding com or threshing it, or turning
wood or sawing it. On the other hand, there is no
work at all to be got from a pond of water that is low
down.
Let us now compare a water-mill driven by a head
of water with a windmill driven by the wind. The
wind is like the cannon-ball, although not moving so
fast, its energy being that of a body which is actually
moving : it is in fact rushing against the sails of the
windmill and driving them round ; and if we throw up
a feather or a straw in a strong gale, we find that it is
hurried away by the wind. But a water-mill has one
decided advantage over a windmill, for in a wmdmill
we must wait for the wind ; but if we have a water-
mill with a good head of water we may turn the water
on and off whenever we choose. We can keep our
stock of energy and draw upon it whenever we have a
mind. In fact, the energy of a body in motion is
like ready money which we are in the act of spending,
but the energy of a head of water, or of any body which
is high up, is like money in a bank, which we may draw
out whenever we want it.
« ^ ■ ' • \
E9
$t
SCIENCE PRIMERS.
[VIBRATINO
VIBRATING BODIES.
40. Sound. — A body that is changing its place
is of course in motion, but it does not follow that
every moving body changes its place as a whole; a
top that spins round very quickly is in motion, but
it does not change its place as a whole.
Experiment 33. — Here is a wire which you see
attached by one end to a support ; now if the other
end be struck it goes backwaids and forwards rapidly,
but the wire as a whok does not change its place.
AVhen the pardcles of such a wire are moving back-
wards and forwards, they are said to be in a state of
vibration. In like manner, when a bell or a drum
is struck the particles of the bell or drum
are in a state of vibration, or when the
string of a musical instrument is pulled
and let go, the string is in a state of
vibration.
Now vibrating motion, just like motion
from place to place, denotes enei^gy, and
indeed the particles of a vibrating body
are moving actively about from side to
side ; if you try to stop them, they will
give you a blow. If anything is in their way, they will
give it a blow — the atmospheric air is, and they conse-
quently give it a blow. Indeed each time the top ^ this
vibrating wire comes back it gives the air a knock in
the same direction. In fact, a vibrating body gives in
a short time a great number of little knocks to the tiir.
When the air is struck, it does not receive the stroke
Fig. to.
, '-r^-
BODIES.]
PHYSICS.
53
quietly, but strikes the air next it, and this in turn
strikes the air next it, and so on, until the blow given
to the air is carried over a great distance. At last
this blow reaches your ear or mine, and we get a blow,
which, however, does not aifect us in the same way as
a blow that knocks us down, and therefore we do not
call it a blow ; but we say that a sound has struck
our ears — in fact we hear a sound.
41. What is noise and what music. — Now
if the body that strikes the air just deals it one
single blow, such as when a cannon is fired, the
air, carries Uiis one blow to our ear, and we say that
we hear a noise. If however the body that strikes the
air be in vibration, and deal it a great number of little
blowr> in one second, the air will carry these on and
give i>ur ears just as many blows in one second, and
then we say that we hear a musical sound. Thus
you see a noise is a single blow given to our
ear, but a musical sound is caused by a series
of little blows following one another at regu-
lar intervals. More than this, if the vibrating body
which is the cause of this disturbance deals the air only
a comparatively small number of blows in one second,
then the air will of course only deal us the same num-
ber in one second, and we shall hear a deep low
note ; but if the vibrating body vibrates very quickly
and deals the air a great number of blows in one
second, the air will of course deal us just as many, and
we shall hear a shrill high note. Thus you see
a deep low note means a small number of
blows dealt to our ears in one second, while
a shrill high note means a great number of
blows in the same time. A very shrill nr^e will
y^
SCIENCE PRIMERS,
[VIBRATIN6
be given by 20,000 blows in one second, and a very
low note by 50 blows in the same time.
42. Sound can do work. — ^A musical note is
pleasant, but a noise or single blow is disagreeable,
and sometimes it hurts or even destroys the ear if
it be a very violent one. Thus if a large cannon
were discharged, the blow to the ear might in some
cases destroy its hearing power; or if the sound
struck against a pane of glass, the concussion mighi
be s*) strong as to shatter the glass, and sometimes;
in such cases as the explosion of a powder magazine
all the windows in the neighbourhood are shattered
to pieces. Thus you see that a loud noise is something
with energy in it, and that it can do work — more
especially work of a destructive nature.
43. It requires Air to carry it. * Experiment
34.— Let us try to ring a bell in a place where
J there is no air, such as an exhausted receiver. There
being no air, there will be nothing which the moving
particles of the bell can give a blow to, and hence
no sound will reach our ears. Jn fact, a bell that
has been struck, or any other vibrating body, has in
it a quantity of energy, of which it parts with some
to the air, while the air in its turn parts with some to
our ear. But if there be no air, there is nothing to
carry to our ear the energy of the vibrating body.
44. Its mode of motion through Air. — Let us
now think a little about the nature of this thing called
sound, which is given out to the air by bodies in vibra-
tion, and which is then carried to a great distance by
the air itself.
In the first place, when a cannon is discharged a
mile or two off, do not imagine that the same partides
BODIES.]
PHYSICS.
55
of air travel all the way from the cannon to your ear.
The particles near the cannon give a blow to those
next ihena and then stop, the particles that have re-
ceived the blow give in their turn a blow to those
next thera and then stop, and so on, till the blow
reaches your ear. What really happens will be made
quite plain by the following experiment
^ * Experiment 35. — Let us take a series of elastic
balls suspended in a row by separate threads, so as to
Fig. 2x.
hang loosely together, just touching one another. Let
us now pull aside endways the first ball, and allovi
it to give a blow to the second. What will happen ?
The first ball having delivered its blow to the
^'^cond, will become quite still. The second will
vv y quicWy transmit the blow to the third, and be-
come still in its turn ; the third will do so likewise,
until the impulse reaches the last ball of the series,
which being the last will be put in motion l^ the blow.
S6
SCIENCE PRIMERS.
[vibrating
Now the first ball may be likened to the particles of
air which are next the cannon, and the last ball to the
particles that are next your ear, and thus you see how
the blow from the air next the cannon is transmitted
to the air next your ear without the necessity of the
same individual particles of air moving all the distance
in order to carry it.
Those of you who have played at croquet must have
noticed what takes place when you croquet your ad-
versary's ball. In this case you hold your own ball
tightly under your foot while your adversary's is just
touching it : you then by means of the mallet give a
blow to your own ball, which docL not however move,
but which transmits the blow to your adversary's ball
with sufficient force to send it a great way off. We
have here, therefore, a result the same as in the series
of balls.
45. Its rate of motion. — Again, this impulse or
blow which we call sound requires time in order to
pass from the cannon to our ear. No doubt it travels
very fast, as fast as a xifle-ball, but yet it does not pass
instantaneously from the cannon to our ear.
Most of you have no doubt seen a cannon fired a
long distance off, and you then saw, first of all the
flash and puff of smoke, and after a few seconds you
heard the noise. Now these few seconds are the time
which the sound or impulse took to travel from the can-
non to your ear. You saw the flash the very moment
the qannon was fired, and therefore, counting from its
appearance, you know how long the sound took to
travel from the cannon to you. Suppose, for instance^
that the cannon was 11,000 feet away, and that you
reckoned ten seconds between Ihe flash and the report,
BODIES.)
PHYSICS,
57
you therefore conclude that sound takes ten seconds to
pass through ii,ooo feet of air, or that it moves at the
rate of i,iob feet a second, which is ]"~ett> near the
truth.
Sound will, however, pass through water much more
quickly than through air, and by means of experiments
made at the Lake of Geneva it has been ascertained
that the rate of progress of sound through water is
nearly four times as great as through air. Sound travels
through wood or iron still faster — through wood, for
instance, it travels from lo to i6 times as fast as
through air, so that it would pass through more than
two miles' length of Avooden logs in one second of time.
46. Echoes. — Suppose now that I stand in the
centre of a large natural amphitheatre, having rocky
cliffs all round me, and from this position let me dis-
charge a gun — the noise or impulse will spread from the
gun to the rocky cliffs and strike them, but something
more will happen after that. The sound when it has
struck the cliffs, finding it can get no further, will come
back again, and in this particular case it will come
back along the very same line that it went, travelling
always at the rate of about r, 100 feet per second. The
result will be that a few seconds after the gun has been
fired I shall hear the sound that has travelled back from
the cliffs just as if another gun had been fired. Now
this sound is called an echo.
You thus see that in the case of echoes we have
the sound or impulse striking an obstacle and then
reflected back from it, but it does not always come
back in the same direction in which it goes ; this
depends upon the shape of the surface against which
it strikes. A very curious expeiiment is that which is
5»
SCIENCE PRIMERS,
[vibrating
shown in the following figure. Place two large hoiiow
reflectors at some distance from one another, and in a
point called the focus of the one put a watch, while
you place your ear in the focus of the other ; you will
then hear the ticking of the watch very distinctly, just
as if it were close to your car. The reason of this is
that the blows given by the watch to the air strike
against the left-hand reflector, and are reflected from
Fig* 29.
it indirections which bring them to the other reflector,
from which they are ^hen all reflected into the ear.
All this is shown in the figure. This property of
sound makes a very nice experiment, but it has some-
times proved inconvenient in practice : for instance, in
the Cathedral of Girgenti in Sicily, it is related that
the slightest whisper is conveyed from the great west-
ern door to the cornice behind the high altar, and that
unfortunately the former station was chosen as the
place of the confessional. The result was that a lis-
tener i^ced at the other station often heard what was
never intended for the public ear, until at length this
caineto be knowD, and another site was chosen. The
BODIES.]
PHYSICS.
59
■****^**i*WiWi«i<nra«J»i-iw
reflection of sound also explains what takes place in
whispering galleries. In that of St. Paul's in London,
for instance, a whisper at one side of the dome is
conveyed to the opposite side across a very consider-
able distance.
47. How to find the number of vibrations
in one second corresponding to any note.
—I have told you that when a vibrating body gives
the air a small number of blov/s in one second, we
have a d^ep note, and that when it strikes the air very
often in one second, we have a shrill high note : what
* Fig. as-
is called the pitch or tone of the note depends there
fore upon the number of blows which is given to the
air in one second. Now we can find out by experimn.nt
how many blows in one second correspond to any
particular note, and I hope by means of the above
figure to make it dear to you how this ia doae»
6o
SCIENCE PRIMERS.
tHEAT£l>
You see a large wheel a to the right, which is
turned by a handle. Over the circumference or rim
of this wheel we have a strong tight strap which passes
over the axle of another wheel b. The result is
that by mean: of the strap the axle of the wheel b
will go round a great many times for a single turn of
A, and the wheel b will itself of course move with
its axle — ^in fact, b may be made to move round very
quickly. You see, too, thai b is full of small teeth.
Now there is a bit of card placed at e against the
teeth of b, so that each tooth strikes the card as it
passes.
Each time the card is struck we hear a sound,
because a blow is given by the card to the air. If
there are loo teeth in the wheel b, there will be loo
blows given to the air in the time that b goes once
round. If b goes round once in a second, loo blows
wiU be given to the air, and in consequence loo
sounds Will strike our ear in one second, each single
sound of which we shaH not be able to distinguish,
but we shall hear an apparently continuous deep note.
Now by driving the handle fast enough I can make b
go round loo times in a second, and during each time
it ^vill strike the card loo times ; the card will in this
case be struck loo times loo, or 10,000 times in one
second : 10,000 little blows will now strike the ear each
second, and we shall hear ai continuous shrill note.
Now when you wish to find the number of blows in
one second corresponding to a given note, what you
have to do is this. Turn the handle more and more
quickly until the instrument by means of the card gives
you a note precisely of the sams pitch as the note
y ju have got to measure ; and when you have once
BODIES.]
PHYSICS.
6i
IS
got the proper speed, keep turning the handle for some
time at the same speed, say for one minute or more.
Now there is connected with the wheel b a dial
(which is shown separately in a large scale lying below),
and the dial registers how many times the card has
been struck since you began to turn. You must
therefore, when you yourself are turning the handle
steadily at the speed which gives the right note, get
another observer to note the position of the hand
in the dial at the b'^Qinning and at the end of one
minute. Suppose he finds out by the dial that during
this minute the card has been struck 60,000 times,
this will correspond to 1,000 times in one second, and
hence you conclude that the note given out is that
which corresponds to 1,000 blows given every second
to the air.
HEATED BODIES.
48. Nature of Heat. — You have seen that a
body in actual motion may be said to possess eneigy,
and also that the same may be said of a body in
vibration. You have further seen that a body in
vibration does not, in consequence, move about from
one place to another, but remains at rest as a whole,
while, however, its various particles are moving about
alternately forwards and backwards.
You have now to consider bodies in a heated state.
First of all, what is heat ? Let us reply by supposing
an iron ball to be put into the fire, and when white-
hot suppose we take it out, put it on the scale-pan
of a bidaiicei counterpoise it^ aii4 allow it to cqq).
6a
SCIENCE PRIMERS.
[heated
Now if heat be something that has entered into the
ball we should expect that as it cools it will grow con-
tinually lighter. If, however, this experiment be
properly made, it will be found that the iron ball does
not lose weight as it cools, and therefore, whatever
heat be, its presence has not made the ball one grain
the heavier.
Let me now suppose that I place myself upon a
very delicate scale-pan, and while I am there, exactly
counterpoised, let some water enter my ear. Of
course I shall now be heavier than I was before.
Suppose, however, that a sound enters my ear. Will
the sound make me heavier ? Not one whit. It will
strike what is called the drum of my ear, and set it
vibrating, and I shall hear the sound, but I shall not
be the least whit heavier in consequence of the
entrance of the sound into my ear. In fact, while the
entrance of water is the entrance of matter, and
makes me heavier, the ^iittiince of sound is only the
entrance of a kind of vibratory motion, and does
not make me heavier. Now may not something of
the kind take place in heated bodies ? May not the
entrance of heat mean the entrance of some kind of
vibratory or backward and forward motion, that does
not add anything to the weight of the body ?
We have strong reasons for thinking that heat is
really a kind of vibratory motiooi so that when a
body is heated each extremely small particle of it is
moving about either backwards and forwards or round
and round. But these particles are so very small, and
cheir motion so very rapid, that the eye has no means
of seeing what really takes place.
Why dien, you will >ay> doe$ pot ^ h^t^ bcdy'
BODIES.]
PHYSICS.
«3
give out a sound, if, as you tell us, its particles are in a
state of rapid motion? Why does not such a body
give a series of small blows to the air around it, just
as a body in ordinary vibration does ? We reply, thai
a heated body does give a series of blows to the
medium around it ; and although these blows are such
that they do not affect the ear, yet they affect the
eye, and give us the sense of light You see now how
great a likeness there is between a sounding body
such as a bell and a hot body such as a white-hot
ball. The particles of both bodies are in a state of
rapid motion : those of the bell strike the air around
the bell, and the air conveys the blows to our ear ; the
particles of the hot ball also deal a succession of blows
to the medium around the ball, and this medium
conveys the blows to our eye. Thus when we experi-
mented on vibrating bodies we used the ear, but when
we experiment on highly heated bodies we use the
eye. And in each case there are two divisions to the
subject : for in vibrating bodies we have to study in the
first place the bodies themselves, how fast they vibrate,
in what way they vibrate, and so on, and in the second
place we have to learn the rate at which the sound
they give out is carried through the air ; so in the case
of heated bodies, we have first of all to study the
bodies themselves, and secondly to learn how fast the
rays of light and heat which they give out travel
through the air.
49. Expansion of bodies when heated. —
When a body is heated, it almost always expands;
that is to say, it gets larger in all directions. To prove
to you that this is the case let us heat a solid, a Uquid|
and a gas.
64
SCIENCE PRIMERS,
[heated
BOD
* Experiment 36.— Let us take (fig. 24) a long
metallic rod held tightly by a screw at one end, b.
The other end is, however, free to expand, and in
doing so it will press against the pointer, p, and in
consequence this pointer will rise; if, therefore, the
bar expands ever so little, this expansion will be seen
very easily, for it will make the pointer alter its posi-
tion, and rise np towards the top. Now let us place
two or three lamps beneath the rod and heat it, and
we shall find that the rod expands, and presses
against the pointer so that it rises. If the lamps be
withdrawn, the rod will cool, and in the course of a
few minutes the pointer will have fallen into its old
position.
Expfii^iMENT 37.— Here is a hollow glass bulb
which 1^ filled with water; let us now heat this
glass bulb, and the water will rise in Ihe fine tube
which is attached to the bulb. In this case both the
glass bulb and the water expand^ but the water ex-
pands more than the glass bulb, and hence it pushes
its way upwards in the fine tube : indeed it expandis
with such force that if there were no empty tube into
which it might rise it would burst the bulb,
now
air
it n
expj
5<
expe
thini
ing.
BODIES.]
PHYSICS,
65
Experiment 38. — To vary the experiment, let us
now take a bladder which is about two-thirds filled with
air and heat it over the fire, turning it round so that
it may not burn. In a short time the air will have
expanded so as to make the bladder appear quite full.
50. Thermometers. — You see from all these
experiments that the tendency of heat is to make
things expand, whether they be solids, liquids, or
gases. And now let us particularly consider mercury
in a bulb of glass, which like water will become
expanded and run up the fine tube when heat is
applied. Here we have in reality two things expand-
ing. In the first place the bulb itself expands, so that
if you were accurately to gauge the bulb when cold
and when hot, you would find it to be slightly
larger when hot. The bulb, however, does not ex-
pand so much as the mercury, and in consequence
the mercury is not content with occupying its old
position in the tube attached to t^^e bulb, — it must
have more room, and to get this it rises in the tube,
xnd, the tube being very fine, a very small expansion
of the mercury causes a very considerable rise in the
tube, and is thus easily seen by the eye. In fact,
the mere warmth of your hand will drive the mercury
rapidly up the tube, and a mere breath of cold air
will drive it down. An instrument of this kind is
therefore very useful for telling whether one thing is
hotter or colder than another, and answers very much
better for this purpose than the feeling of touch.
Suppose, for instance, that we place such an instrument
with its bulb in a vessel of water, and leave it there
for a few minutes, the top of the mercury will then
keep a fixed position in the tube. Let us make a
HI, f
C6
SCIENCE PRIMERS.
[heated
mark, and note this position accurately. Let us
now take the instrument out of this vessel of water
and place it into another vessel also containing
water. If this water be hotter than the first, the
mercury will rise above the mark which we made — that
is to say, the end of its column will now be higher up \
if, however, this water be colder than the first, the
mercury will sink below the mark which we made ;
and thus by observing the height of the meicury in
the tube, we can at once tell whether the second'
vessel of water be hotter or colder than the first.
An instrument of this kind is called a thermo-
meter, and I shall now tell you how a thermometer
is made.
51. How to make them.— To make a thermo-
meter, get a glassblower to blow a hollow bulb
at the end >f a tube of glass, with a very fine bore,
the other end of this fine tube being open to the
air. Next heat the bulb in a flame; in doing
this the air in the bulb expands (just as it did
when we heated the bladder) ; but the other end of
the fine tube being open, the expanded air gets out
tlHFCWgh this end. Next, before the air has had
IIm to cool, plunge the open end of the fine tube
below the surface of a vessel containing mercury.
The bulb, remember, now contains less air than it
^iU m first, part having been driven out by heat.
Jhi^B air cools it shrinks into less bulk, and the
fiiil^ of the air from without drives up the mer-
c«y to occupy the vacant space, just as it drove up
riie water in the water-pump (Art. 34). Part erf this
mercury ^.viH therefore be driven into the bulb. We
hava DO!? got a littk mercury ia the bulb, and w^
BODIES.]
PHYSICS.
67
M-
80
TO-
CO-
90.
30-
n.
-90
-ao
yo
-60
■If-
■40
next take the bulb with the mercury in it and heat
it well above the flame of a lamp — ^bulb, tube, and all
The mercury will soon begin to boil, and its vapour
will drive out the air before it, until bulb and tube
will both be filled with the vapour of mercury. When
this is done, we plunge the open end of^he tube once
more into a vessel of mercury. As there
is now no air in the tube or in the bulb,
but only vapour of mercury, when this
cools it will condense and there will be a
vacuum, and the mercury in which the
instrument is plunged will be driven up
by the pressure of the outside air until it
, fills both tube and bulb. We have thus
filled both tube and bulb with mercury,
and now before it has cooled we seal the
open end by melting the glass, so as to
keep the air out, and this part of the
process is then complete.
Having thus got our thermometer tube,
we plunge it when sufficiently cool into a
box containing pounded ice which is in
the act of melting. The column of mer-
cury of course falls in the tube because
the ice is very cold : (you have been told
that the column of mercury falls when ^v- "s-
the bulb is plunged in a cold substance.) When the
mercury ceases to fall, mark off with a file the posi-
tion of the top of its column in the tule : this is the
position which the top of the column will alway*
occupy when the instrument is put into melting ice>
or something equally cold. Having done this, next
take the thermometer tube and plunge both bulb
F 2
30
20
10
6S
SCTEI^CE PRIMERS.
[heated
and tube into boiling water, and mark off the posi-
tion of the top of the column as before. The column
will now, of course, be very high, for the mercury
will have expanded very much in consequence of
the hot water. You have now got two marks on your
fine tube — the'^one denoting the position of the top of
the column of mercury when you plunge the bulb into
melting ice, the other the position of the top of the
column when you \ 'upf 'Jie bulb and tube into boiling
water. You will . trwv^rds learn that the heat of
boiling water is not <.^ -c . rnstant, but for the present
we may regard boiling water liS having a fixed heat
Having thus got two points marked or scratched
with a file upon the tube of the thermometer cor-
responding to the freezing and to the boiling points
of water, the next operation is to divide the whole
distance between these two points into loo equal
parts. This is done by coating the whole tube with
wax, and then making scratches in the wax-coating
with the point of a needle at the proper places. If
we then dip the whole tube into a solution of hydro-
fluoric acid, this will not affect the wax, but it will
affect the glass where the point of the needle has
cut through the wax. After the tube has been taken
out of the solution we shall therefore find that all
the lines which we made with the point of the needle
have eaten into the gla^s by help of the acid, and
form, in fact, a scale of lines by the aid of which we
may rise from the freezing to the boiling point of water
through ICO steps or stairs, each step denoting some-
thing hotter than the one below it, and colder than the
on ! above it.
Finally, let us call the lower step q degree, the
BODIES.)
piTYsrcs,
^
upper step loo degrees, and let us also number every
ten steps between these two, and our thermometer is
complete.
Such an instrument is called a centigrade ther-
mometer, which means a thermometer with a
hundred steps ; and as this is the most convenient
form of graduation, we shall always use it
If a substance be of such a heat that when a ther-
mometer is placed in it the end of the column rises
to I o or 20 or 30, we say the temperature of the
substance is 10 or 20 or 3* degrees, and so on. Melting
ice therefore has the temperature of o deg ee (written
0°) on the Centigrade scale, and boilir j vater the
temperature of 100 degrees (written lod") on the
same scale; 20"* is a good summer heat, and 35** is
about the heat of our blood, or blood- eat. In fact,
such an instrument gives us a very accurate means
of measuring temperature.
52. Expansion of Solids. — By a method similar
to that of Experiment 36, only more accurate, we
have found out how much rods made of glass or of
metal expand between the freezing and the boiling
points of water, that is to say between o"* and loo' of
the thermometer, and the results are exhibited in the
following table :—
Expanuon between the
freezing and the boiling
points of water of a rod
xoo,ooo inches long.
, , , ♦ 85 inches.
Glass
Copper
Brass
Soft iron
Cast iron
Steel
171
188
120
109
114
»
»>
n
10
SCIENCE PRIMERS,
[heated
BODll
Lead •
Tin
Silver
Gold
Platinum .
Zinc
Expansion between the
freezing and the boiling
points of water of a roc
100,000 inches long.
282 inches
196
192
144
87
298
u
u
»
>f
»
53. Expansion of Liquids. — Liquids expand
more than solids when you increase their heat, but
you cannot make an experiment upon a liquid rod,
because a liquid cannot form a rod. In this case let
us take a definite measure, such as a pint, and find
what would be the overflow in pints of a liquid that
occupied 100,000 pints at the freezing-point of water
if it were raised to the boiling-point
Now, if 100,000 pints of mercury were heated from
o' to 100', or from the freezing to the boiling point,
there would be an overflow of 1,815 pints; and if
100,000 pints of water were heated between the same
range, there would be an overflow of 4,315 pints.
It is found by such experiments that
Liquids expand more than solids for the
same increase of temperature, and that
liquids expand more rapidly at a high than
at a low temperature.
54. Expansion of Gases. — Gases expand
through heat, and that to a great extent; but here
we must bear in mind that other things besides
heat will make gases expand. You remember the
india-rubber ball that was put into a rieceiver and
began to expsmd when the air was withdrawn from the
recel
to si
heatj
the
we
howl
that'
atm<
V,
BODIES.!
PHYSICS.
71
receiver (Experiment 25). When, therefore, we wish
to see how much a quantity of gas expands through
heat, we must take care that the air which surrounds
the gas does not change its pressure : in other words,
we must take a bladder with some air in it, and find
how much it expands when heated in the open air —
that is to say, under the constant pressure of the
atmosphere — between the freezing and the boiling
points of water.
When this is done, it is found that if a bladder
not completely filled with air have a volume equal to
1,000 cubic inches at the freezing-point, its volume at
the boiling-point will be 1,367 cubic inches. If therefore
we have a large quantity of ice-cold water in a vessel
and force this bladder containing 1,000 cubic inches
beneath the water, we shall find the water rise in
the vessel through * a space denoting 1,000 cubic
inches — this being the increase of volume due to the
bladder. But if we take the same vessel, only filled
with boiling water, and plunge the bladder into it, we
shall find the water rise through a space denoting
1,367 cubic inches — this being the volume of the
bladder at this temperature.
55. Remarks on Expansion. — Liquids and
solids expand with immense force. If ycu were to
fill an iron ball quite full of water, shut it tightly
down by means of a screw, and then heat the ball ;
the force of the expansion would be great enough to
burst the ball
In large iron and tubular bridges allowance must be
noade so that the iron has room to expand ; for in the
middle of summer the bridge will be somewhat longer
than in the middle of winter, and if it has not room
71
sc/ej^CjE primers.
[heated
to lengthen out, it will be injured by the force tend-
ing to expand it. There is an arrangement for this
purpoi&e in the Menai Tubular Bridge.
We take advantage of the force of expansion and
contraction in many ways — for instance, in making
carriage wheels. The iron tire is first made red-hot,
and in this state is fitted on loosely upon the wheel ;
it is then rapidly cooled, and in so doing it contracts,
grasps the wheel firmly, and becomes quite tight
56. Specific Heat. — Some bodies require a greater
amount of heat than others in order to raise their tem-
perature one degree. The quantity of heat required
to raise a pound weight of any substance one degree
is called its specific heat. Water has a very great
specific heat ; that is to say, it requires more heat to
raise a pound of water one degree than it does to
raise almost any other substance. The heat required
to raise a pound of water one degree will raise through
one degree 9 lbs. of iron, 1 1 lbs. of zinc, and no less
than 30 lbs. of mercury or gold.
Experiment 39. — To convince you of the great
specific heat of water, let us take 2 lbs. weight of
mercury and heat it to 100°, or the boiling-point of
water, and let us then mix it with i lb. of water at an
ordinary temperature. Now note the height of a
thermometer placed in the water both before and
after the mixture, and you will find that it has hardly
risen more than 5° in consequence of the hot mercury
being poured in.
57. Change of state.— Yqu have already heard
about the three states of matter — the solid, the liquid,
and the gaseous. I have now to tell you that substances
x>'hen heated piss first from the solid to the lioj^id, and
BODIES.]
Pli YSICS.
73
then from the liquid to the gaseous state. You
are told in the Introductory Primer that ic », water,
and steam have precisely the same composition, and
that ice becomes water if it be heaicil, while w?ter
becomes steam if we continue the heat The very
same ':hange will happen to other substances if we treat
them m the same way. Let us, for instance, take a
piece of the metal called zinc and heat it ; after some
time it will melt, and if we still continue to heat it, it
will at last pass away in the shape of zinc vapour. Even
hard, solid iron or steel may be made to melt, and
even driven away in the shape of vapour ; and by
means of an agent called electricity (of which more
hereafter) we can probably heat any substance suffi-
ciently to drive it away in the state of vapour or gas.
We cannot, however, cool all bodies sufficiently to
bring them into the solid or even into the liquid
state. Thus, for instance, pure alcohol has never
been cooled into a solid; but we know very well
that all we have to do is to obtain greater cold
in order to succeed in freezing alcohol In like
manner, we have never been able to cool the atmo-
spheric air sufficiently to bring it into the liquid
form ;. but we know very well that all we require
in order to succeed is to obtain greater cold.
You must not, however, imagine from what I have
said, that cold means anything else than the absence
of heat A cold body is a body which has little
heat, a^d a still colder body has still less heat ; but
even the coldest body which we can produce has a
little heat left. Do not be guided in this respect by
your feeling of touch. Two bodies may be of the
&ame temperature^ as shown by the thermometer;
74
SCIENCE PRIMERS,
[heated
and yet the one may feel much colder to you than
the othei \ and if you keep one hand for some time
ill very cold and the other in very hot water, and
then plunge them both into water of ordinary heat,
this water will seem hot to the one hand and cold
to the other. Do not therefore be guided by any-
thing else than the thermometer, or imagine that
cold is anything else than the absence of heat.
1 o return to our subject. Probably all bodies, if we
could cool them enough — that is to say, take away
enough of their heat — would assume the solid state ;
and then, when each was again heated sufficiently, it
would become liquid, until at last, if still heated, it
would be driven off in the shape of gas or vapour.
There would, however, be a great difference between
the different bodies in the ease with which they would
yield. Ice soon melts if we apply heat ; tin or lead
require to be heated to 200 or 300 degrees before
they will melt ; iron is more difhcult to melt than
leid \ and platinum is more difficult than iron. A
body very difficult to melt is called refractory.
In the following table we have the temperature at
which some of the most useful subs
Ice melts at
Phosphorus
Spermaceti
Potassium
Sodium
Tin .
Lead .
Silver .
Gold .
Iron ,
ances begin to melt,
o"
49"
^r
i,ooo°
. 1,250''
• 1,500^
BODIES.]
PHYSICS,
75
Platinum is so difRcult to melt that we cannot tell
At what temperature it does so. And carbon is still
more difficult to melt — indeed in the very hottest fire
the coal or carbon is always sc'id ; and no one ever
heard of the coals melting down and trickling out
Uirough the furnace bars.
We thus see that the same sort of change takes
place in all bodies through heat ; that is to say, if we
could reach a temperature sufficiently low, all bodies
would become solid like ice, and if we could reach
one sufficiently high, all would become gaseous like
steam : in fact, the change that takes place is always
of the same kind, and we cannot do better than use
water as a type of all other things in this respect, and
study the . behaviour of this substance under heat,
beginning with its solid state when it appears in the
shape of ice.
. 58. Latent heat of Water. — Let us take some
very cold ice, pound it into small pieces, and put the
bulb of our thermometer into this pounded ice. Let
us suppose that the reading of our instrument shews
a temperature 20 degrees below the point we call
0°. Now let us heat the ice, and its temperature will
rise like that of any other solid under like circum-
stances until it comes to 0°, but at this point it will
svop, and rise no fiir^^hcr as loiii^ as any ice remains.
What then does the heat do if it does not raise the
temperature above this point? We reply, it melts
the ice. At first the heat is wholly spent in raising
the temperature of the very cold ice, but when this
temperature has reached o® the heat has quite a
different office to perform ; its power is now wholly
spent ill melting the ice, and when the ice ij ^il aeited
76
SCIENCE PRIMERS.
^tmm
the water has only the temperature o^, being no
hotter than melting ice. In fact, water at o** is equal
to ice at o% together with a laige ametil of heat,
which we call latent heat because it does dot affect
the thermometer.
Experiment 40. — ^You may prove this by putting
some pounded ice into a tin pan and heating it over
a lamp until there is only a little ice left If you then
plunge a thermometer into the melted ice, you will
find that the temperature will hardly be above o", or
in fact the melted ice will be as cold as the ice before
it was melted.
59* Latent heat of Steam.— We have now
changed our ice iuto water, and if we continue to
heat this water its temperature will rise in the ordinary
manner, like that of other bodies, until it reaches the
boiling-point or loo^ Its temperature will then stop
rising, and if we continue to heat the water we shall
only convert it into steam of which the temperature
is 100^ and no more. In fact, just as it took a large
amount of heat to convert ice at the fre<;zing-point into
water at the freezing-point, so does it take a large
amount of heat to convert water at the boiling-point
into steam at the boiling-point So that we are
entitled to say — steam at 100° is equal to water at
100^, together with a large amount of heat which we
call latent, because it does not affect the thermometer.
Experiment 41. — You may prove this by boiling
some water in a flask and putting the thermometer
first into the boiling water and then into the ateam.
They will both be found to have the same te«npe-
rature, or, in other words, steam is no hotter than
boiling; water.
ra
aomss.]
PHYSICS.
77
Thus you see that ice requires latent heat to bring
it into water, while water again requires latent heat to
bring it into steanu Now we can measure how much
heat it will take to bring a pound of ice at o° to a
pound of water at the same temperature, and we find
that it will take as much heat to do this as it would to
raise 79 pounds of water one degree in temperature,
and this is what we mean when we say that the latent
heat of water is equal to 79. In a similar manner it
has been found that the latent heat of steam is 537 ;
that is to say, it will take as much heat to change a
pound of water at 100° into steam of the same
temperature as it would to raise 537 pounds of water
one degree in temperature.
It thus takes a good deal of heat to melt ice, and
it therefore takes a good deal of time to do sa In^
deed it is much better that this is the case, for what
would happen if ice at the melting-point were to
change into water at once when heated ever so little ?
It would render uninhabitable a lai^ge part of the globe,
for the ice of the mountains would on some fine spring
day be at once liquefied, and the water would rush
down in such overwhelming torrents as to sweep every-
thing before it, and large tracts of low-lying land would
be buried under water. In like manner, it is much
better for us that it takes a large amount of heat to
convert water at the boiling-point into steam ; for sup-
pose that water at this point were at once converted
into steam by heating it ever so little, there would then
be an explosion in evei r tea kettle and in every boiler,
while a steam-engine would be an v\tter impossibility.
You have already been told that steam is a gas like
air, and you have learned in the Introductory Primer
J^md^aiL.i.
78
SCIENCE PRIMERS.
[heated
that you cannot see true steam. Wl i a kt ^k \^
boiling l^«^pidly, }.>u may have noticed that you do net
Fee ,i:ythmg quite close to the spoilt of the kf ;s!e,
I it about half an inch beyond it you see a cloud.
Or, again, when a locomotive gives out its steam you
do not see anything quite close to the funnel, bult a
little distance above it you see a cloud. Now this
invisible thing that comes out is true steam, but the
visible cloud consists only of very small drops of
water, formed from the steam as it cools; it is not
therefore steam, but water. True steam is invisible,
like the air or any other gas.
60. Ebullition and evaporation. — I have now
told you something about the steam which is given out
when water boils. I do not, however, mean to say
that no steam is given out before it boils, for this would
be contrary to fact : all of you must have noticed that
a pan of water put on the fire gives out steam long
before it begins to boil. Doubtless, too, you Buist
have noticed that any wet thing or thing full of water
gets dry near the fire —that is ^[f- <;ay, its water goes
away in the shape of sieam. ':m / when steam or
vapour (for both words mean the same thing) is given
out by water which is not boiling we call it evapora-
tion, but if the water boils we call it ebullition.
The difference is simply this. When you heat water
over the fire, the heat has at first two things to do.
It has in the first place to heat the water, and in the
HciXt place it evaporates part of the water ; l>ut when
the temperature of the water has risen to 100° or the
boiling-point, the water cannot be heated above this :
all the t' ngth of the ^xt is then spent in converting
the writ' mto steam, and this stenn) escapes not only
»>r
>*ss]
P//VSICS.
79
*u
imr<.
f'-'ym uia top of the water but from the very bottom
cuso, so that we hear a noise which we ral' boiling as
the bubbles of steam rise through the vvatei and
escape into the air.
6i. The boiling-point depends on pressure.
— ^.I have now to tell you that the temperature or heat at
which water bolis is not a perfectly fixed point like that
of melting :ce, but depends upon the pressure of the
air. If the pressure of the air be lessened, water will
boil below ioo°. You remember you were told that
the pressure of the air is less at the top of a lofty moun-
tain than at the bottom, because at the top you have
a less depth, and therefore *^ less weight or pressure,
of air above you. Well, at the top of Mont Blanc
in Switzerland, which is three miles high, water will
boil at 85°; and if a traveller were to try to boil an
egg in a pan ai the top of Mont Blanc, he might boil
it for hours, but it would not harden, because Ss** is
not high enough to harden the white of an egg.
On the other hand, if we were to boil water at the
foot of a very deep mine the boiling-point would b^^
considerably above 100°.
Experiment 42. — You will see by the fallowing
very simple experiment that the temperature of th«5
boiling-pomt depends upon the pressuu of gas or air
upon the surface of the water. Let us take a glass
flask and fill it half full of water, then cause the water
to boil for some time, until the stea a has driven out
all the air from the upper part of the flask, so that
we have only water, and the vapour of water in the
flask. Now cork it tightly, and, withdrawing it from
the lamp, invert it as in fig. 26. When it has
ceased boiUi^g, lake a sponge and ^.our some cold
8o
SCIENCE PKlAfERS,
[heated
water on the flnsk, when boiling will again begin.
I'he reason of thi« is, that before the cold water was
pou'-ed on there was a considerable pressure of vapour
upon the water of the flask, and this presrare kept it
from boiling, but the effect of the told water was to
condense this vapour, and therefore to lessen its
Fig '3^
pressrire ; and since water boils more easily at a low
pressure than at :i high, the water in the flask began,
as you saw, immediately to boil.
Before leavmg this part of our subject, I ought to
tell you that some bodies expand while others con^
tract in the act of m^^'dng ; that is to say, in passing
from the solid to th 1' -aid state.
Experiment 4^^.-- Here is some ice, for instance,
vrhich is lighter th=^n water, as you will see by the
fact that th ice is at present floating upon the water.
In passing from ice to water there is, therefore, a great
contraction of substance, and in passing from water to
DOMES.]
PI/YSfCS,
8i
ice — in the process of freezing— there is a great
expansion. This expansion takes place with great
force, and if a thick iron vessel be filled with water
and then shut by a stop cock by causing the water to
freeze, you may burst the iron vessel. Casl iron con-
tracts like ice when it melts, or, which is the same thing,
expands like water when it freezes or gets solid, and
in consequence, if liquid iron be run into a mc uld,
when it gets solid it will ex|)4nd so as to fill all the
crevices of the mould ; it can thus be cast in a mould.
On the other hand, gold, silver, and copper expand
when they melt, and contract when they become solid ;
they will not therefore, like cast iron, run into the
crevices of a mould, and therefore coins made of these
metals cannot be cast in a mould, but must be stamped.
All substances, however, expand very greatly when
tlu y are converted into gas, and a cubic inch of boil-
ing water will be converted into steam occupying
nearly 1,700 cubic inches.
C2. Other effects of Heat. — You have now seen
that heat expands bodies or makes them larger, and
tliat it also causes them to change their state, pass-
ing from solids to liquids and from liquids to gases
as the heat continues to be applied. You have seen
how powerful an agent heat is ; how the strongest and
hardest bar of iron will by it be changed into a white-
hot mass as soft as treacle, and if heated still more
will be driven off in the shape of gas.
Heat affects bodies in many other wa5rs, and more
especially it promotes the operation of ciiemical attrac
tion. Thus at a low temperature coal will not combine
with the oxygen of the air, and we may keep o\ir coals
for any length of time in our coal-cellar. But when
heat is applied combination t^Vps nlace ; and lu this
iiL o
«a
SCIENCE PRIMERS.
pHEATE!)
BO
combination in its turn produces heat, the process of
combination goes on, and the coal is said to burn.
In like manner in the experiment (Chemistry Primer,
Art. 6) where sulphur and copper combine together,
heat is first of all applied in order to promote combi-
nation, but v/hen this has begun heat is generated,
and the process goes on of itself, without requiring
any more heat from a lamp.
63. Freezing mixtures. — Again, chemical union,
you have been told (Chemistry Primer, Art. 7), pro-
duces h^at, and this is always true ; nevertheless some-
times two substances which have a tendency to form
a solution mix together with the production of cold
and not of heat. Thus common salt and snow have a
tendency to form a solution, and they will do so with
the production of very considerable cold, or, to speak
more correctly, with the absorption of a very con-
siderable quantity of heat.
ExPEKiMENT 44. — To prove this, let us rapidly
mix some melting ice or snow and some salt together,
and place in ta? mixture the bulb of our thermometer.
The mercury in the tube wiJl soon fall below o*,
thereby showing that this mixture is colder than
melting ice.
Now what is the reason of this ? it is to be found in
the fact that after these two substances have become
mixed together we have a liquid and not a solid — in
iBsict, we have strong brine. Now you have been told
that heat is swallowed up, or becomes latent, when
bodies pass from the solid into the liquid state — for
instance, when ice becomes water. The brine, there-
fore, being a liquid, swallows up part of the heat of the
snow and salt, and. the consequence is that we have a
vel
b(
oti
b^
BODIES. J
Physics.
83
very cold liquid as the result of the union of two solid
bodies. Thus when two solid bodies dissolve each
other, we have very frequently a lowering of tempe-
rature on account of the heat which is swallowed up
by the liquid. Such bodies are said to form freezing
mixtures.
In like manner, if we have a liquid that evaporates
very fast we find it to be intensely cold, because in
order to become a vapour or gas it requires a great
deal of heat, and gets it where it can : thus if you drop
some ether upon your hand it feels very cold, and soon
flies away in the shape of gas ; in fact, it has robbed
your hand of a large quantity of heat in order to pro-
duce this vapour or gas. Ve:y low temperatures, very
intense cold, may sometime^ be produced by causing
certain liquids to evaporate very rapidly.
Experiment 45. — To prove this let me pour some
water into a shallow vessel, and place it along with a
pan containing strong sulphuric acid under the re-
ceiver of the air-pump, and exhaust the air. As
the pressure of the air is withdrawn the water will
evaporate very rapidly, and in order to do so will take
away so much heat from its own substance that ii
will be turned into ice.
64. Distribution of Heat. — Let us now pro-
reed to another part of our subject, and consider
the tendency which heat has to distribute itself.
A hot body will not always remain hot, but it will
part with its heat to the colder bodies that are around
it; and il will always insist upon doing this, but it will
do it in different ways according to circumstances.
Experiment 46. — For instance, let us put a poker
into the fire ; some of the heat of the fire gets into
G 2
84
SCIENCE PRIMERS,
[heated
Vk
that part of the poker which is in the fire, and this
passes along the poker until it heats that end which is
farthest away from the fire, and you will at last find
it too hot to touch. This passage of heat along the
poker is called conduction of heat.
Experiment 47. — Again, let us take a flask two-thirds
full of water, and heat it from below. As the lower
particles of water are heated they expand, and there-
fore get lighter ; they consequently rise tc the top
for the same reason that a cork rises in water, and are
replaced by colder and, therefore, heavier particles
from above. A new set of particles are thus con-
tinually subjected to the heat of the lamp, and in
process of time the whole water will get heated and
begin to boil This process is called convection of
heat.
Neither of these processes will, however, account for
the heat that reac^ies us from the sun. Whether in
conduction or convection the heat is carried by means
of particles of solid or liquid matter, but we have
reason to think that there are no such particles
between us and the sun, while we know that the sun's
light and heat takes less than eight minutes to come
from the sun to us over a distance of 90 millions of
miles. Evidently, then, the heat which comes to us
from the sun moves with an immense velocity, and
does not reach us in virtue of boating up the particles
between the sun and ounp^lves. In fact, in a very
cold day, when the air is very cold and anything but
heated, the sun's rays may be very powerful Now
the process by which heat comes to us from the sun
or any other very hot body is called radiation of
heat.
us
BODIBS.]
PHYSICS.
We have thus three very different ways in which #
heated body communicates its heat to a cold one
namely, conduction, convection, and radiation. Lei
us now consider these in order.
65. Conduction of Heat^We have spoken
about thrusting a poker into the fire, and told you that
at last the other end of the poker will be too hot to
hold. But if, instead of a metal poker or rod, a glass
or stoneware rod were thrust into the fire, the other
end of this rod would never get very hot, because
stoneware does not conduct heat nearly so well as
metal.
Wool and feathers are still worse conductors, and
this is why these substances have been provided by
nature as the clothing of animals ; for the heat of an
animal is generally greater than that of the surrounding
substances, and this heat is not readily conducted off
through the garment of wool, feathers, or fur, with
which the animal is cladc So in the case of boilers of
engines ; when we wish to keep in the heat, we supply
them with steam jackets or coverings made of a non-
conducting substance.
A bad conductor may be used not only to keep in
heat, but also to keep it out ; flannel, for instance,
may be used to wrap round our bodies in order to
keep the heat in, or it may be used to wrap round a
block of ice which we wish to preserve in order to
keep the heat out. Ih fact, heat cannot readily pass
through flannel whether it be going from within out-
wards or from without inwards.
Experiment 48.— It is very easy to show you that
different substances have different conducting powers
for heat You see, as in the figure, two rods or w)reS| one
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SCIENCE PRIMERS.
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of copper and or** of iron, with their ends together, at
which they are K. ted by means of a lamp. After the
lamp has burned for some time, let us take two little
bits of phosphorus, and place one of them at the end
of the copper rod furthest away from the flame. It
will soon take fire. Now place the other piece on
the iron rod at the same distance from the lamp as
the burning phosphorus, and it will not take fire.
This shows us that the beat of the lamp is conducted
more powerfully along the copper than along the
iron*
Fig. 27.
The conduction of heat explains the action of the
safety lamp which was devised by Sir Humphry
Davy for the use of miners, but this very useful lamp
has already been fully described in the Chemical
Primer (Art. 41).
66. Convection of Heat. — If we take a vessel
full of water, and float on its surface a vessel full of
boiling oil, we shall find that the heat of the oil will
be conducted very slowly indeed downwards through
BODIES.]
PHYSICS.
87
the liquid ; in fact, a few inches down, the rise of
temperature will be hardly perceptible. Bui if instead
of heating the vessel with water in it from above we
heat it from below, as in the figure, we shall find that
in a very short time the whole water will be heated
and begin to boil. In fact, as we have already stated,
the heated particles getting lighter rise, and are re-
placed by colc'er and heavier particles from above, so
Fig. 28.
that we have a current as is shown by the arrows in
the figure, the heated water ascending in the middle
and the cold water coming down the sides.
We have several good examples of convection in
nature \ for instance, in a lake which is cooled at its
surface by the action of intense cold. The surface
particles are first cooled, and gi tting heavier they sink
88
SCIENCE PRIMERS. [light from
down and are replaced by lighter and warmer particles
from beneath, so that in a short time the whole body
of T/ater becomes cooled down to a temperature about
4° above the freezing-point; after that temperature
the water, contrary to the usual practice of things,
expands when further cooled instead of contracting ;
and when ice is formed, this ice, being decidedly lighter
than water, floats on the top.
Now, had ice been heavier than water, it would
have fallen down to the bottom as it was formed, a
fresh surface would -thus have been exposed, and the
whole lake would soon have !)ecome one mass of ice.
But as it is, the cold can only freeze the second layer
of water through the ice of the first, and this is a very
slow process, so that there is no danger of a lake
being permanently frozen.
In the air again we have strong convection currents
due to heating ; for it is on this account that the hot
air of a fire goes up the chhnney, being replaced by
cold air from the room ; and we have the very same
thing on a large scale in the great system of wimls, for
at that part of the earth called the equator, where
the sun is most powerful, the air when heated mounts
up just as the air of a fire mounts up the chimney.
This air is then replaced by currents blowing along the
surface of the earth from the poles or colder portions
of the earth. We have thus at the equator, a system
of upward currents which carry off the hot air to the
poles in the upper regions of the air, and we have
also currents bio win <r alon^f^ the surface of the earth,
which bring back this air \i hen cooled to the equator.
These surface-currents blowing from the poles to the
equator are called ibe tmde winds.
HEATED BODIES.]
PHYSICS.
89
%'m
67. Radiant Heat and Light. — The third
method by which a hot body parts with its heat is by
radiation, and it is in virtue of this process that the
heat of the sun reaches our earth. We need not, how-
ever, go farther than our own firesides to get an example
of the process. If we stand opposite a strong iiire, we
find our faces and our eyes suffering from the heat.
Even a kettle containing hot water gives out radiant
heat, although the rays of heat from it cannot pierce
the eye and impress it with the sense of light like those
from the fire or from the sun. Thus when you heat a
body such as a ball of clay, something of the following
kind takes place. The body begins at once to rise in
temperature, and in consequence to give out rays of
heat, but those rays are dark rays, and do not affect the
eye. As the heating process goes on. a few of the
rays which it gives out begin to affect tie eye and the
body becomes red hot ; it next acquires a yellow heat,
next a white heat, and last of all it glows with an
intense light resembling the sun. Let us now devote
ourselves for a short time to the study of these bright
rays which a hot body gives out.
68. Velocity of Light. — Romer, a Danish astro-
nomer, was the first to find out the velocity with
which light travels through space. To understand
what this means let us remember what takes place
when a distant gun is fired off. We see a flash, and
then some seconds after we hear a report. Evidently
then the sound does not reach the ears at the very
moment when the gun is fired, because it lags behind
the light. But does the light reach us at the very
moment ? may not both light and sound start from the
cannon at the same moment, and each take sorae time
96
SCIEME PRIMERS. [iagivt ifRoM
to get to us, the light winning the race and coming in
first ? This point can only be decided by observation
and experiment, and it was by observation that Romer
found it out There is a large planet called Jupiter^
which is sometimes very far from us and sometimes
comparatively near, and this large planet has several
satellites, or small attendants, ne of which passes across
the disc or surface of Jupiter at regular intervals, so
that when we use a powerful telescope we see the small
satellite like a black body crossing the large disc of the
planet Now Romer found that when Jupiter was very
far away from us the satellite seemed to be later in
crossing than it ought, and he argued from this that
we on the earth do not see the crossing of the satellite
over the disc or surface of Jupiter at the very moment
when it takes place, but that light takes some time to
get from Jupiter to our eyes, just as the report of a
distant gun takes some time after the explosion to
reach, our ears.
You thus see that light as well as sound takes time
to travel, only light travels much faster than sound — •
light travels at the enormous rate of 186,000 miles a
second, while sound creeps along at the rate of 1,100
feet in the same time. Light only takes 8 minutes
to come from the sun to us, although the distance is
90 millions of miles. If, therefore, the sun were to
be suddenly extinguished, we should not find it out
until 8 minutes afterwards.
Do not, however, suppose that light consists of
small particles shot out by hot bodies, and flying
through space at the enormous rate of 186,000 miles
a second. If this were the case, we should be knocked
to piec^ by a ray of light A ray of light may be said
Heated bodies.]
PHYSICS.
^
to enter the eye just as sound may be said to enter
the ear. We have already explained that when we
hear the report of a gun, it does not mean that small
particles of air travel all the way from the gun to
our ear. And so when we view a ray of light it does
not mean that a small particle is shot from the bijght
body into our eye. An impulse or wave in each case
passes over the medium between us and the "body, and
the blow goes from particle to particle after the manner
we have already explained in the experiment with
ivory balls (Art. 44).
69. Reflection of Light. — Wlien light strikes a
polished surface of metal, it is reflected from it If you
Fig. 29.
hold a lighted candle before a mirror, you will see the
image of the candle in the mirror, which means that
the rays from the candle strike the mirror and are re-
flected from it to your eye, just as if they came from
the mirror itself and not from the candle.
* Experiment 49. — In order to understand how
reflection acts, let us take a horizontal polished mcr
tallic surface—that is to say, let us pour mercury into
a shallow flat-bottomed vessel. Now place a bent
tube open at the bottom above the mercury as in fig.
29, and let the light of a candle enter the tube at the
right end ; if we place our eye at the left end, we shall
95
SCIENCE PklMERS. [LiGrtt froIh
see the light from the candle as it conies reflected
from the surface of mercury.
In this experiment, therefore, the light of the candle
goes down the one tube, strikes the surface of mercury
and then ascends the other tube to the eye. But in ordei
that the light may do this, two things are necessary.
In the first place, the two tubes must have the same
inclination or slope ; and secondly, the one tube
must be exactly opposite the other, so that if they
were suddenly to fall flat down they would be in a line
with one another. Whenever, therefore, a ray of light
strikes a polished surface, the reflected ray rises from the
surface with the same slope as the ray that strikes the
surface falls towards it, and both rays, if you could
imagine them squeezed flat against the surface, would
be found to make one line.
You cannot understand the laws of reflection com-
pletely without geometry, but the following figure will
perhaps enable you to do so to some extent. In the
figure, A is supposed to be a bright point giving out
light, and m m is a mirror. Let a b, a b' be two of the
rays of light from a, striking the mirror at b and b'.
These will then rise into the eye of the observer in the
directions bd, B'D',the falling slope of the ray a b being
equal to the rising slope of b d, and the falling slope of
A b' equal to the rising slope of b'd'. Now if you imagine
the direction of the two rays b d, b'd', prolonged beneath
the mirror, they would meet at a', a point as much below
the mirror as the bright point a is above it. To the eye,
therefore, the rays will in point of fact appear to proceed
from a', so that the apparent position of the reflected
image a' is as much behind the mirror as th« bright
point A is itself before it
HEATED BODIES.]
PHYSICS.
93
Whenever, therefore, you stand in front of a mirror,
you see your own image in the mirror as much behind
the mirror on the other side as you yourself are in front
of it ; if you go close to the mirror, the reflected figure
goes close also, if you draw back the reflected figure
draws back, and so oh. You will, however, notice
the difference—namely, that your right hand is
the left hand of the image, and your right
Fig. 3a
I
side generally the left side of the image, but
in other respects the image is precisely a copy of
yourself.
In fig. 31 you see in the lower part the image of
the upper part, and you notice how, in the image, the
letters g^ from right to left, and not from left to
right
When the bright reflecting surface is not flat, curious
images are sometimes produced. Take, for instance,
the bright surface of mercury in the bulb of the thermo-
meter and look into it You will th^re see a very sipalt
:
W
SCIENCE PRIMERS, [light from
distorted image of yourself, and indeed of the whole
room, only the far away parts of the room will be
exceedingly small.
' Take again a couple of bright concave mirrors like
those of fig. 22, only, instead of putting a watch at
the focus of the one mirror, and your ear at that of the
other, place a red-hot ball in the one focus, and your
hand in the other focus, and you will soon find it too
hot. Indeed, if you had two large reflectors of this kind
Fig. 31. '
and had a fire burning in the focus of the one, you might
cook a beef-steak in that of the other, even tho^igh the
two reflectors were fifty feet apart. The reason is that
the rays of heat from the fire in the one focus strike the
mirror near it, and are reflected from it in lines that
bring them to the other reflector, and they are then
again reflected in such lines as to bring them all to-
gether into the focMs of this reflector, We thu| h^vci
HEATED BODIES.]
PHYSICS,
9S
as it were, the fire itself burning at the one focus, and
an image of the fire at the other, the image being
sufficiently hot to cook a beef-steak.
70. Bending or refraction of Light. Expe-
riment 50. — Put a small, heavy body at the bottom of
a stoneware or pewter jug, and put your eye in such a
position that the side of the jug just hides the body from
your eye ; then let some one fill the jug full of water, and
the small body at the bottom will now become visible.
Why is this ? It is because the ray of light from the
)■;
Fig. 32.
small object at the bottom of the water after it leaves
the surface of the water is bent in a different direction,
so that you can in fact see it round a corner ; and if
the small body at the bottom were a little fish, it could
also see you. 1
It thus appears that if a slanting ray of light strikes
a surface of water, it is bent so as to be less slanting
after it enters the water ; or again, if a ray of light comes
out from the water, it is bent so as to be more slanting
after it enters the air. The same thing would happen
if the ray of light entered a surface of transparent glass
s
if
^
SCIENCE PRIMERS, [f.lOHT FROM
instead of a surface of water, — a slanting ray would
become less slanting after it entered the glass. If you
had a flat, thick piece of glass, the ray of light would
take the course that is shown in the preceding figure,
in which we see that its path before it enters the glass,
and its path after it leaves the glass, are intbe same
direction (though not in the same line), wliile, how-
ever, its path in the glass is quite different.
Suppose, however, that the piece of glass is not flat
but shaped like a wedge ; in fact, that it stands straight
up above the page on a bottom like fig. 33, and that
«r
when viewed standing up it has the appearance of fig.
34. Such a piece of glass is called
a prism. Let us now see in what
manner a ray of light will be bent
in passing through a prism. This
is exhibited in figure 33, from
which you see that the ray is bent
towards the thick part of the prism;
in fact, the direction of the ray is
entirely changed.
You thus see that whenever a ray of light passes
Fig. 34.
■MM
HEATED BODIES.]
PHYSICS,
91
through a wedge-shaped piece of glass, it is bent to-
wards the thick part of the wedge.
71. Lenses, images given by thcr, , — Now let
us vary the shape of the piece of glass in the following
manner. Let the piece of glass be circular like a cake,
only thickest in the middle and thinnest all round the
edge ; in fact, appearing like a circle if viewed in one
direction, but if viewed endwise appearing like the
following figure.
Such a piece of glass is called a lens. Now
let a bundle of rays of light from a distance
fall upon the lens. What will happen ? The
lens will act like a circular wedge ; it is in truth
a circular wedge, and being thickest in the ^
middle the rays of light will be bent towards the ^*^- 35-
middle all round the lens. In fact, the rays
of light will come to a point, or nearly so, as will be seer
from the following figure.
Now suppose that when the sun is shining, you place
a lens so as to allow the rays firom the sun to strike iti
full on the surface, these rays will be brought to a
point, or nearly so, on the other side of the lens ; and if
you place a sheet of paper at this point, you will see ^
III. u
98
SCIENCE PRIMERS. [tiGHt prom
small bright image of the sun, which will be so intensefiy
hot as to set fire to the sheet of paper ; in fact, the lens '
will now act as a burning-glass.
Experiment 51. — Such a lens will give an image
of anything else as well as of the sun ; for instance, I
have here an arrangement by which the rays of light
from a candle are allowed to fall full upon a lens, and
I obtain upon a piece* of oiled paper placed on the
other side of the lens an image of the candle, only as
you see upside down. In fact, if you place anything
at all bright in front of a lens some distance off,
behind the lens you will get a small image of this
thing. If you place your face in front of the lens,
behind the lens there will be a small image of your
face. Now this is precisely what the photographer
does. He has a black box with a lens at one end of
it, such as you see in the following figure. He points
Fig. 37.
the iens to a landscape or to the face of a person,
and in the dark box there is a little image of the land-
scape or of the face, which he first of all allows to fall
upon ground glass, so that he can see it and know
if it be right. He then takes out this ground-glass
plate and puts in its stead a plate of glass having its
surface covered over with a peculiar substance that is
3^ct^d on by light The image inside th^ box noir
HEATED BODIES]
PHYSICS,
99
falls right upon this sensitive chemical substance, and
the bright parts of the image act upon and change
the nature of the surface, while, however, the dark parti^
do not affect it. By this means the image stamps ail
impression of itself upon the substance, but in this
impression the bright parts of the image appear dark
and the dark parts bright, and it is therefore called a
negative. From this negative the ordinary pictures
or positives are afterwards taken.
72. Magnifying glasses. — ^A lens may be used
for magnifying anything very small, thus forming a
magnifying glass with which most of you are no doubt
familiar. In this case you must place the glass very
near th^ thing that you wish to magnify. For instance,
you could not by means of a magnifying glass of this
kind magnify a distant object such as a planet or the
moon, but you can only magnify sometliing close to
you. If you wish to magnify a planet or the m.oan,
you must use two glasses; one a large glass, by means
of which you get an image of the planet or of the
moon — ^just as by means of a burning -glass you get
an image of the sun — and the other a magnifying
glass, by means of which you examine and enlarge
this image, which the other glass has given you.
Thus, if you wish to magnify a near object you use
a magnifier, but if you wish to magnify a distant object
you must first of all, by means of a lens, obtain near at
hand an image of the distant object, and then treating
thb image just as you would the object itself, you may
scrutinize and magnify it by means of a magnifying
glass. This combination of two glasses, one giving you
an knage of a distant object, and the other magnifying
thui Image, is callecl 9, telescope ; in practice the
F ?
SCIENCE PRIMERS. [light from
glasses are shut up in tubes so as to keep oitt stray
light
73. Different kinds of Light are differently
bent. — I have shown you how a ray of light *5 bent
in passing through a prism. I have now to tell you
that this bending is not the same for every kind of
light. In fig. 38 we see how a ray of red light is bent
after passing through a prism. If the ray had been
orange instead of red, it would have been somewhat
more bent out of its original course ; if yellow, still
more ; if green, still more than the yellow ; if blue
still more than the green ; if indigo, still more than
the blue; and if violet, still more than the indigo.
Now if the ray were a compound ray containing
mixed together all these seven colours (red, orange,
yellow, green, blue, indigo, and violet), each of these
as it came out of the prism would be bent differently
from its neighbours, and would therefore be separated
from them, and the eye would therefore see all these
colours separate, although they were mixed together
when they entered the prism.
A prism thus breaks up a compound ray of light into
its elements, separating the various colours from one
another.
Now you will perhaps be surprised when I tell yoq
that white light, such as that of the sun, is in reality com^
posed of a mixture of all the various colours which 1
have given you above — red, orange, yellow^ and so on \
a little reflection will, however, convince you that such
is really the case.
We are all of us familiar with the magnificent dkh
play of colours seen in drops of dew, in crystals and in
^ems, when rays of light are allowed to f<ill upon tlu^^, •
HEATED BODIES.]
PHYSICS,
lOI
On such occasions they sparkle with all the colours
of the rainbow, and this very allusion bids us ask if
the hues of the rainbow be not due to the same cause
as the colours of gems. Does not its very name imply
the presence in the sky of a multitude of muiute drops
of water such as would shine forth in the grass like in-
numerable diamonds ? Are not all these displays due
to the same cause ; and, if so, what is the cause ? The
disco very^of it is due to Sir Isaac Newton, who was the
first to show that white light is in reality composed of
a great many differently coloured rays mixed together,
and that those rays are in their passage through cer-
tain substances separated from one another. The
prism, in fact, as we have already said, gives us the
means of separating the variously coloured elements of
a compound ray from one another.
Fig. 38.
Suppose, for instance, that we have a narrow vertical
or up-and-down slit in the shutter of a darkened room
through which the full sunlight is allowed to pass ; — in
fig. 38 we have a plan of this arrangement looking
down upon it from above, or taking, as it were, a bird's-
eye view of it Now if we have no prism to commence
Virith, and look from i towards the slit in the shutter at
102
SCIENCE PRIMERS. \uQVit >Roil
s, we shall see a bright slit and nothing more; in fact,
the slit will serve as an opening through which we may
see the bright sun beyond. Let us now introduce the
prism as in the figure \ when we have done so, our eye
at E will no longer see the slit If, however, we move
our eye towards the thick part of the prism, we shall at
last catch hold of the light from the slit, but it will be
now very much changed in appearance. It will not now
reach our eye in the shape of a bright thin slit, as
formerly, but it will appear as a broad band or ribbon
of light of many colours, beginning with red at the
one end, and passing gradually and in order through
orange,, yellow, green, blue, and indigo, into violet at
the other extremity.
All this may be easily explained by what we have
already said, bearing in mind that white sunlight is in
reality composed of all the different colours mixed to-
gether. Not only, therefore, are the rays bent in their
passage through the prism, but they are unequally
bent. And we shall have for each variety of light its
own appropriate slit in its own appropriate position.
We shall therefore have a multitude of little bright
images of the slit lying side by side, forming, in fact,
a band or ribbon of light rather than a i^it ; the red
being at one end, because the red rays are least bent,
and the violet at the other end, because the violet rays
are most bent. This variously coloured ribbon of
light is called a spectrum ; and if it be the light
of the sun which we employ to light up our slit, then
we get the solar spectrum^ *
74. Recapitulation. — I have now told you a good
deal about radiant light and heat You have in the ftni
place learned that» as you begin to heat bodiesi diey glv#
HEATEJD BODIE.'>.]
PHYSICS.
i03
out first of all dark rays, but that as you raise their
lemperature, the rays become luminous and capable of
affecting the eye. You have then been told something
about the reflection of these rays from polished sur-
faces. You have also been told how their direction is
bent when they pass through water and glass; and how
a glass prism bends the rays towards its thickest part.
You have next been told that a lens bends the rays all
round towards its centre or thickest part ; and how,
if you allow sun-light to fall upon a lens, you get a
small bright image of the sun ; which image will set
fire to a sheet of paper or bum the hand.
You have also learned that the moon or a planet
will give by means of a lens an image of the same
kind ; and how, if you approach such an image with a
magnifying glass and look into it, you really see a very
large moon or a very large planet, and that this com-
bination of two glasses is called a telescope. Finally,
you have been told that differently coloured rays of
light are bent by a prism into different places, so that
a prism separates all the elements of a compound ray
of light
And now, before concluding, let us study a little the
nature of heat.
75. Nature of Heat. — We have already compared
heat to sound, and told you that a heated body is an
energetic body. Let us now take up this comparison
once more. In sound we have two things to study:
first, the body which vibrates ; and secondly, the im-
pulses which this body sends through the air to our
ear, imd which make us hear a sound.
How you wore told that a heated body is one in
iirliidi ^ snifti particles are in very rapid vibration^
tD4
SCIENCE PRIMERS,
[naturi
and that just as a vibrating body gives out sound, which
strikes the ear, so a heated body gives out light, which
strikes the eye. But how is a body made to vibrate ;
a bell or a drum, for instance ? — only by giving it a
blow. You bring the heavy hainmer or tongue quickly
against the side df thie bell, and the bell begins te
vibrate ; now this hammer or tongue before it strike?
the bell is a body in rapid motion, and therefore pos-
sesses energy, or can do work. Well, what becomes
of its energy after it strikes the bell ? It has, in truth,
given up its own energy to the bell, for the bell is now
vibrating, and you have already been told that a vibrat-
ing body is one with energy in it. Thus the energy of
the blow given to the bell has not been lost, but has
only been transferred from the hammer to the bell.
Now let us suppose a blacksfaith places a piece of
lead upon his anvil and brings down his hammer
upon it with a heavy blow. You hear a dull thud,
but there is no vibration like that of the bell. What
becomes therefore of the enerjgjr of the blow? It is
not transformed into vibrations like those of the bell,
which can strike the ear — into what therefore is it
changed ? or is it changed into anything ? We reply
that it is changed into heat. The blow has heated
the lead and set all its particles vibrating, although
n©t in the same way as those of the bell ; and if the
blacksmith strikes the piece of lead long enough, I
dare say he will even melt the ipad.
No doubt some of you have spent much energy in
rubbing a bright button on a piece of wood. Nowvlwit
has become of all the energy you have:^nt upcm^e
button ? We reply, it ha»been tnmsformed itito hei^ a»
you will easily find out by put^g ^e bt&tton qukldy
^^.
OF HEAT.]
PIJYSICS.
105
on the back of your own hand or on the back of your
n^hbour's.
Experiment 52.— To show you how the energy of a
blow is changed into that other kind of energy which
we call heat, let us take one of those wax matches
tipped with phosphorus called vestas, and, placing it
upon the hearthstone, strike it a blow with a hammer
or stone ; you will now find that the heat developed
has been sufficient to set the phosphorus on fire.
You thus see that friction produces heat, and you may
have noticed that on a dark night sparks Hy out from the
break-wheel, wluch stops the motion of a railway train.
In all such cases> actual visible energy is being changed
into that form of energy which we call heat, the differ-
ence being that in visible energy the body moves as a
whole, and all its particles move in the same direction
at the saipe moment, while in heat the various particles
move backwards and forwards rapidly, while the body,
as a whole, is at rest. You thus see that visible energy
can be changed into heat, and I have further to tell you
that he^t can to some extent be transformed back into
visible energy. In the case of a steam-engine what is it
that does all the work ? Is it not the fire that heats
the water of the boiler? and in this case part of the
heat-energy of the burning coals actually and truly
changes itself into the visible energy with which the
piston moves up and down, and the fly-wheel moves
round and round.
In fact, all the work done by steam-engines is work
got put of heat Thus you see we can not only change
actual energy into beat, but, in the steam-engine, we
can change heat back again into actual eneigy.
io6
SCIENCE PRIMERS, [ELEcrRiFXKD
ELECTRIFIED BODIES.
76. Conductors and Non-conductors. — It
was known more than two thousand years ago that
when a piece of amber is rubbed with silk, it attracts
light bodies ; and Dr. Gilbert, about three hundred
years ago, showed that many other things, such as sul-
phur, sealing-wax and glass, have the same property as
amber.
Here you see the faint and small beginning of our
knowledge of electricity, a knowledge which has of
late years grown so wonderfully i« to enable us to send
messages between Europe and America in less than one
second of time;
*ExpERiMENT 53.— Let us take a metal rod, having a
glass stem, and rub the glass with a piece of silk, both silk
and glass being well heated and quite dry. The glass
will now have the power of attracting little bits of paper
or elder pith, but only at that place where it has been
rubbed. The glass has, in fact, by rubbing, acquired
a new property, but this property cannot spread itself
over its surface. So much for glass. Suppose now
that we take the metal rod and touch with it the prime
conductor of an electric machine in action, we shall
find that the metal rod has acquired the same proper-
ties as the glass; that is to say, it will attract light
bodies like paper or elder pith, but all parts of the
rod of metal will have the same property, and not
merely that part which touched the electric itmchine.
In fact, the electiic influence can spread itself oveir a
surface of metal, though it cannot r.ver one of gi|U^»
Glass, therefore, is said to be a non*cosiductor tf
frRlFlED
BCfDIES.]
PHYSICS.
V0I
'S It
\o that
ittracts
mdred
as sul-
[erty as
of our
has of
osend
an one
.1
ivinga
)th silk
^ glass
paper
5 been
juired
t itself
5 now
prime
shall
oper-
light
f the
I not
hine.
vttsc
electricity, iivhile metal is called a conductor. In
fact, neither heat nor electricity can easily spread itself
over glass, but both c" > easily spread themselves
over metal ; charcoal, acids, soluble salts, water, and
the bodies of animals are likewise good conductors
of electricity, although not so good as the metals;
while, on the other hand, india-rubber, dry air, silk,
glass, wax, sulphur, amb^r, shellac, are all very bad
conductors.
If we wish to succeed in experiments with elec-
tricity, it is absolutely necessary to keep the electricity
once we have got it ; we must, in fact, surround it on
all sides by non-conducting bodies. It is, therefore, of
great importance to make our experiments in dry air,
and to make the body which has the electricity stand
upon a glass support.
7|. Two kinds of Electricity. Experiment
54. — I have now to convince you that there are two
opposite kinds of electricity. To prove this let us
make^use of the apparatus you see in fig. 39, con-
sisting of a small pith ball suspended by means
of a silk thread to a glass support. First of all
let us rub a glass rod with silk, and with the rod so
rubbed touch the pith ball. The glass end will com-
municate electricity^ to the pith ball, and it will not
be able to get away^ because the silk thread, the glass
support, and the air (if dry) around the pith ball are
all non-conductors. Now, if you notice, you will $ee
that after the glass rod has been made to touch the
pith ball, this ball will no longer be attracted to the
glass rod, but will,. on the other hand, be repelled by
it Let us next rub a stick of sealing-wax with a piece
of warm, dry flannel, and bring the stick so rubbed.
io8
SCIENCE PRIMERS, [ELECTRIFIE^
near to the pith ball. It will now be found that the
pith ball, which was repelled by the excited glass, will
be attracted to the excited sealing-wax.
It thus appears that a pith ball first touched with
excited glass will be afterwards repelled by excited glass,
but will be attracte<^ ' excited sealing-wax.
Now if we had reversed oijr plan of operations, and
had first of all touched the pith ball with excited seai-
F'g 39-
ing.wax, instead of excited glass, it would then have
been repelled by excited sealing-wax, but attracted by
excited glass.
We learn from this that there are two kinds of elec-
tricity; namely, that which we get from excited glass,
and that which we get from excited sealing-wax.
Now when we touched the pith ball with excited
glass, we communicated to it part of the electricity of
the glass j and as it was afterwards repelled by excited
the
rill
[ith
JS,
Id
BODIES.]
PHYSICS.
to9
glass, we conclude that bodies charged with the
same kind of electricity repel one another.
On the other hand, the pith ball, if charged with
excited glass, will be attracted to excited sealing-wax ;
or if charged with excited sealing-wax, it will be at-
tracted to excited glass, and hence we conclude that
bodies charged with opposite kinds of elec-
tricity attract one another.
78. They exist combined in unexcited
bodies. — We may suppose that every substance
has in it a quantity of these two kinds of electricity
mixed together, and that what we do in rubbing
is merely to separate the two electricities from one
another. Accordingly, when we rub a piece of seal-
ing-wax with a piece of flannel, all that we do is to
separate the two kinds of electricity — the one kind
keeping to the sealing-wax, while the other remains
behind upon the flannel. In like manner all that we
do when we excite glass with silk is to separate the
two electricities, one remaining on the glass while
the other adheres to the silk. The same thing holds
in all cases where electricity is developed by friction,
and it is impossible to produce the one electricity
ivithout, at the same time, producing just as much of
the other also. In fine, we do not create electricity ;
but, according ,;o this view of it, we merely separate
the two opposite kinds from one another.
Tlie electricity which appears in a stick of glass when
it has been rubbed with silk is called positive, and
that which appears in a stick of sealing-wax, when it
has been rubbed with flannel, is called negative.
These are merely terms used in order to distinguish
i^lireen the two kindi of electricity.
116
SCIENCE PktMERS, (electrifieB
79. Action of excited on uncxcited bodieb.
—We have seen that electricities of the same kind
rapel, while electricities of opposite kinds attract each
other, but we have still to learn what will happen in
the following case. Let a (fig. 40), be a large ball
of hollow brass, and let the tube to the left hand of
ft be also of brass ; also let these stand upon a glass
support, so that any electricity which a has may not
be able to get away.
Now let B and c be two vessels having' their
upper parts made of brass, only capable of being
separated from one another at the middle part, where
Fig. 40.
you see the line in the figure ; and let both b and C
stand upon glass supports, so that any electricity which
either of them has, may not be able to get away.
Let us begin by supposing that A has received f
charge of positive electricity, and that in thf flie^
time B and c are unelectrified. Now push B d^d € ^
towards a. Since b and c are not electnfied* tlM»ir M|^
BODIES.]
PHYSICS.
Ill
Mr
electricities arc not sejyarated from each other, but
mixed together; however, when you push them up to
A, the positive electricity of a attracts the negative
electricity of b to its side, and repels the positive away
to the extreme right of c, as you see in the figure.
If we now pull c away from b, and finally pull B
away from a, we shall thus have got a quantity of
negative electricity in b, and a quantity of positive
in c, both separate from each other, while the elec-
tricity in A will be the same as before.
We have, in fact, made use of the electricity in a to
separate part of the two electricities of b and c from
each other, and a is still as ready as ever to help us
again. Now this distant action or help, rendered by
the electricity of a in separating that of b and c, is
called electric induction.
80. The electric spark. — We may, however, per-
form our experiment iu a somewhat different manner.
Let us now bring b and c slowly towards a, and con-
tinue to do so. When a and b are very near together,
we shall have the positive electricity of a and the nega-
tive electricity, which has been induced to appear oxi b,
separated firom t ach other by only a small thickness of
air until at last they will be so strong and the film of
air so ^n, diat they will rush together and unite in the
^oifm of a ispark. The consequence will be that a will
ha^ lost a portion of its positive electricity, and b will
Hii^lost all its negative. If we now pull b and c away
li^e will still be the positive charge at c, which has
nCMt gone away ; in fact, while a has lost part of its posi-
ItVie electricity, c will have gained just as much, so that
die result is virtually the same as if part of the elec-
Idd^ of a hsMl fone over to c.
112
SCIENCE PRIMERS, [%\XJ^mmm
%mmM\
:aitimtiiiitm
Fig. 41.
8i. Sundry experiments. — Whatire
said about electrfc
may be easily illustrated by
a few simple and striking
experiments; but it wmk be
remembered that in all these
experiments the glass of the
apparatus must be quite dry
and warm.
Experiment 55.~-Here you
see in the figure an instru-
ment by which we can detect
electricity, called the gold
leaf electroscope. In or-
der to show you its action,
let me first of all communicate to the knob at the top
(see Appendix) a slight charge of positive electricity.
Now this charge runs to the gold leaves which
are electrically connected with the knob, and then
these leaves, being both charged with the same
kind of electricify, begin to repel each other as
you see in the figure. The electroscope is now in
action. ^ ^
Experiment 56.— Having thus charged the elec^o-
scope with positive electricky, let us bring near its knob
an excited glass rod, when the gold leaves will diverge
still more. The reason of this is that the positive elec-
tricity of the excited glass decomposes the neutiid
electricity of the knob attracting the negative to it«i||
and repelling the positive to the gold leaves. 3^
therefore, the leaves had been previously chargpl
with jwsitive electridty, they will now ^ms^
widely.
iieeleis
tricity, s
that the
The res
sealing*
knob a
negativ
were p
of this
tricity
collapj
Exi
or cot
Let ui
electri
spark,
with I
farthe
bally
Tb
cause
the II
of tli
as p
posit
two ^
cons
4*
eleci
lotl
and
S
BOt>{£S.]
PHYSICS,
113
Experiment 57. — If we now bring near the knob of
die electroscope, charged as before with positive elec-
tricity, a stick of excited sealing-wax, we shall first find
that the gold leaves will collapse instead of diverging.
The reason is that the negative electricity of the excited
sealing-wax decompcses the neutral electricity of the
knob attracting the positive to itself, and driving the
negative to the gold leaves. But since the gold leaves
were previously charged with positive electricity, part
of this charge will be cancelled by the negative elec-
tricity driven towards them, and they will consequently
collapse.
Experiment 58. — Here we havfe a hollow brass ball
or conductor, supported on an insulating glass stand.
Let us now bring this insulated conductor near the
electric machine when in action, and we shall get a
spark, but it will be very feeble. Let us now touch
with our finger that part of the hollow ball which is
farthest from the m>xhine, and the spark given to the
ball will now be much more intense.
This illustrates what we said in Art So about the
cause of the spark. In fact, the positive electricity of
the machine pulls towards itself the negative electricity
of the hollow ball, and drives away the positive as far
as possible. If, however, this ball is insulated, the
positive cannot be driven away sufficiently far, nor the
two electricities be separated sufficiently well, and the
consequence is you have but a feeble spark. But
when you touch the hollow brass ball, the positive
electnctty of the ball is driven through your body
to the earth, the electricities are thus well separated,
and there is a |^1 spark.
Sa^ Actiim of points.— In the last experiment,
HI. I
114
SC/ENCJE PRIMERS, Ielectrifieo
BODIES.)
if you cor'uiue to touch the brass ball, and the electric
machine is worked it the same time, a succession of
sparks will pass through your body to the earth, and
these will cause yoii to feel a somewhat unpleasant
sensation. The spark from the electric machine may
in truth be compared to a flash of lightning — a flash
of lightning being, in fact, a very long spark. Now,
just as when a man is struck by lightning the elec-
tricity passes through his body to the earth, so when
we grasp or touch the ball of the last experiment, the
electricity goes through our body to the earth.
Experiment 59. — Now let us attach a point to the
hollow ball, and place this point next the conductor of
the machine, touching the ball as before with our flnger.
It will now be impossible to get a spark from the
machine, but there will be instead a continuous rush
of electricity. In facit, anything pointed carries off
the electricity just as rapidly as it i? produced, and
does not give it time to gather so as to form a spark.
We now see the use of the pointed metallic conductors
that are placed above lofty buildings, to protect them
from lightning strokes. These pointed metallic con-
ductors, running down into the earth, carry off the
electricity in a silent manner, just as the point did in
Experiment 59 ; and ju$t a^ the point protected my
finger from a spark in the one case, so does the light-
ning conductor protect the building from a flash m
stroke of lightning in the other.
Franklin, an American philosopher, was the first to
find out that lightning and electricity are the same thhif
— the only difference being that a flash of lightning is
often several miles in length, whereas an electric spiik
is only a few inches.
83. :
positioi
niachin
we ha^
electric
lecting
One
the ek
revolvi
rcvo
onf
UfHU
id \
cm
leal
of
^WMKO
BODIES.]
pnvsiai.
»t5
to the
:tor of
5nger.
n the
5 rush
es off
, and
park.
ctors
them
con-
■ the
id in
I my
gbt-
i Of
tto
83. Electrical Machine.— You are now in a
position to understand the construction of an electric
machine. Such a machine is composed of two parts ;
we have first of all an arrangement for producing
electricity, and we have next an arrangement for col-
lecting it.
One of the best known machines is that in which
the electricity is produced by a large plate of glass
revolving, as ivL fig. 42. As the plate of glass
Fig. 4»,
revolves, it is rubbed against by two sets of rubbers,
m% above and the other below. These rubbers are
UjHl^Uy made of leather stuffed with horse-hair, so as
^ press rather tightly against the glass. They are
coated with a soft metal, which is spread over the
leatli^', and this metal is generally made of one part
of m^ one of tiD» and two of mercury melted
a
tt6
SCIENCE PRIMERS, [electrifi^
BODIES.]
together. There is a metallic chain which connects
these rubbers with one another, and with the earth.
Now when the glass plate is turned round, positive
electricity is produced in the glass, while negative
electricity is produced in the rubbers. The negative
electricity of the rubbers then passes along the me-
tallic chain which is connected with the rubbers, and
is conducted by means of this chain to the earth,
through which it spreads until it is scattered and
diffused — in fact completely lost. We have thus got
rid of the negative electricity, and there is now
left the positive electricity on the glass. Now sur-
rounding the glass we have two brass rods, which are
united to a latge metallic surface called the con-
ductor, which you see in the figure. This conductor
stands upon ^ass supports, so that it is able to keep
what electricity it gets. The two large rods near the
glass plate are moreover armed with metallic points.
Now you have already been told that points have a
great tendency to draw off electricity. The consequence
is that these points draw off, or collect, the positive
electricity of the glass and carry it to the conduct<»r,
where it remains, since the conductor stands upon
glass supports. By turning the glass plate sufficiently
long, we may thus accumulate a large amoual of posi-
tive electricity in%is condiK:tor.
Experiment 6o. — If, when Hie conductor of the
elec^ madiine is charged with ^ectricity, I {ilace
my i«nger near it, a spark passes between the c^-
ductor and my finger. The reason is that the posilivt
electricity of the conductor separates tc^ two <le^
tricities which are together in my finger, driviiig #«^
the positive, which is of the same kind as it^elt t|l tike
!tRIFlBt)
BODIES.]
PHYSICS.
117
>nnects
earth.
)ositiVe
Jgative
igative
le roe-
's, and
earth,
I and
us got
now
V sur-
:h are
con-
luctor
keep
arthe
ointa
avea
lence
sitive
ICtOf,
upon
ently
posi-
^e
»Iace
C0n-
itive
ilec-
ear^ dirough my feet, but, on the other hand, attract-
ing the negative to itself.
The two electricities — ^namely the positive in the
conductor, and the negative in my finger — then rush
together through the air and unite with each other,
and in so doing they form a spark.
84. Leyden jar. Experiment 61. — When you
thus approach your finger or your knuckle to* an
electric machine, you feel a pricking sensation when
the spark passes, but that is all; you do not get a
severe shock. In order to
get a shock you must use a
Leyden jar, such as you see
in fig. 43. This is a glass
jar, the inside of which is
coated with tinfoil, as well
as the outside up to the
neck. A brass rod with a
knob at the end is con-
nected with ihe inside coat-
ing, and is kept tight by
being passed through a cork
which covers the mouth of the jar. Thus the jar ha s two
coatings, an inside andun ou'side one, and these are
quite separated from each other, as far as electricity is
cofii^med, inasmuch as glass does nol^onduct electri-
^Mb^ Now suppose I take the jar by its outside coating
in )||yhand, and h«ld the inside knob to the conductor
oC A fleets machine at work. The positive elec-
ir^H irom the ccmductor will then get into the inside
coa^g of the j«r. It will then decompose the two
electrt|iities of the outside coating, driying away the
ponlti^ through my hand and body generally to the
1
Fix- 43»
irS
SCIENCE PRIMERS, [BtiCTRiFiED
earth, and attracting the negfttive. In fact thipg will be
a battalion of positive electricity in the inside coatit^
facing an opposite battalion of negative electricity in
the outside coating, the two longing very much to
meet, but unable to do so for the glass. So intent are
these two electricities on watching each other that
they will remain close at their post while I put some
more positive electricity into the interior. This second
charge will then act precisely like the first ; it will
decompose anew the two electricities of the outside
coating, driving positive electricity from the outside
through my hand to the earth, while negative elec-
tricity will remain in the outside coating, facing the
new battalion of positive electricity which has been
introduced inside. We have now two inside and two
outside battalions watching one another, and by con-
tinuing this process we accumulate a large quantity of
opposite electricities in the two coatings of such a jar.
If we wish to discharge the jar, we make use of a
discharging rod, such as you see in the figure. It
should be . held by its glass handles, and one of the
knobs should be made to touch the outer coating of
the jar while the other is gradually brought near the
knob connected with the
interior of the jar \ wh«n
the two knobs are near to-
gether a bright i|)ark is
seen, accompanied with a
report, and the \2x 10 i^is-
charged. If we wiitr to
feel the shock oui
let us grasp t'.e
coating by one of our hands, and 4|pj^roacli tbe
Fig. 44.
nODIES.
..■""."■—"■—
towafc
the di
Or if 1
hands,
tiide c
the in!
the be
—Fro
vincec
in it
the 3:
is ac<
is ver
last I
a seel
Now
a jar
electi
whicl
Ag
quire
the e
hard
see 1
widi
work
t
wht]
CTRIFIED
MODIES.]
PHYSICS,
tI9
ft will be
coadi^
ricity in
luch to
tent are
ler that
ut some
second
it will
outside
outside
VQ elec-
ing the
IS been
md two
by con-
ntity of
h a jar.
ise of a
ire. It
of the
ting of
ear the
ith the
; when
ear to-
ark is
with a
ii#is-
to
towards the knob connected with the inside coating,
the discharge will then take place through our body.
Or if several wish to feel the shock, let them all joih
hands, and let the one at the one end grasp the out-
dde coating, while the ow^ at the other end touches
the inside knob, and the shock will then pass through
the bodies of all.
{5. Energetic nature of electrified bodies.
— from what has been said jom must now be con-
vinced that electricity is something which has energy
in it You see that the two opposite electricities of
the jar rush together and unite, and that the union
is accompanied by a flash and a report. This, flash
is very bright while it lasts ; and although it does not
last longer than the twenty-four thousandth part of
a second, it nevertheless implies considerable heat.
Now heat means energy, and we thus see that when
a jar is discharged, that kind of energy which we call
electricity is changed into that other form of energy
which we call heat and light.
Again, since electricity is an energetic thing, it re-
quires labour or work to produce it; ^ou do so by turning
the electric machine^ but such a machine is particularly
hard to turn on account of the electricity. You thus
see that there is nothing for nothing ; if you
widi to obtain an energetic agent, you must spend
wcark in doing so. On the other hand, there is no
^ifppearance of energy when the two electricities
pipRbine, but only, a change from the form of' elec-
iPppity- inta that of heat
86. Electric curi-ents.— You have seen that
when you hold a pointed conductor tiear an electric
madiitie at work (Art 82) there is a continuous stream
ISO
SCIENCE PRIMERS, [electrified
BODIES.
or current of electricity, which passes through die
point and through your hand to the ground.
We have, however, a much better means than the
electric machine gives us of obtaining powerful
Fig- 45.
electric currents. We shall now briefly describe this
method, which was first discovered by -n Italian
called Volta, and which has therefore been named the
Voltaic battery. This arrangement is shown in the
above figure. Here you see to the extreme left a
plate marked Cy which means a plate of copper. Next
you see a plate of zinc marked z, which is soldered to
a wire connecting it with the plate of copper in the
second vessel In the second vessel you have another
plate of zinc, which is similarly connected with the
copper in the third vessel. Finally, to the extreme right
you have a Single plate of zinc. Suppose now that
the vessels are filled with a mixture of su^^uric acid
and water, and that we attach wires to the copper at
the left-hand end, and also to the zinc at the right-
hand end, and that we bring these wires together.
(These wires are called the pole-wires of the batteiy;)
It will now be found that there is a current of positfye
electricity passing round and round through the cii^i^
in the direction of the arrow-heads. Let us tmt^
how it
wire a
and g(
it ent<
then ]
coppei
next
the m
by the
and fi
the li
origin
87.
descri
many
of ob
It
currei
but a
curre
batte
batte
Gro\
singl
beinj
eartl
parti
wel
asy
hav<
this
tal;(
l-'5
' 'i^^Hfi^fitm^.y*'''-^ " "". ',v- .■'
BODIESj
p//ys/cs.
Ul
how it goes. In the first place, it comes firom the
wire attached to the extreme left-hand copper plate,
and goes, as in the figure, through the long wires until
it enters the extreme right-hand plate of zinc ; it
then passes through the liquid till it reaches the
copper plate, from which it passes along the wire to the
next zinc plate ; it then passes through the liquid of
the middle vessel to the copper plate, and from that
by the wire to the zinc plate of the left-hand vessel ;
and finally from the zinc plate of this vessel through
the liquid to the same plate from which it started
originally. '
87. Grove's Battery. — ^The arrangement now
described was that used by Volta, but since his time
many improvements have been made in the method
of obtaining a current of electricity.
It was found that with Volta's arrangement the
current, though strong at first, very soon became weak ;
but a method has been devised by which the electric
current can always be kept at the same strength. A
battery by which this is done is called a constant
battery, and one of the best is that invented by
Grove (see fig. 48). In this battery, instead of a
single vessel we use a double one, the outer vessel
being made of glass, while the inner is made of porous
earthenware. The outer glass or stoneware vessel is
partly filled with diluted sulphuric acid. Within it
we have a plate of zinc (amalgamated on the outside),
as you see in the figure,, while within the glass vessel we
have a porous vessel, made of unglazed porcelain. Into
this porous vessel is poured strong nitric acid, and into
thi$ nifric acid is put a thin plate of platinum, which
takes flic place of the copper in Volta's arrangement.
12»
SCIENCE PRIMERS, [electrified
^DIES.
Now when this battery is in action, the zinc dissolves
in the dilute sulphuric acid, and during this process
hydrogen gas is given off, But this hydrogen does not
rise up in the sh?ipe of bubbles, but finds its way into
the porous vessel which contains the strong nitric acid.
It there decomposes the nitric acid, taking some oxygen
to itself, so as to become water (hydrogen and oxygen
forming water), and thereby turning the nitric into
nitrous acid, which shows its presence by strong orange-
coloured fumes. Thus the hydrogen does not reach
the platinum plate ; indeed it is to prevent its doing so
that this arrangement was made, for it was found that
in Volta*s original battery the hydrogen given out as
the zinc dissolved adhered to the copper plate, in con-
sequence of which the force of the battery became
much weakened. 9
What we have now described is only a single vessel
or cell, as it is called, of Grove's battery. In a large
battery of this kind there may be 50 or loo cells —
the wire that is attached to the platinum of one cell
being connected with the zinc of another, in a man-
ner precisely similar to that of fig. 45, the only differ-
ence being that instead of copper we have platinum,
and instead of a single vessel a double one of the
nature now described. Also, the positive current
passes through the liquid from the zinc to the plati-
num plate, just as it passed through the liquid from
the zinc to the copper plate in Volta's arrangement.
88. Properties of the current. — Let us now
see what an electric current can do ; that is to say,
let us perform a few simple experimqits.
Experiment 62.—Make a Grove's battery tteodjr
for action, and introduce a bit of very fine p!*«,tinti»j
wire bl
the c\
will
come
Ex]
for a^
'RIFJBD
[solves
jrocess
|es not
into
acid
ygen
ygen
into
|ange-
each
pgso
tliat
it as
con-
-ame
esse!
arge
Is^
cell
lan-
fer.
im,
the
?nt
iti.
w
Y
fODIES.]
PHVS/CS.
123
wire between the two pole wires of the battery ; when
the connection is made, and the current passes, it
will be found that the fine platinum wire will be-
come red-hot.
Experiment 63. — Make a Grove*s battery ready
for action, and insert its two pole wires into two
Fig. 46
inverted vessels containing water, as in fig. 46.
It will be found that the current decomposes the
water, a*id that oxygen gas will appear in the one
vesisei and hydrogen gas in the other. The oxygen
gas will appear at the pole which is connected with
the platinum plate, while the hydrogen will appear at
tfteat which is connected with the zinc plate. Thus
j^u see that a voltaic battery has the power of
Iteiomposing water. It has also the power of de-
^^f^isiposing very many compound liquids.
Experiment 64. — Here we have some copper
wire covered with thread so as to insulate it, and this
cc^per wire is wound round a thick piece of irou
124
SCIENCE PRIMERS, [ELECTRiFiEn
BOWl
shaped like a horse-shoe ; now let us connect the two
proles of our battery with the two extremities of the
copper wire which goes round the iron. If the battery
be now in action, it will be found
that the iron has acquired the power
of attracting other iron towards it, so
that a plate of iron will be held up,
as in the figure, with a heavy weight
attached to it As soon, however,
as the connection between the horse-
shoe and the battery is broken this
power is lost, and the weight which
the iron has been supporting will
drop down* at once.
Experiment 65. — -Take a bit of
^'^ '*^* hard steel, such as a knitting-needle,
and attach it to the iron of the horse-shoe in the last
experiment while the current is passing. This needle
will have gained certain properties which (unlike the
soft iron) it will not lose when the current is broken,
but will retain ever afterwards. For instance, if we
suspend the needle round the middle by means of a
very fine silk threat, and let it swing horizontally, it
will always point in one direction, and this direction
will be nearly north and south. The needle will, in
fact, have become a compass needle, always pointing
in one direction, and thus enabling the mariner when
out at sea to steer his vessel in the proper course. A
piece of hard steel possessing these properties is called
a magnet.
Experiment 66.— rLet us now suspend a magnetic
needle horizontally upon a pivot It will poiixl nearly
north and south. But let us now bring near it n wjtf
tltfot
foul
soutl
righj
II
its 11
' .u^aMWMIWe^'^'M*!^'-'^^''*™*'^''''**''^"''*
n
RiFiEn
BODIES.]
PHYSICS,
!15
through which a current is passing, and it will be
found that the needle will no longer point north and
south, but it will place itself so as to lie across or at
right angles to the wire conveying the current.
If we now break the current, the needle will resume
its usual direction.
Experiment 67. — We may render the last experi-
ment more marked by means of an arrangement such
-T
Fig 48.
•
as is sketched in the above figure. Let us aup^- /
pose that we have our battery at one end of tW^
room, while two wires covered with thread are carri^
from the two poles of the battery quite to^ jjther
end of the room, and are there joined together, so th^
the battery is now in action. Furthermore ypiirsee at
the end most remote from the battery arslppended
magnetic needle, which is placed ne^r the^ire,;«nd ,
which will be violently deflected wh^n th^ current
passes. Now if anyone at the very opposite comer
of the room should disconnect the wire fro^ one
of the poles of the battery, that very moment the cur-'
rent will cease to flow, and the magnetic needle will
resume its ordinary position.
89. Ele^ctric Telegraph. — It thus appears that
4
-}'-<:■■
■ jft"
126
JSCIEJ^CE PRIMERS, t^LECTkiFiEb
BODll
by disconnecting the wire from the battery at one end
of the room the needle is made to move at the other
at the very same moment This action would take
place even if the wires connected with th6 poles were
carried loo or even i,ooo miles away before they were
joined together. If a magnetic needle were placed
beside the wire conveying the current, even though
the wire should be i,ooo miles from the battery, it
would be deflected, but as soon as the other extremity
of this wire i,ooo miles away was disjoined from the
pole of the battery, the current would cease to pass,
and the magnetic needle would return to its usual
position. You thus see how it is possible, by
making and breaking contact of a wire with
the pole of a battery, to move a magnetic
needle i,ooo miles away.
In fact we have here the principle of the electric
telegraph, which performs such wonders in the way
of information, telling us what takes place in America
a few seconds after it happens. I cannot, however,
enter more fully into the subject, but at least you see
that it is possible to agitate a magnetic needle i,ooo
miles away, and, just as in the alphabet for the deaf
and dumb, these signals may be made the means of
conveying inf^nnation.
90. Cone usion. — You* have now learned what
the electric current can do. How, in the first
place, it can heat a fine wire through which it
passes; how, secondly, it can decompose wat^
and other compounds; how, thirdly, it can make
a piece of soft iron into a powerful though tempo-
rary magnet; how, fourthly, it can make a pi|pet
of hard steel into a permanent mai^et; and fiMiiy
and|
ren(
dist
.«*iKiirt.i
>ne end
le other
W take
'S were
ywere
placed
though
tery, it
remity
)m the
pas^
usual
e, by
with
netic
Jctric
e way
aerica
^ever,
u see
t^ooo
deaf
is of
vhat
i k
ake
BODI£S.]
PHYSICS,
127
Illy
and lastly, how it can deflect the compass needle,
rendering 'it thereby possible to telegraph to great
distances.
We cannot enter more fully into this very interesting
subject, but in conckision let me remind you that
you have now learned something about the active
moods of matter. We spoke first of all about moving
bodies, then about vibrating bodies, then about
heated bodies, and lastly about electrified bodies;
and we have tried throughout to show you that the
activity which a body may possess is never really lost.
It may, no doubt, pass to some other body, or it may
change its form, going from visible energy into sound,
or into heat, or into electricity, or changing about in
many different ways, b tit it is really lost no more than
a particle of matter is lost.
Indeed just as the science of Chemistry is built
upon the principle that matter only changes form,
going from one combination to another, and does not
absolutely disappear, so the science of Physics is
founded upon the principle that activity or energy
only changes form, and never absolutely disappears.
This, however, is a principle the full development of
which must be reserved for a future stage.
THINGS TO BE REMEMBERED.
A POUND avoirdupois is equal to 7,000 grains.
If a stone be dropped from the hand, it will fall
through 16 feet during the first second of time.
Steel is the strongest metal, but gold is the most
malleable; for a cubic inch of gold can be beaten
out so as to cover the floor of a room 50 feet long
and 40 feet wide.
The diamond is the hardest solid ; that is to say,
it can scratch everything else, but nothing else can
scratch it
A cubic inch of water weighs nearly 252 grains;
and, therefore, four cubic inches weigh nearly 1,000
grains.
100 cubic inches of air weigh 31 grains.
100 cubic inches of carbonic acid weigh 47 grains.
100 cubic inches of hydrogen only weigh 2 grains.
The pressure of the atmosphere will support a
column of mercury 30 inches high, and a column of
water more than 30 feet high.
Sound travels through air at a velocity of about
1,100 feet in one second of time.
If a musical string vibrates 50 times in one secon^,
it emits a deep, low note; if it vibrates 10,000 tkatB
in one second, it emits a shrill, high note.
Tlj
heat
quire
heat
Lil
Tl
twei
#
THINGS 10 BE REMEMBERED,
129
The heat required to melt a pound of ice would
heat 79 pounds of water one degree. The heat re-
quired to boil away a pound of boUing water would
heat 537 pounds of water one degree.
Light travels through space nearly at the rate of
190,000 miles in one second of time.
The spark from a Leyden jar lasts only the
twenty-four-thousandth part of one second.
INSTRUCTIONS REGARDING APPARATUS.
The apparatus to be used should be set up on the
table before the lesson, and the teacher should make
sure that he can perform without difficulty the various
experiments. After the lesson the apparatus ought to
be put away carefully into its appropriate place.
Care must be taken that the piston of the air-pump
is rendered tight in its cylinder by means of lard.
Care must also be taken that the receiver fits well upon
its bed-plate, and for this purpose it must be well
greased with lard. , When this is done, the receiver
ought to move smoothly and without noise on its
bed-plate ; but if there is a grinding noise it shows
that some hajd substance is present, and the bottom
of the receiver must then be carefully cleaned and
greased anew. This remark applies to the hemi-
spheres (fig. 15), as well as to the glass receivers.
In order to fill the box of Experiment 28 with
Qarbonic acid gas, the tube conveying the gas should
descend very nearly, but not quite, to the bottom of
the box.
To fill the same box (Experiment 29) with hydrogen,
the tube conveying the gas should ascend very nearly
to the bottom of the box, which is now uppermost
The whole apparatus for Experiment 45 must be
placed in a cool room some hours before the ex-^
periment is made. «
Gi
phoi
phosl
little!
pap<
imTkUCTIONS REGARD WG APPARA TVS. 131
n the
make
arious
ght to
pump
lard.
upon
well
ceiver
)n its
!h0W5
>ttom
and
leniK
with
3uld
1 of
:en,
irly
be
ex-
•
«
Great care ought to be taken in handling phos-
phorus, which very easily takes fire. The stock of
phosphorus should be kept under water, and the
little bits cut off should be well dried in blotting-
paper before being used.
When the mercury is tarnished, take a piece of
paper and make it into a funnel, having a pin-hole
at its bottom. Pour the mercury gently into this
funnel, and let it run through the pin-hole into a
vessel prepared for it It will now be quite bright
Care must be taken that the mercury is not con-
taminated with other metals. A small portion should
be kept separate for amalgamation in the battery.
Before the electrical machine is used the glass plate
ought to be well warmed. For this purpose it ought
to be placed endwise towards the fire, and the handle
ought to be turned round occasionally, so as to expose
to the fire the various parts of the plate. If these
instructions be not attended to, the glass may probably
crack.
The electroscope ought not to be charged highly,
otherwise the gold leaves will be driven to the sides
of the jar and be torn. To charge the electroscope
give the Leyden jar a single small spark from the
machine— then touch the electroscope with its knob.
The insulating glass supports of the conductor
ought also to be warm and dry.
Finally, the Leyden jar and everything made of
glass with which any electrical experiment is to be
made, ought to be warm and dry.
In the Grove's battery the zinc ought to be well
amalgamated (see Chemical Primer), and the varic is
K 2
n^
SCIENCE PRIMERS.
metals ought to be quite bright at the points where
they are connected with the battery.
The outer cells ought to be charged with one
part by measure of strong sulphuric acid and eight
parts of water.
The porous vessels of the Grove's battery ought
to be well steeped in water after the battery has
been in use ; and the zinc and platinum plates ought
likewise to be well cleaned.
In Experiment No. 66, it is necessary to fill with
mercury the two little brass cups into which the ends
of the battery wires are plunged.
DESCRIPTION OF APPARATUS.
Ko. ofEx- r^"r\/
periment. A '• »•
I, 2.— Tin pan, with peas ........ o i o
3. — Iron plate with four strings o I 6
4. — ^Balance to carry 2 lbs. in each scale ; beam
two feet long i 12 o
Piece of metal weighing 200 grains ...010
Set of weights, 600 grains to | grain ... o 10 6
5.— 2 lbs. mercury in a bottle . . . . . • o 10 o
Two pieces of glass two inches square . . 004
6. — Apparatus unnecessary.
9, 10. — Beam of w^ood 019
Two 4-lb. weights ...030
15.— Plumoline ...........010
Stoneware dish for mercury .......006
16.— Tube for showing level of water ....026
17.— Metal cylinder with two tubes and stoppers .060
Tube with moveable bottom and cord ..030
Water-jar for tube . ........010
Indigo solution .... * 005
18, 19.— Substance weighing 1,000 gmins, same spe-
cific gravity as water ... ....026
20. — Hollow brass cylinder .......026
^^m^ Bucket to contain it .......,•026
^W Apparatus for attaching the bucket to balance 01 6
21. — See Experiment 18.
22.- Block to illustrate flotation O o 3
24. — ^Apparatus unnecessary.
134
SCIENCE PRIMERS.
No. of Ex- p .
periment. / V
25. — Tate's air-pump . , 3 13 6
Bell-jar receiver ..026
Two india-rubber balls . . , . . . . 003
26.— Jar with neck and flange ......026
Two pieces of india-rubber for it . . . . o 10
27, 28, 29. — Box with strings 006
30.— Magdeburg hemispheres 056
Brass cock for ditto ........030
31' — Tube for barometer ........010
Glass mortar for cistern .......010
Funnel for filling barometer . . . . . . 002
33- — Vibrating wire on support ...... o i o
37. — Model thermometer ...050
Centigrade thermometer 040
38.— Bladder two-thirds filled with air ...,006
",9. — Further apparatus unnecessary.
40. —Use tin pan of Experiment I.
41. — Use flask of Experiment 42.
42.— Flask for boiling water, and cork in dupli-
cate ...,,,, 030
Triangleand wire gauze to support flak . .015
•3» 44* — No special apparatus necessary.
45* — ^^^ to Jiold sulphuric acid in vacuo^ and
shallow vessel to hold water 038
46. — No apparatus necessary.
47.— Use flask of Experiment 42.
48. — Wires to show unequal power of iron and
copper to conduct heat ......010
50.— Use tin pan of Experiment i.
51.— Apparatus to show image of candle , . . o lo 6
52. — Apparatus unnecessary.
54. — Electric pendulum .....,.,.020
Several pieces of elder-pith ...... o o 6 •
55.— Electroscope o 12 o
DESCRIPTION OF APPARA TUS,
135
No. of Ex-
periment.
Electrical machine, 16-inch plate .
Box of amalgam
56.— Rod, half brass, half glass , . .
Rod of glass covered with jed-wax
Piece of silk
Piece of flannel ......
57.— No additional apparatus.
58^ 59.— Brass ball, with point, on insulated
60. — No apparatus required.
61. — Leyden jar, pint size ....
Discharger
62.— Grove's battery, 4 cells in frame
Yard of fine platinum wire . .
63. — Voltameter
64. — Electro-magnet
65.— Knitting-needle and thread , .
66 —Apparatus for Oersted's experiment
67.— Thirty feet of covered wire . . .
stand
rrice.
£ s. d,
o
4
o
o
o
o
o
4
I
2
2
o
o
o
6
6
6
6
.036
.040
.030
. I 18 o
.006
. o 10 6
.060
.002
.056
.013
£19 3 8
\
X
As a wish has been expnsst i to havs a cheaper if iesi
compute set of apparatus^ the following is offered as an
alternative list.
DESCRIPTION OF APPARATUS.
No. of Ex- Pitct.
piiriiMnt /^ ». d
I, 2. — ^Tin pan, with peas (supplied by experi-
menter). ooo
3. — Iron plate with four strings ......016
4.»-13alance to carry 2 lbs. in each scale ; beam
twa feet long. I 12 O
Piece of nietal weighing 200 grains . . . o I O
Set of weights, 600 grains to ) grain . . . o 10 6
5. — 2 lbs. mercuiy in a bottle 0x00
Two pieces of glass two inches square • .004
6. — Apparatus unnecessary.
9, 10. — Beam of wood (supplied by experimenter) .000
Two 4-lb. weights Ditto ....000
15. — Plumbline '..oio
Stoneware dish for mercury 006
16. — Tube for showing level of water ....026
17. — Metal cylinder with two tubes and stoppers
(not supplied) , 000
Tube with moveable bottom and cord ...030
Water-jar for tul/e 010
Indigo solution ..005
18, 19. — Substance weighing 1,000 graiiis, same spe-
cific gravity as water ....... o 2 6
Carried forward > • 3 6 3
No. oi £x-
pcriment.
20. — H
B
A
21. — S
22. — B
24. — A
25.—^
]
r
26.— J
I
17, 28, 29
30.— 1
31-'
33.—
37.-
38.-
39-
40.—
41.-
42.-
43»44*-
45--
SCIENCE PRIMERS.
137
No. oi Ex- Price,
pcriment {^ s. d.
Brought over ..363
20. — Hollow brass cylinder * o 2 j
Bucket to contain it 026
Apparatus for attaching the backet to balance o i 6
21. — See Experiment 18.
22. — Block to illustrate flotation 003
24. — Apparatus unnecessary.
25. — Air-pump .......••••.220
Bell-jar receiver •••026
Two india-rubber balls ..•••••003
26. — Jar with neck and flange 026
Two pieces of india-rubber for it • • . • o I o
If, 28, 29. — Box with strings ........006
30. — Magdeburg hemispheres .......05^
Brass cock for ditto ....•••.030
31. — Tube for barometer 010
Glass mortar for cistern 010
Funnel for filling barometer ......002
33. — Vibrating wire on support o I c
37. — Model thermometer 050
Centigrade thermometer .......040
38. — Bladder two-thirds filled with air ....00 6
39. — Further apparatus unuecesrory.
4a — Use tin pan of Experiment i.
41.- Use flask of Experiment 42.
42. — Flask for boiling water, and cork in duph-
cate 030
Triangle and wire gauz . to support flask . . c i 5
43,44. — No special apparatus necessary.
45. — This experiment cannot be shown with the
air-pump of this list . 000
Carried forward ..774
1 38 DESCRIPTION OF APPARA TUS,
No. ol Ex- Priee.
(teriment £ g. d.
Brought over ..774
46. — No apparatus necessary.
47. — Use flask of Experiment 42.
48. — Wires to show unequal power of iron and
copper to conduct heat ...... o I o
50. — Use tin pan of Experiment i.
51. — Apparatus to show image of candle ... o 10 6
52. — Apparatus unnecessary.
54. — Electric pendulum 020
Several pieces of elder-pith ......006
55. — Electroscope *, .076
Electrical machine ..iibo
Box of amalgam ...010
56. — Rod, half brass, half glass .......026
Rod of glass covered with red-wax ...026
Piece of silk ..* 006
Piece of flannel 006
57. — No additional apparatus.
58, 59. — Brass ball, with point, on insulated stand . o 3 <f
60 — No apparatus required.
61. — Leyden jar, pint size 040
Discharger 030
62. — Battery o 12 6
Yard of fine platinum wire ......006
63. — This experiment cannot be shdwn with the
battery of this list 000
64. — Electro-magnet 060
65. — Knitting-needle and thread o o 2
66. — Apparatus for Oersted's experiment . . . .0 $ 6
67. —Thirty feet of covered wire ......013
£1^ g 3
Price for the whole of the apparatus . ;£ii" o o
Priee.
€ 9. d,
7 7 4
> I
) lo b
> 2
> o
> 7
10
» I
' 2
> 2
> O
' O
o
6
6
o
o
6
6
6
6
3 ^
4 o
3 o
12 6
o 6
o o
6 o
O 2
5 6
» o
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