EXPERIMENTS RELATING TO THE ALLEGED INFLUENCE OF COLOUR ON THE RADIATION OF NON-LUMINOUS HEAT. © ©*«♦*— BY A. D. BACHE, Prof, of Nat. Philos, and Chem. University of Pennsylvania, One of the Secretaries Am. Phil. Soc. &c. INQUIRY In relation to the alleged Influence of Colour on the Radiation of Non-lumi - nous Heat . BY A. D. BACHE, Prof, of Nat. Philos, and Chem. TJniv. of Penn. [From the Journal of the Franklin Institute for November, 1835.] In the following essay I propose to submit a few remarks upon a pa¬ per by Doct. Stark of Edinburgh, first published in the Transactions of the Royal Society of London, for 1833, together with an experimental inquiry into the alleged influence of colour on the radiation and absorp¬ tion of non-luminous heat. The experiments were commenced soon after the paper referred to, reached this country, and in them was adopted what seemed to me the less exceptionable of two methods used by Doctor Stark, which actually bear upon the question of the radiation of non-luminous heat. It was my intention to examine the matter more fully than had been done by Dr. Stark, and to procure a more satisfactory induction by experimenting on a considerable variety of substances. In this I had the kind assist¬ ance of my colleague, Prof. Courtenay* While these experiments were in progress, the remarks of the Rev. Professor Powell, of Oxford, on the paper of Doctor Stark, appeared in the Edinburgh New Philosophical Journal. They confirmed me en¬ tirely in the view of the inapplicability of most of the experiments made by Doct. Stark, to the determination of the question of the influence of colour on the radiation or the absorption of heat. Of this class were the absorption of heat, radiant heat being understood, as tested by the in¬ verse of Count Rumford’s method for comparing the conducting powers of substances used for clothing; also as tested by the effect of the heat from the flame of an argand gas burner, thrown by a mirror upon the bulb of an air thermometer, which was variously coated. Of the same class were the experiments on radiation, as tested by the method used by Count Rumford, above referred to; the enveloping materials on the inner thermometer being wools of different colours, and coloured wheat- en paste. Not included in this class were the methods of ascertaining the rate of cooling of a thermometer of which the bulb was coated with different pigments, and of a glass globe filled with warm water and variously coat¬ ed. I give the preference to a modification of this latter method from the greater extent of radiating surface which may, without inconvenience, be commanded by it. The glass globe used by Dr. Stark, was one inch and a quarter in diameter; it was coated at different times with Prus¬ sian blue, red lead, and white lead, and in a room at 50° Fah., the fall of temperature from 120°, through 25 degrees, was with the coatings re¬ spectively, in seventeen minutes, eighteen minutes, and nineteen minutes. I am constrained to differ from Professor Powell in his remarks upon the method just referred to, and, with great deference to so high autho¬ rity, would state why I consider them inconclusive. Professor Powell deems it necessary, or at least highly important to the determination of the question, that the radiating coatings of the globe should be equalized 1 4 in respect to thickness, conducting power, density, &c., and refers to the experiments of Prof. Leslie, in which equal quantities of different radiating substances were dissolved and spread upon a surface, for com¬ parison. That equal thicknesses of substances possessing different ra¬ diating powers, should be compared together, seems to me to be disprov¬ ed by the law established by Sir John Leslie’s own experiments, name¬ ly, that radiation takes place not only from the surface, but in a thick¬ ness which is appreciable in good radiators. This thickness not being the same for different substances. Thus when different coatings of jelly were applied, in succession, upon one of the sides of the cube in Professor Leslie’s experiments, the radiation increased with the thick¬ ness, up to a certain point. The effect of conducting power appears, by this same experiment, to be so small that an increase of the thickness in the bad conductor was actually more than compensated for by the in¬ creased radiating power. The influence of density on conducting pow¬ er is well known, but the effect of either as controlling the radiating pow¬ er of a substance, or as modifying it, is, I apprehend, yet to be appre¬ ciated. If these views be correct, and they are, I believe, founded upon the authorities so ably illustrated by Professor Powell in his report on radiant heat, to the British Association, the radiating powers of substan¬ ces would not be rightly compared by equalizing their thicknesses upon I a given surface, nor by equalizing their weights; but by ascertaining, for each substance, that thickness beyond which radiation does not take place. This will be placed in a clearer point of view in the sequel. I do not, however, consider the question at issue as the less difficult to determine; “no substance can be made to assume different colours with¬ out at the same time changing its internal structure,”* and I believe with Professor Powell, that “a very extensive induction is perhaps the only means open to us of ascertaining this, (the circumstances and properties [wherein the coatings differ) considering how totally ignorant we are of the peculiarities on which their colour depends.” This very extensive induction I do not pretend to have made, but I think to have multiplied experiments so much beyond the number made by Dr. Stark, as to be able to show that the supposed influence of colour on the absorption and radiation of heat remains yet to be demonstrated, and thus to prevent the admission as proved of what is more than doubtful. The principal object was to select a considerable variety of pigments of the same colour differing chemically, and of different colours chemi¬ cally allied, and, as subsidiary, to ascertain the effect of changes of colour, produced by chemical means, on different substances, and the effect of the material used to apply the pigment to the radiating body. Several tin cylinders were procured, two inches high, and an inch and a half in diameter, closed at the bottom, and having fitted to the top a slight¬ ly conical tube, to receive a perforated cork, through which to pass the stem of a thermometer. One of these vessels having been selected, was coated in successive layers with a pigment. Water which was boiling in a porcelain capsule was then poured into the cylinder, which was suspended by means of two lateral hooks to cords attached to the cano¬ py covering the lecture table. A thermometer introduced through a cork had its bulb nearly in the middle of the axis of the cylinder, and the thermometer by displacing part of the water assured that the quan- * Prof. Leslie’s Essay on Heat. 5 tity contained was the same in each case. A temperature was selected for beginning the experiments, sufficiently below that which the intro¬ duction of boiling water produced, to permit the rate of cooling to have become uniform, and one for ending which was high enough to prevent uncertainty from the slowness of the fall of temperature. The instant of the arrival of the mercurial column at any degree on the scale, and of its leaving the same, was noted, and a mean taken for the time of being at that temperature; a precaution which though superfluous in such experiments as these, will, I am persuaded, be found of importance where minute accuracy is desired in investigating the motion of heat. One of us observed the thermometer, the other noted the time by a pocket chronometer. The time of cooling of the cylinder coated with colouring matter having been ascertained, an additional layer of the same substance was put upon it and the cooling again observed. The time of cooling di¬ minished, of course, until that thickness was obtained beneath which no radiation takes place, the time then slowly increased with each ad¬ ditional coat, the conducting power entering as an appreciable element into the rate of cooling. To show the decided nature of the results, I subjoin an account of one series towards the beginning of our experi¬ ments, when a want of experience rendered us cautious in applying the successive coatings, lest we should pass the thickness of determinate radiation. The necessity for thus feeling our way, rendered the labour of the experiments very considerable. Cylinder coated with Prussian blue: Time of cooling from 180° to 140° Fah. 1. Thick coating, - - 101 lj 2. ditto added, - 965 3. Additional coat, - - 910| seconds. 4. do. do. - 829J 5. do. do. - 805 6. do. do. - 824 Another series, in a further advanced stage of our experiments is subjoined: Cylinder coated with Litmus blue: Time of cooling from 180° to 1403 Fah. 1. First thick coating, 2. Additional coat, 3. do. do. 4. do. do. 985 seconds. 855 82 7 \ 8341 Besides the necessity of making several experiments to obtain a sin¬ gle result, it sometimes occurred that particular results required to be repeated for verification, when apparent discrepancies occurred; this was done to ascertain if they were real or not. As it was obvious that the experiments must necessarily extend through a considerable time, during which the circumstances attending the cooling of the cylinders could not be expected to remain uniform, a standard for comparison was provided, in a cylinder of which the coating was not changed, and which was observed in regular turn with the other cylinders. At first a vessel without coating was used for this purpose, but as it was found liable to tarnish, it was substituted by a cylinder having a coating of aurum musivum, which was one of the 6 smoothest and most uniform of the coloured coatings used. The numbers obtained on the different days from a mean of the trials made of the cooling of the standard cylinder, were applied to compare the results of one day with those of another. This assumes that the times of cooling of the different vessels would be affected proportionately by a given change in the circumstances of the experiment. The inability to preserve the circumstances constant is the real objection to this method, and one which most affects the certainty of the results.* The following example shows the application of this method. The observed times of cooling of the standard cylinder, from 180 to 140° in two experiments on the 31st of October, were 969} and 968} seconds, mean 969. Three experiments on the first of November, gave 898, 892, and 893} seconds, mean 894}. Cylinder, number four, coated with cochineal, (crimson,) gave for the time of cooling from 180° to 140°, on the 1st of November, 848}. To compare this with a result obtained with the same cylinder, on the 31st of October, we have 894} : 969 :: 848} : x, the equivalent number for October 31st, 916.3 seconds. The results obtained with the same cylinder, on different occasions of experiment, having been thus rendered comparable, the comparison of experiments with different cylinders was effected by determining the time of cooling with the same coating upon different cylinders. Thus, numbers one and two having been coated with carbonate of lead, and their times of cooling through forty degrees having been ascertained, all the results with the various other coatings applied to these cylinders were comparable. The numbers thus obtained will not be strictly proportional to the radiating power of the substance used, for the whole surface of the cy¬ linders, including the ends, was not coated, and the contact of the air, and its consequent circulation, exert a most important influence on the rate of cooling. This latter element has been shown by the experiments of Petit and Dulong, to be independent of the nature of the surface, and as the amount of uncoated surface remains constant, the greater effect of radiation will appear by the more rapid rate of cooling, and the less by the less rapid rate. I proceed now to examine the degree of approximation which maybe expected from the results of the experiments. First, a comparison of different observations on the same day, under the same circumstances of the cylinders, and nearly or quite the same as to the temperature of the room, will show how far accuracy is possible under the most favourable suppositions. The following table presents the results of this kind obtained during the entire series of experiments, with the ratios of the times of cooling. * If the circumstances could be retained the same, three observations of the tem¬ perature at equal known intervals, would give a numerical expression for the radiating power of the coating. 1 Nature of Coating. Time in CPP*C Ratio. Nature of Coating. Time in QPP*Q Ratio. Cylinder No. 3. OC O• Cylinder No. 1. No coating. 1281$ 1300 1.000 1.014 Sulphuret of Antimony. 849J 972f 1.000 1.145 Chalk. 909* 939! 1.000 1.034 do. additional. Coating on another occ’n. 871! 878! 1.000 1.008 Prussian blue. 909! 932! 1.000 1.025 Red lead. 886! 894! 1.000 1.009 Litmus blue. 920! 956 1.000 1.038 do. blackened by sulphuretted hydrogen 911! 924! 1.000 1.014 Cylinder No. 5. Cylinder No. 4. Aurum Musivum. 892 1.000 Gamboge 932 1.000 893! 1.001 942! 1.011 898 I 007 938! 1.000 do. on another 937! 1.000 Chromate of lead. 954! 1.017 occasion. 959 1.023 845 1.000 943! 1.000 Vermilion. 850 1.006 do. 957 1.014 740! 1.000 818 1.000 Sulphate of Baryta. 778 1.051 do. 820! 1.003 — 850 1.000 Cylinder No. 1. do. 860 1 012 897 1.055 No coating. 1396! 1.000 — 1425! 1.020 851 1.000 1445! 1.035 do. 872! 1.025 1313! 1.000 do. another occasion. 1315! 1.002 do. 1303 1.000 1320 1.013 In the foregoing table, ten of the ratios are about l.ol to l,six 1.02 to 1, three 1,03 to 1, one 1.04 to 1, and two 1.05 to 1: it is, therefore, fair to infer that the single ratio of 1.14 to 1 results from an error of record, or observation, and the table fully shows, that, under the same circumstances 9 the results could readily he reproduced within about two per cent. Second. The correction for the altered circumstances of temperature . V 16 ,.!? 01 * 1 ’ &c :>™y k e tested by comparing the experiments made with different cylinders, having the same coatings, on different days. In the annexed table is given the various results of this kind furnished throughout the series of experiments. The date is given in the left hand column, and applies to all the results on the same horizontal line with it. A comparison of the numbers in the columns marked ratio, and on the same horizontal lines, will show how far the same reduction to a stand¬ ard would have been given by different cylinders; in other words, how 8 far the influence of currents of air, local temperature, and radiation from or to adjacent bodies, might have interfered with the particular results. Si o O o « •< p r-^ to to mm mm MM MM mm 00 oo to to to to to to CD *-*■ o 00 “'I On •-* o> 05 MM Ml VO 00 o> MM 1 III V. •" II II = No. ofcylind. 1 P ET > PS 1 > c- a. p »-3 ft* a. o 3 o 3 3 u o r+ p p o z 3 " 3 3 o o HH 3 cd sr 2 3* ° o • o 3 7? 3 fr g P a. cd 2. 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IV. • < III No. of cylind. > w > o o s; T3 3 c 3* o o p »-* PT »-s o 3 c* 3 O P o 3* o C/3 c 3 g s Pf nT #» a. 3 cd rf 3 p <-P CD CD 2. o p r—* 3 ccq do. w p* 3 MJ CD O C/3 « o • c o *-n CD o p s 3 CD P 3 • CL, cr? 00 vo 00 00 ~ mm 05 m- VO to 00 ^ O OlM^ Mm ca on mm T* VO VO VO O 05 O IfM 00 “'I VO MM to O 00 oo ^ K^m ■MICMIOli- (OlM 1C|m10|m KD|m k0|M Mm mm M— o <— »-» M- H* Hr-H mm mm o o b o b o b b o b o o boo b o o o Ratio. 05 O o> o o vo On o ^ o O CO ^ o to o on ifM O y. V. < y. No. of cylind. ► ►* > r 2$ 3 3 3 r*- p »"* e 3 •-* P 3 3 3 3 3 p: C/3 *■* CD g g g o «> 3 3 3 3 w t C/3 f C/3 CD < P 3 3 3 «-* 3 3 3 3 • cr? vo vo oo 00 vo CO 00 Reduced time on O 00 •*4- Wh ^ VO oo (oPp(m VO 05 if- 00 to on On of cooling. mm mm MM M- m- (-1 O MM • • o o • o o • • o o b b Ratio. o o 05 O O 00 -