UC-NRLF 
 
 CULAR THEORY, 
 
 PAPERS, 
 
 ' I "!.: ' , 
 
 fOSEPH-LOUIS G4 ; i 
 
 Club 
 No, I, 
 
REESE LIBRARY 
 
 OF THK 
 
 UNIVERSITY OF CALIFORNIA. 
 
 
 Deceived AP8 >#M895 - l8 ? 
 
 Accessions No . v Z # C/^-ss No... 
 
(gfemfiic fu6 (geprtnfe ($o. 4. 
 
 FOUNDATIONS 
 
 OF THE 
 
 MOLECULAR THEORY 
 
 COMPRISING 
 
 PAPERS AND EXTRACTS 
 
 BY 
 
 JOHN DALTON, 
 JOSEPH-LOUIS GAY-LUSSAC, 
 
 AND 
 
 AMEDEO AVOGADRO, 
 (1808-1811.) 
 
 OF THE 
 
 UNIVERSITY 
 
 OF 
 
 IFOR 
 EDINBURGH : 
 
 WILLIAM F. CLAY, 18 TEVIOT PLACE. 
 
 LONDON : 
 
 SIMPKIN, MARSHALL, HAMILTON, KENT, & CO. LTD. 
 
 1893. 
 
PREFACE. 
 
 papers here reprinted in chronological order 
 JL serve to exhibit the historical development of the 
 idea of a connection existing between the number of 
 particles in different gases and the volume they occupy. 
 
 It will be seen that Dalton from the first entertains the 
 notion that equal volumes of different gases may contain 
 the same number of ultimate particles at equal tempera- 
 ture and pressure, but that he is legitimately forced to 
 reject this assumption, conceiving no distinction between 
 the atom and the molecule of an element. Gay-Lussac's 
 important experimental work on the combining volumes 
 of gases then shows the necessity of a simple relation 
 between the ultimate particles of gases and their volumes, 
 although he does not point this out in his paper. Dalton, 
 however, perceives the necessity, and characteristically 
 concludes by doubting the accuracy of Gay-Lussac's 
 experiments. Avogadro, finally, accepts both Dalton's 
 theory and Gay-Lussac's data, and teaches how to recon- 
 cile them by distinguishing between the atom and the 
 molecule of an elementary gas. 
 
 It has npt been thought necessary to reprint the letter 
 of Ampere to Berthollet (Annales de Chimie, 90, 43-86, 
 1814), since that paper contains no advance on the views 
 of Avogadro published three years earlier, its author 
 simply drawing the same conclusions from the same 
 premises. 
 
 The English version of the French originals will pro- 
 bably be found more faithful than elegant, especially so 
 in the case of Avogadro's paper, where the French is 
 always clumsy and occasionally obscure. 
 
 J. W. 
 
FOUNDATIONS OF THE 
 MOLECULAR THEORY. 
 
 i. EXTRACTS FROM A NEW SYSTEM OF 
 CHEMICAL PHILOSOPHY. PART I. 
 (1808). BY JOHN DALTON. 
 
 I. 
 
 (pp. 70-72.) 
 
 3. The quantity of heat belonging to the ultimate par- 
 tides of all elastic fluids ; must be the same under the same 
 pressure and temperature. 
 
 It is evident the number of ultimate particles or mole- 
 cules in a given weight or volume of one gas is not the 
 same as in another : for, if equal measures of azotic and 
 oxygenous gases were mixed, and could be instantly 
 united chemically, they would form nearly two measures 
 of nitrous gas, having the same weight as the two original 
 measures ; but the number of ultimate particles could at 
 most be one half of that before the union. No two 
 elastic fluids, probably, therefore, have the same number 
 of particles, either in the same volume or the same weight. 
 Suppose, then, a given volume of any elastic fluid to 
 be constituted of particles, each surrounded with an 
 atmosphere of heat, repelling each other through the 
 medium of those atmospheres, and in a state of equili- 
 
6 Dalton. 
 
 brium under the pressure of a constant force, such as the 
 earth's atmosphere, also at the temperature of the sur- 
 rounding bodies ; suppose further, that by some sudden 
 change each molecule of air was endued with a stronger 
 affinity for heat ; query the change that would take place 
 in consequence of this last supposition ? The only 
 answer that can be given, as it appears to me, is this. 
 The particles will condense their respective atmospheres 
 of heat, by which their mutual repulsion will be dimin- 
 ished, and the external pressure will therefore effect a 
 proportionate condensation in the volume of air : neither 
 an increase nor diminution in the quantity of heat around 
 each molecule, or around the whole, will take place. 
 Hence the truth of the supposition, or as it may now be 
 called, proposition, is demonstrated. 
 
 II. 
 
 (pp. 187-189.) 
 
 I shall now proceed to give my present views on the 
 subject of mixed gases, which are somewhat different 
 from what they were when the theory was announced, in 
 consequence of the fresh lights which succeeding ex- 
 perience has diffused. In prosecuting my enquiries into 
 the nature of elastic fluids, I soon perceived it was 
 necessary, if possible, to ascertain whether the atoms or 
 ultimate particles of the different gases are of the same 
 size or volume in like circumstances of temperature and 
 pressure. By the size or volume of an ultimate particle, 
 I mean in this place, the space it occupies in the state of 
 a pure elastic fluid ; in this sense the bulk of the particle 
 signifies the bulk of the supposed impenetrable nucleus, 
 together with that of its surrounding repulsive atmo- 
 sphere of heat. At the time I formed the theory of 
 mixed gases, I had a confused idea, as many have, I 
 
New System of Chemical Philosophy. 7 
 
 suppose, at this time, that the particles of elastic fluids 
 are all of the same size ; that a given volume of oxygenous 
 gas contains just as many particles as the same volume of 
 hydrogenous ; or if not, that we had no data from which 
 the question could be solved. But from a train of reason- 
 ing, similar to that exhibited at page 71, I became con- 
 vinced that different gases have not their particles of the 
 same size : and that the following may be adopted as a 
 maxim, till some reason appears to the contrary : namely, 
 
 That every species of pure elastic fluid has its particles 
 globular and all of a size ; but that no two species agree in 
 the size of their particles, the pressure and temperature 
 being the same. 
 
Gay-Lussac. 
 
 2. MEMOIR ON THE COMBINATION OF 
 GASEOUS SUBSTANCES WITH EACH 
 OTHER. BY M. GAY-LUSSAC.* 
 
 Read before the Philomathic Society ', ^\st December 1808, 
 
 QUEST ANCES, whether in the solid, liquid, or gaseous 
 O state, possess properties which are independent of 
 the force of cohesion ; but they also possess others which 
 appear to be modified by this force (so variable in its 
 intensity), and which no longer follow any regular law. 
 The same pressure applied to all solid or liquid sub- 
 stances would produce a diminution of volume differing 
 in each case, while it would be equal for all elastic fluids. 
 Similarly, heat expands all substances; but the dilata- 
 tions of liquids and solids have hitherto presented no 
 regularity, and it is only those of elastic fluids which are 
 equal and independent of the nature of each gas. The 
 attraction of the molecules in solids and liquids is, there- 
 fore, the cause which modifies their special properties; 
 and it appears that it is only when the attraction is 
 entirely destroyed, as in gases, that bodies under similar 
 conditions obey simple and regular laws. At least, it is 
 my intention to make known some new properties in 
 gases, the effects of which are regular, by showing that 
 these substances combine amongst themselves in very 
 simple proportions, and that the contraction of volume 
 which they experience on combination also follows a 
 
 * Memoires de la Societe d'Arcueil, II. (1809), pp. 207-234. 
 
Combination of Gaseous Substances. 9 
 
 regular law. I hope by this means to give a proof of an 
 idea advanced by several very distinguished chemists 
 that we are perhaps not far removed from the time when 
 we shall be able to submit the bulk of chemical pheno- 
 mena to calculation. 
 
 It is a very important question in itself, and one much 
 discussed amongst chemists, to ascertain if compounds 
 are formed in all sorts of proportions. M. Proust, who 
 appears first to have fixed his attention on this subject, is 
 of opinion that the metals are susceptible of only two de- 
 grees of oxidation, a minimum and a maximum ; but led 
 away by this seductive theory, he has seen himself forced to 
 entertain principles contrary to physics in order to reduce 
 to two oxides all those which the same metal sometimes 
 presents. M. Berthollet thinks, on the other hand 
 reasoning from general considerations and his own ex- 
 periments that compounds are always formed in very 
 variable proportions, unless they are determined by 
 special causes, such as crystallisation, insolubility, or 
 elasticity. Lastly, Dalton has advanced the idea that 
 compounds of two bodies are formed in such a way that 
 one atom of the one unites with one, two, three, or more 
 atoms of the other.* It would follow from this mode of 
 looking at compounds that they are formed in constant 
 proportions, the existence of intermediate bodies being 
 excluded, and in this respect Dalton's theory would 
 resemble that of M. Proust; but M. Berthollet has 
 already strongly opposed it in the Introduction he has 
 written to Thomson's Chemistry, and we shall see that 
 in reality it is not entirely exact. Such is the state of the 
 
 * Dalton has been led to this idea by systematic considerations ; 
 and one may see from his work, A New System of Chemical 
 Philosophy, p. 213, and from that of Thomson, Vol. 6, that his 
 researches have no connection with mine. 
 
io Gay-Lussac. 
 
 question now under discussion ; it is still very far from 
 receiving its solution, but I hope that the facts which I 
 now proceed to set forth, facts which had entirely escaped 
 the notice of chemists, will contribute to its elucidation. 
 
 Suspecting, from the exact ratio of TOO of oxygen to 
 200 of hydrogen, which M. Humboldt and I had deter- 
 mined for the proportions of water, that other gases might 
 also combine in simple ratios, I have made the following 
 experiments. I prepared fluoboric,* muriatic, and carbonic 
 gases, and made them combine successively with ammonia 
 gas. 100 parts of muriatic gas saturate precisely 
 100 parts of ammonia gas, and the salt which is formed 
 from them is perfectly neutral, whether one or other of 
 the gases is in excess. Fluoboric gas, on the contrary, 
 unites in two proportions with ammonia gas. When the 
 acidt gas is put first into the graduated tube, and the 
 other gas is then passed in, it is found that equal volumes 
 of the two condense, and that the salt formed is neutral. 
 But if we begin by first putting the ammonia gas into the 
 tube, and then admitting the fluoboric gas in single 
 bubbles, the first gas will then be in excess with regard to 
 the second, and there will result a salt with excess of 
 base, composed of 100 of fluoboric gas and 200 of 
 ammonia gas. If carbonic gas is brought into contact 
 with ammonia gas, by passing it sometimes first, some- 
 times second into the tube, there is always formed a 
 sub-carbonate composed of 100 parts of carbonic gas and 
 200 of ammonia gas. It may, however, be proved that 
 neutral carbonate of ammonia would be composed of 
 equal volumes of each of these components. M. Ber- 
 
 * M. Thenard and I have given the name of fluoboric gas to that 
 particular gas which we obtained by distilling pure fluoride of lime 
 with vitreous boracic acid. 
 
 t ["Alkaline" in the original.] 
 
Combination of Gaseous Substances. 1 1 
 
 thollet, who has analysed this salt, obtained by passing 
 carbonic gas into the sub-carbonate, found that it was 
 composed of 73.34 parts by weight of carbonic gas and 
 26.66 of ammonia gas. Now, if we suppose it to be 
 composed of equal volumes of its components, we find 
 from their known specific gravity, that it contains by 
 weight 
 
 71.81 of carbonic acid, 
 
 28.19 of ammonia, 
 
 100.0 
 
 a proportion differing only slightly from the preceding. 
 
 If the neutral carbonate of ammonia could be formed 
 by the mixture of carbonic gas and ammonia gas, as 
 much of one gas as of the other would be absorbed ; and 
 since we can only obtain it through the intervention of 
 water, we must conclude that it is the affinity of this 
 liquid which competes with that of the ammonia to over- 
 come the elasticity of the carbonic acid, and that the 
 neutral carbonate of ammonia can only exist through the 
 medium of water. 
 
 Thus we may conclude that muriatic, fluoboric, and 
 carbonic acids take exactly their own volume of ammonia 
 gas to form neutral salts, and that the last two take twice 
 as much to form sub-salts. It is very remarkable to see 
 acids so different from one another neutralise a volume 
 of ammonia gas equal to their own ; and from this we 
 may suspect that if all acids and all alkalis could be 
 obtained in the gaseous state, neutrality would result 
 from the combination of equal volumes of acid and 
 alkali. 
 
 It is not less remarkable that, whether we obtain a 
 neutral salt or a sub-salt, their elements combine in simple 
 ratios which may be considered as limits to their propor- 
 tions. Accordingly, if we accept the specific gravity of 
 
 X 
 
 f OF THE 
 
 (UNI 
 
 V ^ 
 
1 2 Gay-Lussac. 
 
 muriatic acid determined by M. Biot and myself,* and 
 those of carbonic gas and ammonia given by MM. Biot 
 and Arago, we find that dry muriate of ammonia is 
 composed of 
 
 Ammonia, 100.0 38.35 
 
 Muriatic acid, 160.7 61.65 
 
 100. oo 
 
 a proportion very far from that of M. Berthollet 
 100 of ammonia, 
 2 1 3 of acid. 
 
 In the same way, we find that sub-carbonate of 
 ammonia contains 
 
 Ammonia, 100.0 43.98 
 
 Carbonic acid, 127.3 56.02 
 
 100.00 
 and the neutral carbonate 
 
 Ammonia, 100.0 28.19 
 
 Carbonic acid, 254.6 71.81 
 
 TOO.OO 
 
 It is easy from the preceding results to ascertain the 
 ratios of the capacity of fluoboric, muriatic, and carbonic 
 acids; for since these three gases saturate the same 
 volume of ammonia gas, their relative capacities will be 
 inversely as their densities, allowance having been made 
 for the water contained in muriatic acid. 
 
 We might even now conclude that gases combine with 
 each other in very simple ratios ; but I shall still give 
 some fresh proofs. 
 
 * As muriatic gas contains one-fourth its weight of water, we 
 must only take three-fourths of the density for that of real muriatic 
 acid. 
 
Combination of Gaseous Substances. 1 3 
 
 According to the experiments of M. Amedee Berthollet, 
 ammonia is composed of 
 
 100 of nitrogen, 
 300 of hydrogen, 
 by volume. 
 
 I have found (ist vol. of the Societe d'Arcueil) that 
 sulphuric acid is composed of 
 
 100 of sulphurous gas, 
 50 of oxygen gas. 
 
 When a mixture of 50 parts of oxygen and 100 of 
 carbonic oxide- (formed by the distillation of oxide of 
 zinc with strongly calcined charcoal) is inflamed, these two 
 gases are destroyed and their place taken by 100 parts 
 of carbonic acid gas. Consequently carbonic acid may 
 be considered as being composed of 
 
 100 of carbonic oxide gas, 
 50 of oxygen gas. 
 
 Davy, from the analysis of various compounds of 
 nitrogen with oxygen, has found the following propor- 
 tions by weight : 
 
 Nitrogen. Oxygen. 
 
 Nitrous oxide - - 63.30 36.70 
 
 Nitrous gas - - 44.05 55.95 
 
 Nitric acid - - 29.50 70.50 
 
 Reducing these proportions to volumes we find 
 
 Nitrogen. Oxygen. 
 
 Nitrous oxide - - 100 49.5 
 
 Nitrous gas - 100 108.9 
 
 Nitric acid - 100 204.7 
 
 The first and the last of these proportions differ only 
 
 slightly from 100 to 50, and 100 to 200 ; it is only the 
 
 second which diverges somewhat from 100 to 100. The 
 
 difference, however, is not very great, and is such as we 
 
 might expect in experiments of this sort; and I have 
 
 assured myself that it is actually nil. On burning the 
 
14 Gay-Lussac. 
 
 new combustible substance from potash in 100 parts by 
 volume of nitrous gas, there remained over exactly 50 
 parts of nitrogen, the weight of which, deducted from 
 that of the nitrous gas (determined with great care by M. 
 Berard at Arcueil), yields as result that this gas is com- 
 posed of equal parts by volume of nitrogen and oxygen. 
 We may then admit the following numbers for the pro- 
 portions by volume of the compounds of nitrogen with 
 oxygen : 
 
 Nitrogen. Oxygen. 
 
 Nitrous oxide - 100 50 
 
 Nitrous gas - - 100 100 
 
 Nitric acid - 100 200 
 
 From my experiments, which differ very little from 
 those of M. Chenevix, oxygenated muriatic acid * is com- 
 posed by weight of 
 
 Oxygen - 22.92 
 
 Muriatic acid 77.08 
 
 Converting these quantities into volumes, we find that 
 
 oxygenated muriatic acid is formed of 
 
 Muriatic gas - 300.0 
 
 Oxygen gas - 103.2 
 
 a proportion very nearly 
 
 Muriatic gas - 300 
 
 Oxygen gas - ioof 
 
 * [Chlorine.] 
 
 f In the proportion by weight of oxygenated muriatic acid, the 
 muriatic acid is supposed to be free from water, whilst in the pro- 
 portion by volume it is supposed to be combined with a fourth of 
 its weight of water, which, since the reading of this paper, M. 
 Thenard and I have proved to be absolutely necessary for its exist- 
 ence in the gaseous state. But since the simple ratio of 300 of acid 
 to loo of oxygen cannot be due to chance, we must conclude that 
 water by combining with dry muriatic acid to form ordinary muriatic 
 acid does not sensibly change its specific gravity. We should be 
 led to the same conclusion from the consideration that the specific 
 
Combination of Gaseous Substances. 1 5 
 
 Thus it appears evident to me that gases always com- 
 bine in the simplest proportions when they act on one 
 another ; and we have seen in reality in all the preceding 
 examples that the ratio of combination is i to i, i to 2, or 
 i to 3. It is very important to observe that in considering 
 weights there is no simple and finite relation between the 
 elements of any one compound ; it is only when there is 
 a second compound between the same elements that the 
 new proportion of the element that has been added is a 
 multiple of the first quantity. Gases, on the contrary, in 
 whatever proportions they may combine, always give rise 
 to compounds whose elements by volume are multiples 
 of each other. 
 
 Not only, however, do gases combine in very simple 
 proportions, as we have just seen, but the apparent con- 
 traction of volume which they experience on combination 
 has also a simple relation to the volume of the gases, or 
 at least to that of one of them. 
 
 I have said, following M. Berthollet, that 100 parts of 
 carbonic oxide gas, prepared by distilling oxide of zinc 
 and strongly calcined charcoal, produce 100 parts of car- 
 bonic gas on combining with 50 of oxygen. It follows 
 from this that the apparent contraction of the two gases 
 is precisely equal to the volume of oxygen gas added. 
 The density of carbonic gas is thus equal to that of car- 
 bonic oxide gas plus half the density of oxygen gas ; or, 
 conversely, the density of carbonic oxide gas is equal to 
 that of carbonic gas, minus half that of oxygen gas. 
 Accordingly, taking the density of air as unity, we find 
 
 gravity of oxygenated muriatic acid, which from our experiments 
 contains no water, is exactly the same as that obtained by adding the 
 density of oxygen gas to three times that of muriatic gas, and taking 
 half of this sum. M. Thenard and I have also found that oxygen- 
 ated muriatic gas contains precisely half its volume of oxygen, and 
 that it can destroy in consequence its own volume of hydrogen. 
 
 j OF THE ^w 
 
 (XJNIVERSITTT) 
 
 x. OF S 
 
1 6 Gay-Lussac. 
 
 the density of carbonic oxide gas to be 0.9678, instead of 
 0.9569 experimentally determined by Cruickshanks. 
 We know, besides, that a given volume of oxygen pro- 
 duces an equal volume of carbonic acid ; consequently 
 oxygen gas doubles its volume on forming carbonic oxide 
 gas with carbon, and so does carbonic gas on being 
 passed over red-hot charcoal. Since oxygen produces an 
 equal volume of carbonic gas, and the density of the 
 latter is well known, it is easy to calculate the proportion 
 of its elements. In this way we find that carbonic gas 
 is composed of 
 
 27.38 of carbon, 
 72.62 of oxygen, 
 and carbonic oxide of 
 
 42.99 of carbon, 
 57.01 of oxygen. 
 
 Pursuing a similar course, we find that if sulphur ta\kes 
 100 parts of oxygen to produce sulphurous acid, it takes 
 150 parts to produce sulphuric acid. As a matter of 
 fact, we find that sulphuric acid, according to the experi- 
 ments of MM. Klaproth, Bucholz, and Richter, is com- 
 posed of 100 parts by weight of sulphur and 138 of 
 oxygen. 
 
 On the other hand sulphuric acid is composed of 2 
 parts by volume of sulphurous gas, and i of oxygen gas. 
 Consequently the weight of a certain quantity of sulphuric 
 acid should be the same as that of 2 parts of sulphurous 
 acid and i of oxygen gas, i.e., 2 x 2.265, P^ us I - 10 359 = 
 5- 6 3359 ', seeing that, according to Kirwan, sulphurous 
 gas weighs 2.265, tne density of air being taken as unity. 
 But from the proportion of 100 of sulphur to 138 of 
 oxygen, this quantity contains 3.26653 of oxygen, and if 
 we subtract from it 1.10359 there will remain 2.16294 for 
 the weight of oxygen in 2 parts of sulphurous acid, or 
 1.08147 for the weight of oxygen contained in i part. 
 
Combination of Gaseous Substances. 17 
 
 Now as this last quantity only differs by 2 per cent, 
 from 1.10359, which represents the weight of i part of 
 oxygen gas, it must be concluded that oxygen gas, in 
 combining with sulphur to form sulphurous gas, only ex- 
 periences a diminution of a fiftieth of its volume, and this 
 would probably be nil if the data I have employed were 
 more exact. On this last supposition, using Kirwan's 
 value for the specific gravity of sulphurous gas, we should 
 find that this acid is composed of 
 
 100.00 of sulphur, 
 95.02 of oxygen. 
 
 But if, adopting the preceding proportions for sulphuric 
 acid, we allow, as appears probable, that 100 of sulphur- 
 ous gas contain 100 of oxygen gas, and that 50 have 
 still to be added to convert it into sulphuric acid, we 
 shall obtain for the proportions in sulphurous acid 
 100.00 of sulphur, 
 92.0 of oxygen. 
 
 Its specific gravity calculated on the same suppositions, 
 and referred to that of air, would be 2.30314, instead of 
 2.2650 as Kirwan found directly.* 
 
 * In order to remove these differences it would be necessary to 
 make new experiments on the density of sulphurous gas, on the 
 direct union of oxygen gas with sulphur to see if there is contraction, 
 and on the union of sulphurous gas with ammonia gas. I have 
 found, it is true, on heating cinnabar in oxygen gas, that 100 parts 
 of this gas only produce 93 of sulphurous gas. It also appeared as 
 if less sulphurous gas than ammonia gas was necessary to form a 
 neutral salt. But as these experiments were not made under suit- 
 able conditions, especially the last, which could only be made in 
 presence of water, the sulphurous gas decomposing and precipitating 
 sulphur immediately on being mixed with the ammonia gas, I 
 intend to repeat them and determine exactly all the conditions 
 before drawing any conclusion from them. This is all the more 
 necessary, as sulphurous gas can be used to analyse sulphuretted 
 hydrogen gas, if its proportions are well known. 
 
 B 
 
1 8 Gay-Lussac. 
 
 Phosphorus is very closely connected with sulphur, 
 seeing that both have nearly the same specific gravity. 
 Consequently phosphorus should take up twice as much 
 oxygen to become phosphorous acid, as to pass from this 
 state into phosphoric acid. Since the latter is composed, 
 according to Rose, of 
 
 100.0 of phosphorus, 
 114.0 of oxygen, 
 
 it follows that phosphorous acid should contain 
 
 i oo.o of phosphorus, 
 76.0 of oxygen. 
 
 We have seen that 100 parts of nitrogen gas take 50 
 parts of oxygen gas to form nitrous oxide, and 100 of 
 oxygen gas to form nitrous gas. In the first case, the 
 contraction is a little greater than the volume of oxygen 
 added ; for the specific gravity of nitrous oxide, calculated 
 on this hypothesis, is 1.52092, while that given by Davy 
 is 1.61414. But it is easy to show, from some of Davy's 
 experiments, that the apparent contraction is precisely 
 equal to the volume of oxygen gas added. On passing 
 the electric spark through a mixture of 100 parts of 
 hydrogen and 97.5 of nitrous oxide the hydrogen is 
 destroyed, and 102 parts of nitrogen remain, including 
 that quantity which is almost always mixed with the 
 hydrogen, and a little of the latter gas which has escaped 
 combustion. The residue, after making all corrections, 
 would be very nearly equal in volume to the nitrous 
 oxide employed. Similarly, on passing the electric spark 
 through a mixture of 100 parts of phosphuretted hydrogen 
 and 250 of nitrous oxide, water and phosphoric acid are 
 formed, and exactly 250 parts of nitrogen remain, 
 another evident proof that the apparent contraction of 
 the elements of nitrous oxide is equal to the whole volume 
 
Combination of Gaseous Substances. 19 
 
 of oxygen added. From this circumstance, its specific 
 gravity referred to that of air should be 1.52092. 
 
 The apparent contraction of the elements of nitrous 
 gas appears, on the other hand, to be nil. If we admit, 
 as I have shown, that it is composed of equal parts of 
 oxygen and nitrogen, we find that its density, calculated 
 on the assumption that there is no contraction, is 1.036, 
 while that determined directly is 1.038. 
 
 Saussure found that the density of water vapour is to 
 that of air as 10 is to 14. Assuming that the contraction 
 of volume of the two gases is only equal to the whole 
 volume of oxygen added, we find instead of this a ratio 
 of 10 to 1 6. This difference, and the authority of a 
 physicist so distinguished as Saussure, would seem to be 
 enough to make us reject the assumption I have just 
 made; but I shall mention several circumstances that 
 render it very probable. Firstly, it has a very strong 
 analogy in its favour; secondly, M. Trales found by 
 direct experiment that the ratio of the density of water- 
 vapour to air is 10 to 14.5, instead of 10 to 14 ; thirdly, 
 although we do not know very exactly the volume occu- 
 pied by water on passing into the elastic state, we do 
 know, from the experiments of Watt, that a cubic inch of 
 water produces nearly a cubic foot of steam, i.e., a volume 
 1728 times as great. Now, adopting Saussure's ratio, we 
 find only 1488 for the volume occupied by water when it 
 is converted into steam ; but adopting the ratio of 10 to 
 1 6, we should have 1700.6. Finally, the refraction of 
 water-vapour, calculated on the assumption of the ratio 
 10 to 14, is a little greater than the observed refraction; 
 but that calculated from the ratio 10 to 16 is much more 
 in harmony with the results of experiment. These, then, 
 are the considerations which go to make the ratio 10 to 
 1 6 very probable. 
 
 Ammonia gas is composed of three parts by volume of 
 
2O Gay-Lussac. 
 
 hydrogen and one of nitrogen, and its density compared 
 to air is 0.596. But if we suppose the apparent contrac- 
 tion to be half of the whole volume, we find 0.594 for the 
 density. Thus it is proved, by this almost perfect con- 
 cordance, that the apparent contraction of its elements is 
 precisely half the total volume, or rather double the 
 volume of the nitrogen. 
 
 I have already proved that oxygenated muriatic gas is 
 composed of 300 parts of muriatic gas and 100 of oxygen 
 gas. Admitting that the apparent contraction of the two 
 gases is half the whole volume, we find 2.468 for its 
 density, and by experiment 2.470. I have also assured 
 myself by several experiments that the proportions of its 
 elements are such that it forms neutral salts with the 
 metals. For example, if we pass oxygenated muriatic gas 
 over copper, there is formed a slightly acid green muriate, 
 and a little oxide of copper is precipitated, because the 
 salt cannot be obtained perfectly neutral. It follows from 
 this that in all the muriates, as in oxygenated muriatic 
 acid, the acid reduced to volume is thrice the oxygen. 
 It would be the same for carbonates and fluorides, the 
 acids of which have for equal volumes the same saturation 
 capacity as muriatic acid. 
 
 We see, then, from these various examples, that the 
 contraction experienced by two gases on combination is 
 in almost exact relation with their volume, or rather with 
 the volume of one of them. Only very slight differences 
 exist between the densities of compounds obtained by 
 calculation and those given by experiment, and it is 
 probable that, on undertaking new researches, we shall 
 see them vanish entirely. 
 
 Recalling the great law of chemical affinity, that every 
 combination involves an approximation of the elementary 
 molecules, it is difficult to conceive why carbonic oxide 
 gas should be lighter than oxygen. Indeed, that is the 
 
Combination of Gaseous Substances. 2 1 
 
 principal reason which has led M. Berthollet to assume the 
 existence of hydrogen in this gas, and thus explain its low 
 density. But it seems to me that the difficulty arises 
 from supposing that the approximation of the elementary 
 molecules is represented in gases by the diminution of 
 volume which they suffer on combination. This supposi- 
 tion is not always true, and we might cite several gaseous 
 combinations, the constituent molecules of which would 
 be brought very close together, although there is not only 
 no diminution of volume, but even a dilatation. Such, 
 for example, is nitrous gas, whether we consider it as 
 being formed directly from nitrogen and oxygen, or from 
 nitrous oxide and oxygen. In the first case, there is no 
 diminution of volume; and in the second, there would be 
 dilatation, for 100 parts of nitrous oxide and 50 of oxygen 
 would produce 200 of nitrous gas. We know too that 
 carbonic gas represents an exactly equal volume of oxygen, 
 and that the affinity which unites its elements is very 
 powerful. Nevertheless, if we admitted an immediate 
 relation between the condensation of the elements and 
 the condensation of volume, we should conclude, con- 
 trary to experiment, that there is no condensation.- 
 Otherwise it would be necessary to suppose that if 
 carbon were in the gaseous state it would combine in 
 equal volumes (or in any other proportion) with oxygen, 
 and that the apparent condensation would then be equal 
 to the whole volume of the gaseous carbon. But if we 
 make this supposition for carbonic acid, we may also 
 make it for carbonic oxide, by assuming, for instance, 
 that 100 parts of gaseous carbon would produce 100 
 parts of the gas on combining with 50 parts of oxygen. 
 However it may stand with these suppositions, which 
 only serve to make it conceivable that oxygen can pro- 
 duce a compound lighter than itself by combining with a 
 solid substance, we must admit, as a truth founded on 'a 
 
 OF THE 
 
 UNIVERsi'i 
 
2 2 Gay-L us sac. 
 
 great number of observations, that the condensation of 
 the molecules of two combining substances, in particular 
 of two gases, has no immediate relation to the condensa- 
 tion of volume, since we often see that whilst one is very 
 great the other is very small or even nil. 
 
 The observation that the gaseous combustibles com- 
 bine with oxygen in the simple ratios of i to i, i ,to 2, 
 i to J, can lead us to determine the density of the 
 vapours of combustible substances, or at least to approxi- 
 mate closely to that determination. For if we suppose 
 all combustible substances to be in the gaseous state, a 
 specified volume of each would absorb an equal volume 
 of oxygen, or twice as much, or else half; and as we 
 know the proportion of oxygen taken up by each com- 
 bustible substance in the solid or liquid state, it is suffi- 
 cient to convert the oxygen into volumes and also the 
 combustible, under the condition that its vapour shall be 
 equal to the volume of oxygen, or else double or half this 
 value. For example, mercury is susceptible of two degrees 
 of oxidation, and we may compare the first one to nitrous 
 oxide. Now, according to MM. Fourcroy and Thenard, 
 100 parts of mercury absorb 4.16, which reduced to 
 gas would occupy a space of 8.20. These TOO parts 
 of mercury reduced to vapour should therefore occupy 
 twice the space, viz., 16.40. We thence conclude that 
 the density of mercury vapour is 12.01 greater than that 
 of oxygen, and that the metal on passing from the liquid 
 to the gaseous state assumes a volume 961 times as great. 
 
 I shall not discuss more of these determinations, 
 because they are only based on analogies, and it is 
 besides easy to multiply them. I shall conclude this 
 Memoir by examining if compounds are formed in con- 
 stant or variable proportions, as the experiments of which 
 I have just given an account lead me to the discussion of 
 these two opinions. 
 
Combination of Gaseous Substances. 23 
 
 According to Dalton's ingenious idea, that combina- 
 tions are formed from atom to atom, the various 
 compounds which two substances can form would be 
 produced by the union of one molecule of the one with 
 one molecule of the other, or with two, or with a greater 
 number, but always without intermediate compounds. 
 Thomson and Wollaston have indeed described experi- 
 ments which appear to confirm this theory. Thomson * 
 has found that super-oxalate of potash contains twice as 
 much acid as is necessary to saturate the alkali; and 
 Wollaston, t that the sub-carbonate of potash contains, on 
 the other hand, twice as much alkali as is necessary to 
 saturate the acid. 
 
 The numerous results I have brought forward in this 
 Memoir are also very favourable to the theory. But M. 
 Berthollet, who thinks that combinations are made con- 
 tinuously, cites in proof of his opinion the acid sulphates, 
 glass, alloys, mixtures of various liquids, all of which 
 are compounds with very variable proportions, and he 
 insists principally on the identity of the force which pro-| 
 duces chemical compounds and solutions. 
 
 Each of these two opinions has, therefore, a large 
 number of facts in its favour; but although they are 
 apparently utterly opposed, it is easy to reconcile them. 
 . We must first of all admit, with M. Berthollet, that 
 chemical action is exercised indefinitely in a continuous 
 manner between the molecules of substances, whatever ( 
 their number and ratio may be, and that 'in general we 
 can obtain compounds with very variable proportions. 
 But then we must admit at the same time that, apart 
 from insolubility, cohesion, and elasticity, which tend to 
 produce compounds in fixed proportions, chemical action 
 
 * [See Alembic Club Reprints, No. 2, p. 41.] 
 f [See Alembic Club Reprints, No. 2, p. 35.] 
 
24 Gay-Lussac. 
 
 is exerted more powerfully when the elements are in 
 simple ratios or in multiple proportions among them- 
 selves, and that compounds are thus produced which 
 separate out more easily. In this way we reconcile the 
 two opinions, and maintain the great chemical law, that 
 whenever two substances are in presence of each other 
 they act in their sphere of activity according to their 
 masses, and give rise in general to compounds with very 
 variable proportions, unless these proportions are deter- 
 mined by special circumstances. 
 
 Conclusion. 
 
 I have shown in this Memoir that the compounds of 
 gaseous substances with each other are always formed in 
 very simple ratios, so that representing one of the terms 
 by unity, the other is i, or 2, or at most 3. These ratios 
 by volume are not observed with solid or liquid sub- 
 stances, nor when we consider weights, and they form a 
 new proof that it is only in the gaseous state that sub- 
 stances are in the same circumstances and obey regular 
 laws. It is remarkable to see that ammonia gas neutralises 
 exactly its own volume of gaseous acids; and it is probable 
 that if all acids and alkalies were in the elastic state, they 
 would all combine in equal volumes to produce neutral 
 salts. The capacity of saturation of acids and alkalies 
 measured by volume would then be the same, and this 
 might perhaps be the true manner of determining it. 
 The apparent contraction of volume suffered by gases on 
 combination is also very simply related to the volume 
 of one of them, and this property likewise is peculiar to 
 gaseous substances. 
 
New System of Chemical Philosophy. 25 
 
 3. EXTRACT FROM A NEW SYSTEM OF 
 CHEMICAL PHILOSOPHY. PART II. 
 (1810). BY JOHN DALTON. 
 
 (Appendix, pp. 555~559-) 
 
 SOME observations on nitric acid, and the other com- 
 pounds of azote and oxygen, have been made by Gay 
 Lussac, in the 2d vol. of the Memoires d'Arcueil. He 
 contends that one measure of oxygenous gas unites to 
 two measures of nitrous gas to form nitric acid, and to 
 three measures to form nitrous acid. Now I have shewn 
 that i measure of oxygen may be combined with 1.3 of 
 nitrous gas, or with 3.5, or with any intermediate quantity 
 whatever, according to circumstances, which he seems to 
 allow : what, then, is the nature of the combinations below 
 2, and above 3, of nitrous gas ? No answer is given to 
 this ; but the opinion is founded upon an hypothesis that 
 all elastic fluids combine in equal measures, or in measures 
 that have some simple relation one to another, as i to 2, 
 i to 3, 2 to 3, &c. In fact, his notion of measures is 
 analogous to mine of atoms ; and if it could be proved \ 
 that all elastic fluids have the same number of atoms in j 
 the same volume, or numbers that are as i, 2, 3, &c. the | 
 two hypotheses would be the same, except that mine is 
 universal, and his applies only to elastic fluids. Gay 
 Lussac could not but see (page 188, Part i. of this work) 
 that a similar hypothesis had been entertained by me, 
 and abandoned as untenable ; however, as he has revived 
 the notion, I shall make a few observations upon it, 
 though I do not doubt but he will soon see its inadequacy. 
 Nitrous gas is, according to Gay Lussac, constituted of 
 equal measures of azote and oxygen, which, when com- 
 
26 Dalton. 
 
 bined, occupy the same volume as when free. He quotes 
 Davy, who found 44.05 azote, and 55.95 oxygen by 
 weight, in nitrous gas. He converts these into volumes,, 
 and finds them after the rate of TOO azote to 108.9 oxygen. 
 There is, however, a mistake in this ; if properly reduced, 
 it gives 100 azote to 112 oxygen, taking the specific 
 gravities according to Biot and Arago. But that Davy 
 has overrated the oxygen 12 per cent, he shews by 
 burning potassium in nitrous gas, when 100 measures- 
 afforded just 50 of azote. The degree of purity of the 
 nitrous gas, and the particulars of the experiment, are not 
 mentioned. This one result is to stand against the mean 
 of three experiments of Davy, and may or may not be 
 more correct, as hereafter shall appear. Dr. Henry's 
 analysis of ammonia embraces that of nitrous gas also ; 
 he finds TOO measures of ammonia require 120 of nitrous 
 gas for their saturation. Now this will apply to Gay 
 Lussac's theory in a very direct manner ; for, according 
 to him, ammonia is formed of i measure of azote and 3 
 of hydrogen, condensed into a volume of 2 ; it follows, 
 then, that 100 ammonia require 75 oxygen to saturate 
 the hydrogen; hence, 120 nitrous gas should contain 
 75 ox yg en > or I0 should contain 62.5, instead of 50. 
 Here either the theory of Gay Lussac, or the experience 
 of Dr. Henry, must give results wide of the truth. In 
 regard to ammonia too, it may farther be added, that 
 neither is the rate of azote to hydrogen i to 3, nor is the 
 volume of ammonia doubled by decomposition, according 
 to the experiments of Berthollet, Davy, and Henry, made 
 with the most scrupulous attention to accuracy, to which 
 may be added my own. There is another point of view 
 in which this theory of Gay Lussac is unfortunate, in 
 regard to ammonia and nitrous gas ; i measure of azote 
 with 3 of hydrogen, forms 2 of ammonia ; and i measure 
 of azote with i of oxygen, forms 2 of nitrous gas : now, 
 
New System of Chemical Philosophy. 27 
 
 according to a well established principle in chemistry, 
 i measure of oxygen ought to combine with 3 of hydrogen, 
 or with one half as much, or twice as much ; but no one 
 of these combinations takes place. If Gay Lussac adopt 
 my conclusions, namely, that 100 measures of azote 
 require about 250 hydrogen to form ammonia, and that 
 100 azote require about 120 oxygen to form nitrous gas, 
 he will perceive that the hydrogen of the former would 
 unite to the oxygen of the latter, and form water, leaving 
 no excess of either, further than the unavoidable errors of 
 experiments might produce ; and thus the great chemical 
 law would be preserved. The truth is, I believe, that' 
 gases do not unite in equal or exact measures in any one 
 instance ; when they appear to do so, it is owing to the 
 inaccuracy of our experiments. In no case, perhaps, is 
 there a nearer approach to mathematical exactness, than 
 in that of i measure of oxygen to 2 of hydrogen ; but 
 here, the most exact experiments I have ever made, gave 
 1.97 hydrogen to i oxygen. 
 
28 Avogadro. 
 
 4. ESSAY ON A MANNER OF DETER- 
 MINING THE RELATIVE- MASSES OF 
 THE ELEMENTARY MOLECULES OF 
 BODIES, AND THE PROPORTIONS IN 
 WHICH THEY ENTER INTO THESE 
 COMPOUNDS. BY A. AVOGADRO * 
 
 I. 
 
 MGAY-LUSSAC has shown in an interesting 
 . Memoir (Memoires de la Societe* d'Arcueil, 
 Tome II.) that gases always unite in a very simple pro- 
 portion by volume, and that when the result of the union 
 is a gas, its volume also is very simply related to those of 
 its components. But the quantitative proportions of sub- 
 stances in compounds seem only to depend on the rela- 
 tive number of molecules t which combine, and on the 
 number of composite molecules which result. It must 
 then be admitted that very simple relations also exist 
 between the volumes of gaseous substances and the 
 numbers of simple or compound molecules which form 
 
 * Journal de Physique, LXXIII. (1811), pp. 58-76. 
 
 t [Avogadro has been accused of inconsistency in his use of the 
 term " molecule," but a careful perusal of his paper will show that 
 he uses it with its qualifying adjectives quite consistently, as 
 follows : 
 
 MoUcule (translated " molecule ") without qualification means in 
 modern chemical phraseology either atom or molecttle. 
 
 MoUcule inttgrante (translated "integral molecule") means 
 molecule in general, but is usually applied only to compounds. 
 
 MoUcule constitiiante (translated "constituent molecule") is em- 
 ployed to denote the molecule of an elementary substance. 
 
 Molecule elementaire (translated "elementary molecule ") stands 
 for the atom of an elementary substance.] 
 
Relative Masses of Elementary Molecules. 29 
 
 them. The first hypothesis to present itself in this con- 
 nection, and apparently even the only admissible one, is 
 the supposition that the number of integral molecules in 
 any gases is always the same for equal volumes, or 
 always proportional to the volumes. Indeed, if we were 
 to suppose that the number of molecules contained in a 
 given volume were different for different gases, it would 
 scarcely be possible to conceive that the law regulating 
 the distance of molecules could give in all cases relations 
 so simple as those which the facts just detailed compel 
 us to acknowledge between the volume and the number 
 of molecules. On the other hand, it is very well conceiv- 
 able that the molecules of gases being at such a distance 
 that their mutual attraction cannot be exercised, their 
 varying attraction for caloric may be limited to condens- 
 ing a greater or smaller quantity around them, without 
 the atmosphere formed by this fluid having any greater 
 extent in the one case than in the other, and, conse- 
 quently, without the distance between the molecules vary- 
 ing ; or, in other words, without the number of molecules 
 contained in a given volume being different. Dalton, it 
 is true, has proposed a hypothesis directly opposed to 
 this, namely, that the quantity of caloric is always the 
 same for the molecules of all bodies whatsoever in the 
 gaseous state, and that the greater or less attraction for 
 caloric only results in producing a greater or less con- 
 densation of this quantity around the molecules, and 
 thus varying the distance between the molecules them- 
 selves. But in our present ignorance of the manner in 
 which this attraction of the molecules for caloric is 
 exerted, there is nothing to decide us a priori in favour 
 of the one of these hypotheses rather than the other ; 
 and we should rather be inclined to adopt a neutral 
 hypothesis, which would make the distance between the 
 molecules and the quantities of caloric vary according 
 
 (UNIVERSITY) 
 \'^S 
 
3O Avogadro. 
 
 to unknown laws, were it not that the hypothesis we have 
 just proposed is based on that simplicity of relation 
 between the volumes of gases on combination, which 
 would appear to be otherwise inexplicable. 
 
 Setting out from this hypothesis, it is apparent that we 
 have the means of determining very easily the relative 
 masses of the molecules of substances obtainable in the 
 gaseous state, and the relative number of these molecules 
 in compounds ; for the ratios of the masses of the mole- 
 cules are then the same as those of the densities of the 
 different gases at equal temperature and pressure, and 
 the relative number of molecules in a compound is given 
 at once by the ratio of the volumes of the gases that 
 form it. For example, since the numbers 1.10359 an d 
 0.07321 express the densities of the two gases oxygen 
 and hydrogen compared to that of atmospheric air as 
 unity, and the ratio of the two numbers consequently 
 represents the ratio between the masses of equal volumes 
 of these two gases, it will also represent on our hypo- 
 thesis the ratio of the masses of their molecules. Thus 
 the mass of the molecule of oxygen will be about 15 
 times that of the molecule of hydrogen, or, more exactly, 
 as 15.074 to i. In the same way the mass of the mole- 
 cule of nitrogen will be to that of hydrogen as 0.96913 to 
 0.07321, that is, as 13, or more exactly 13.238, to i. 
 On the other hand, since we know that the ratio of the 
 volumes of hydrogen and oxygen in the formation of 
 water is 2 to i, it follows that water results from the 
 union of each molecule of oxygen with two molecules of 
 hydrogen. Similarly, according to the proportions by 
 volume established by M. Gay-Lussac for the elements 
 of ammonia, nitrous oxide, nitrous gas, and nitric acid, 
 ammonia will result from the union of one molecule of 
 nitrogen with three of hydrogen, nitrous oxide from one 
 molecule of oxygen with two of nitrogen, nitrous gas 
 
Relative Masses of Elementary Molecules. 3 1 
 
 from one molecule of nitrogen with one of oxygen, and 
 nitric acid from one of nitrogen with two of oxygen. 
 
 II. 
 
 There is a consideration which appears at first sight to 
 be opposed to the admission of our hypothesis with respect 
 to compound substances. It seems that a molecule 
 composed of two or more elementary molecules should 
 have its mass equal to the sum of the masses of these 
 molecules ; and that in particular, if in a compound one 
 molecule of one substance unites with two or more 
 molecules of another substance, the number of compound 
 molecules should remain the same as the number of mole- 
 cules of the first substance. Accordingly, on our hypothesis 
 when a gas combines with two or more times its volume 
 of another gas, the resulting compound, if gaseous, must 
 have a volume equal to that of the first of these gases. 
 Now, in general, this is not actually the case. For 
 instance, the volume of water in the gaseous state is, as 
 M. Gay-Lussac has shown, twice as great as the volume 
 of oxygen which enters into it, or, what comes to the 
 same thing, equal to that of the hydrogen instead of 
 being equal to that of the oxygen. But a means of explain- 
 ing facts of this type in conformity with our hypothesis 
 presents itself naturally enough : we suppose, namely, 
 that the constituent molecules of any simple gas what- 
 ever (i.e., the molecules which are at such a distance 
 from each other that they cannot exercise their mutual 
 action) are not formed of a solitary elementary molecule, 
 but are made up of a certain number of these molecules 
 united by attraction to form a single one ; and further, 
 that when molecules of another substance unite with the 
 former to form a compound molecule, the integral mole- 
 cule which should result splits up into two or more parts 
 (or integral molecules) composed of half, quarter, &c., the 
 
32 Avogadro. 
 
 number of elementary molecules going to form the con- 
 stituent molecule of the first substance, combined with 
 half, quarter, &c., the number of constituent molecules 
 of the second substance that ought to enter into com- 
 bination with one constituent molecule of the first sub- 
 stance (or, what comes to the same thing, combined with 
 a number equal to this last of half-molecules, quarter- 
 molecules, &c., of the second substance) ; so that the 
 number of integral molecules of the compound becomes 
 double, quadruple, &c., what it would have been if there 
 had been no splitting-up, and exactly what is necessary to 
 satisfy the volume of the resulting gas.* 
 
 On reviewing the various compound gases most gene- 
 rally known, I only find examples of duplication of the 
 volume relatively to the volume of that one of the consti- 
 tuents which combines with one or more volumes of the 
 other. We have already seen this for water. In the 
 same way, we know that the volume of ammonia gas is 
 twice that of the nitrogen which enters into it. M. Gay- 
 Lussac has also shown that the volume of nitrous oxide 
 is equal to that of the nitrogen which forms part of it, 
 and consequently is twice that of the oxygen. Finally, 
 nitrous gas, which contains equal volumes of nitrogen and 
 oxygen, has a volume equal to the sum of the two con- 
 stituent gases, that is to say, double that of each of them. 
 Thus in all these cases there must be a division of the 
 molecule into two ; but it is possible that in other cases 
 the division might be into four, eight, &c. The possi- 
 bility of this division of compound molecules might have 
 been conjectured a priori ; for otherwise the integral 
 molecules of bodies composed of several substances with 
 a relatively large number of molecules, would come to 
 
 * Thus, for example, the integral molecule of water will be com- 
 posed of a half-molecule of oxygen with one molecule, or, what is 
 the same thing, two half-molecules of hydrogen. 
 
Relative Masses of Elementary Molecules. 33 
 
 have a mass excessive in comparison with the molecules 
 of simple substances. We might therefore imagine that 
 nature had some means of bringing them back to the 
 order of the latter, and the facts have pointed out to us 
 the existence of such means. Besides, there is another 
 consideration which would seem to make us admit in 
 some cases the division in question ; for how could one 
 otherwise conceive a real combination between two 
 gaseous substances uniting in equal volumes without con- 
 densation, such as takes place in the formation of nitrous 
 gas ? Supposing the molecules to remain at such a 
 distance that the mutual attraction of those of each gas 
 could not be exercised, we cannot imagine that a new 
 attraction could take place between the molecules of one 
 gas and those of the other. But on the hypothesis of 
 division of the molecule,* it is easy to see that the com- 
 bination really reduces two different molecules to one, 
 and that there would be contraction by the whole volume 
 of one of the gases if each compound molecule did not 
 split up into two molecules of the same nature. M. Gay- 
 Lussac clearly saw that, according to the facts, the 
 diminution of volume on the combination of gases cannot 
 represent the approximation of their elementary mole- 
 cules. The division of molecules on combination 
 explains to us how these two things may be made inde- 
 pendent of each other. 
 
 III. 
 
 Dalton, on arbitrary suppositions as to the most 
 likely relative number of molecules in compounds, has 
 endeavoured to fix ratios between the masses of the 
 molecules of simple substances. Our hypothesis, supposing 
 it well-founded, puts us in a position to confirm or rectify 
 his results from precise data, and, above all, to assign the 
 magnitude of compound molecules according to the 
 
 c 
 
34 Avogadro. 
 
 volumes of the gaseous compounds, which depend partly 
 on the division of molecules entirely unsuspected by this 
 physicist. 
 
 Thus Dalton supposes* that water is formed by the 
 union of hydrogen and oxygen, molecule to molecule. 
 From this, and from the ratio by weight of the two com- 
 ponents, it would follow that the mass of the molecule of 
 oxygen would be to that of hydrogen as 7^ to i nearly, 
 or, according to Dalton's evaluation, as 6 to i. This 
 ratio on our hypothesis is, as we saw, twice as great, 
 namely, as 15 to i. As for the molecule of water, its 
 mass ought to be roughly expressed by 15 + 2 = 17 (taking 
 for unity that of hydrogen), if there were no division of 
 the molecule into two ; but on account of this division it 
 is reduced to half, 8|, or more exactly 8.537, as may also 
 be found directly by dividing the density of aqueous 
 vapour 0.625 (Gay-Lussac) by the density of hydrogen 
 0.0732. This mass only differs from 7, that assigned to 
 it by Dalton, by the difference in the values for the com- 
 position of water ; so that in this respect Dalton's result 
 is approximately correct from the combination of two 
 compensating errors, the error in the mass of the molecule 
 of oxygen, and his neglect of the division of the molecule. 
 
 Dalton supposes that in nitrous gas the combination 
 of nitrogen and oxygen is molecule to molecule : we have 
 seen on our hypothesis that this is actually the case. 
 Thus Dalton would have found the same molecular 
 mass for nitrogen as we have, always supposing that 
 of hydrogen to be unity, if he had not set out from a 
 different value for that of oxygen, and if he had taken 
 precisely the same value for the quantities of the elements 
 
 * In what follows I shall make use of the exposition of Dalton's 
 ideas given in Thomson's System of Chemistry. [See Alembic Club 
 Reprints, No. 2, p, 42.] 
 
Relative Masses of Elementary Molecules. 35 
 
 in nitrous gas by weight. But by supposing the mole- 
 cule of oxygen to be less than half what we find, he has 
 been obliged to make that of nitrogen also equal to less 
 than half the value we have assigned to it, viz., 5 instead 
 of 13. As regards the molecule of nitrous, gas itself, his 
 neglect of the division of the molecule again makes his 
 result approach ours; he has made it 6 + 5 = 11, whilst 
 
 according to us it is about - = 14, or more exactly 
 15.074+ i3-238 y diyiding 
 
 2 
 
 1.03636, the density of nitrous gas according to Gay- 
 Lussac, by 0.07321. Dalton has likewise fixed in the 
 same manner as the facts have given us, the relative 
 number of molecules in nitrous oxide and in nitric acid, 
 and in the first case the same circumstance has rectified 
 his result for the magnitude of the molecule. He makes 
 it 6 + 2x5 = 1 6, whilst according to our method it should 
 
 be- 
 
 2 
 
 obtained by dividing 1.52092, Gay-Lussac's value for the 
 density of nitrous oxide, by the density of hydrogen. 
 
 In the case of ammonia, Dalton's supposition as to the 
 relative number of molecules in its composition is on our 
 hypothesis entirely at fault. He supposes nitrogen and 
 hydrogen to'be united in it molecule to molecule, whereas 
 we have seen that one molecule of nitrogen unites with 
 three molecules of hydrogen. According to him the 
 molecule of ammonia would be 5 + 1 = 6: according to 
 
 us it should be ^ ^ = 8, or more exactly 8.119, as ma Y 
 
 also be deduced directly from the density of ammonia 
 gas. The division of the molecule, which does not enter 
 into Dalton's calculations, partly corrects in this case also 
 the error which would result from his other suppositions. 
 
 OF THE 
 
 UNIVERSITY 
 
36 Avogadro. 
 
 All the compounds we have just discussed are produced 
 by the union of one molecule of one of the components 
 with one or more molecules of the other. In nitrous 
 acid we have another compound of two of the substances 
 already spoken of, in which the terms of the ratio 
 between the number of molecules both differ from unity. 
 From Gay-Lussac's experiments (Societe d'Arcueil, same 
 volume), it appears that this acid is formed from i part 
 by volume of oxygen and 3 of nitrous gas, or, what comes 
 to the same thing, of 3 parts of nitrogen and 5 of oxygen ; 
 whence it would follow, on our hypothesis, that its mole- 
 cule should be composed of 3 molecules of nitrogen and 
 5 of oxygen, leaving the possibility of division out of 
 account. But this mode of combination can be referred 
 to the preceding simpler forms by considering it as the 
 result of the union of i molecule of oxygen with 3 of 
 nitrous gas, i.e. with 3 molecules, each composed of a 
 half-molecule of oxygen and a half-molecule of nitrogen,, 
 which thus already includes the division of some of the 
 molecules of oxygen which enter into that of nitrous 
 acid. Supposing there to be no other division, the 
 mass of this last molecule would be 57.542, that of 
 hydrogen being taken as unity, and the density of nitrous 
 acid gas would be 4.21267,^6 density of air being taken 
 as unity. But it is probable that there is at least another 
 division into two, and consequently a reduction of the 
 density to half : we must wait until this density has been 
 determined by experiment. 
 
 IV. 
 
 We may now look at a few more compounds, which on 
 our hypothesis can give us at least conjectural informa- 
 tion concerning the relative masses of the molecules and 
 their number in these compounds, and compare our 
 results with the suppositions of Dalton. 
 
Relative Masses of Elementary Molecules. 37 
 
 M. Gay-Lussac has shown that if we assume that dry 
 sulphuric acid is composed of 100 parts of sulphur and 
 138 of oxygen by weight, as the most recent work of 
 chemists has established, and that the density of sul- 
 phurous acid gas is 2.265 referred to air as unity 
 (Kirwan's determination), and if we admit, as the result 
 of Gay-Lussac's experiments, that sulphuric acid is com- 
 posed of two parts by volume of sulphurous acid gas 
 and one of oxygen, then the volume of sulphurous acid 
 is nearly equal to that of the oxygen which entered into 
 it ; and this equality would be exact if the bases on 
 which the calculation rests were the same. If we 
 suppose Kirwan's determination to be exact, throwing 
 the whole error on the analysis of sulphuric acid, we 
 find that in sulphurous acid 100 parts of sulphur take 
 95.02 of oxygen, and consequently in sulphuric acid 
 
 95.02+^ = 142.53, instead of 138. If, on 'the con- 
 trary, we suppose the analysis of sulphuric acid to be 
 exact, it follows that sulphurous acid contains 92 of 
 oxygen for 100 of sulphur, and that its specific gravity 
 should be 2.30314, instead of 2.265. 
 
 One consideration would appear to weigh in favour 
 of the first assumption, until the density of sulphurous 
 acid gas has been confirmed or rectified by. fresh experi- 
 ments, namely, that there must have been in the deter- 
 mination of the composition of sulphuric acid, a source 
 of error tending to increase the quantity of the radical, 
 or, what is the same thing, diminish the quantity of 
 oxygen. The determination was made from the quantity 
 of dry sulphuric acid produced. Now it seems almost 
 certain that ordinary sulphur contains hydrogen ; the 
 weight of this hydrogen, which must have been con- 
 verted into water in the operation, has therefore been 
 added to the true weight of the radical. I shall therefore 
 
38 Avogadro. 
 
 assume sulphurous acid to be composed of 95.02 * of 
 oxygen to 100 of sulphur, or rather of sulphuric radical, 
 instead of 92. t 
 
 In order now to determine the mass of the molecule 
 of the sulphuric radical, it would be necessary to know 
 what proportion by volume this radical in the gaseous 
 state would bear to the oxygen in the formation of sul- 
 phurous acid. The analogy with other combinations 
 already discussed, where there is in general a doubling 
 of the volume or halving of the molecule, leads us to 
 suppose that it is the same in this case also, i.e. that the 
 volume of the sulphur as gas is half that of the sulphurous 
 acid, and consequently also half that of the oxygen with 
 which it combines. On this supposition the density of 
 
 sulphur gas will be to that of oxygen as 100 to ^^' 2 , or 
 
 2 
 
 47.51 ; which gives 2.323 for the density of gaseous 
 sulphur, taking that of air as unity. The masses of the 
 molecules being according to our hypothesis in. the same 
 ratio as the densities of the gases to which they belong, 
 the mass of the molecule of the sulphuric radical will be 
 to that of hydrogen as 2.323 to 0.07321, or as 31.73 to i. 
 One of these molecules combined, as we have said, with 
 two of oxygen, will form sulphurous acid (division of the 
 molecule being left out of account), and combined with 
 yet another molecule of oxygen, will form sulphuric acid. 
 Accordingly, sulphurous acid should be analogous to 
 
 * [Erroneously 92.02 in the original.] 
 
 f This was written before I had seen the Memoir of Davy on 
 oxymuriatic acid, which also contains new experiments on sulphur 
 and phosphorus. In it he determines the density of sulphurous 
 acid gas, and finds it to be only 2.0967, which gives new force to 
 the above considerations. If we adopt this density, we find that in 
 sulphurous acid 100 parts by weight of sulphur take in of oxygen, 
 and in sulphuric acid 167 instead of 138 ; but perhaps this density 
 of sulphurous acid, according to Davy, is somewhat too low. 
 
Relative Masses of Elementary Molecules. 39 
 
 nitric acid, with regard to the relative number of 
 molecules of its constituents, sulphuric acid having no 
 analogue amongst the nitrogen compounds. The mole- 
 cule of sulphurous acid, having regard to division, will 
 be equal to 31-73 + 2x15.074 Qr ^ ^ ^ wQuld algo 
 
 be obtained directly by dividing the density 2.265 f 
 sulphurous acid gas by that of hydrogen gas. As for the 
 molecule of sulphuric acid, it cannot be determined, for 
 we do not know whether there is further division of the 
 molecule on its formation, or not* 
 
 * Davy in the Memoir alluded to, has made the same supposi- 
 tions as to the relative number of molecules of oxygen and radical in 
 sulphurous and sulphuric acids. From his determination of the 
 density of sulphurous acid gas, the density of the sulphuric radical 
 would be 1.9862, and its molecule 27.13, that of hydrogen being 
 taken as unity. Davy, by a similar calculation, fixes it at about 
 half, viz., 13.7, because he supposes the molecule of oxygen to be 
 equal to about half our molecule, using Dalton's hypothesis with 
 respect to water. 
 
 He finds nearly the same mass, viz., 13.4, by taking as his start- 
 ing-point the density of sulphuretted hydrogen, which his experi- 
 ments make equal to 1.0645, a result only slightly different from 
 Kirwan's, and by assuming that this gas (which contains, as we 
 know, its own volume of hydrogen combined with sulphur) is com- 
 posed of one molecule of sulphur and one of hydrogen. As we 
 suppose the molecule of sulphur to be nearly twice as great, we 
 must assume that this gas is the product of the union of one mole- 
 cule of the radical with two at least of hydrogen, and that its 
 volume is twice that of the gaseous radical, as in so many other 
 cases. I say at least with two molecules of hydrogen, for if there 
 were hydrogen already in ordinary sulphur, as known experiments 
 on this substance indicate, its quantity also must be added. If, for 
 instance, ordinary sulphur were composed of one molecule of sul- 
 phuric radical and one of hydrogen, sulphuretted hydrogen would be 
 composed of three molecules of hydrogen and one of radical. This 
 could be decided by the comparison of the specific gravities of 
 sulphuretted hydrogen and sulphurous acid gas, if both were known 
 exactly. For example, supposing Davy's determination for sul- 
 
4O Avogadro. 
 
 Dalton had supposed that sulphuric acid was com- 
 posed of two molecules of oxygen to one of radical, and 
 sulphurous acid of one molecule of oxygen to one of 
 sulphur. These two assumptions are incompatible, for 
 according to Gay-Lussac's results the quantities of 
 oxygen in these two acids for a given quantity of radical, 
 are represented by i and ij. Besides, in his determina- 
 tion of the molecule he set out from a wrong value for 
 the composition of sulphuric acid, and it is only by 
 chance that the mass 15 which he assigns to it, bears to 
 his value for the mass of the oxygen molecule a ratio which 
 approaches that presented by these two substances on 
 our hypothesis. 
 
 Phosphorus has so much analogy with sulphur that we 
 might apparently assume that phosphoric acid also is 
 composed of three molecules of oxygen to one of 
 radical, and phosphorous acid of only two of oxygen to 
 one of radical. On this assumption we may calculate 
 approximately the mass of the molecule of the phosphoric 
 radical. Rose found by a method analogous to that 
 which had been employed for sulphuric acid, that phos- 
 phoric acid contains about 115 parts by weight of 
 oxygen to 100 of phosphorus. There ought to be a 
 little more oxygen in it if we suppose that phosphorus, 
 like sulphur, contains hydrogen. As an approximation 
 
 phuretted hydrogen to be exact, the molecule of the sulphuric 
 radical, on the supposition of only two molecules of hydrogen, would 
 be 27.08, that of hydrogen being taken as unity ; but on the sup- 
 position of three molecules of hydrogen, 27.08 would still be the 
 sum of one molecule of radical and one of hydrogen, so that the 
 former would be reduced to 26.08. If the exact density of sul- 
 phurous acid gas confirmed one or other of these results, it would 
 confirm by that means one or other of these hypotheses : but we 
 are not sufficiently agreed about these densities to be able to draw 
 any conclusion in this respect from the determinations hitherto 
 existing. 
 
Relative Masses of Elementary Molecules. 41 
 
 we can make this increase in the same proportion as we 
 have seen holds good for sulphuric acid in accordance 
 with the specific gravity of sulphurous acid, and thus 
 bring the quantity of oxygen up to 120. We then find 
 from our hypotheses that the mass of the molecule of 
 the phosphoric radical is about 38, that of hydrogen 
 being taken as unity. Dalton also has adopted for phos- 
 phorous and phosphoric acids, hypotheses analogous to 
 those he had made for sulphurous and sulphuric acids ; 
 but since he used different values for the elements of 
 these acids by weight, he arrived at a determination of 
 the molecule of phosphorus, which does not bear the 
 same ratio to his determination of the molecule of 
 sulphur as ought to exist, according to us, between these 
 molecules : he has fixed that of phosphorus as 8, 
 hydrogen being unity.* 
 
 Let us now see what conjecture we may form as to the 
 mass of the molecule of a substance which plays in 
 nature a far greater part than sulphur or phosphorus, 
 namely, that of carbon. As it is certain that the 
 volume of carbonic acid is equal to that of the oxygen 
 which enters into it, then, if we admit that the volume 
 of carbon, supposed gaseous, which forms the other 
 element, is doubled by the division of its molecules into 
 two, as in several combinations of that sort, it will be 
 necessary to suppose that this volume is the half of that 
 of the oxygen With which it combines, and that con- 
 
 * Davy has adopted the same suppositions as ourselves for the 
 number of molecules of oxygen and radical in phosphorous and 
 phosphoric acids ; and by still taking the molecule of oxygen nearly 
 half ours, he finds 16.5 for the molecule of phosphorus, which would 
 .give about 33 on our evaluation of the molecule of oxygen, instead 
 of 38. The difference arises from Davy having taken from his own 
 experiments 34 parts to 25, i.e. 136 to 100 of phosphorus, as the 
 quantity of oxygen in phosphoric acid, instead of 120 as we have 
 assumed. Further experiments will clear up this point. 
 
 /<&&> LIBM^. 
 
 f P OF THE ^ \ 
 
 UNIVERSITY) 
 
 .. ^/ 
 
42 Avogadro. 
 
 sequently carbonic acid results from the union of one 
 molecule of carbon and two of oxygen, and is therefore 
 analogous to sulphurous and phosphorous acids, accord- 
 ing to the preceding suppositions. In this case we find 
 from the proportion by weight between the oxygen and 
 the carbon, that the density of carbon as gas would be 
 0.832 with respect to that of air as unity, and the mass 
 of its molecule 11.36 with respect to hydrogen. There 
 is, however, one difficulty in this supposition, for we give 
 to the molecule of carbon a mass less than that of 
 nitrogen and oxygen, whereas one would be inclined to 
 attribute the solidity of its aggregation at the highest 
 temperatures to a higher molecular mass, as is observed 
 in the case of the sulphuric and phosphoric radicals. 
 We might avoid this difficulty by assuming a division of 
 the molecule into four, or even into eight, on the forma- 
 tion of carbonic acid ; for in that way we should have 
 the molecule of carbon twice or four times as great as 
 that we have just fixed. But such a composition would 
 not be analogous to that of the other acids ; and, be- 
 sides, according to other known examples, the assump- 
 tion or not of the gaseous state does not appear to depend 
 solely on the magnitude of the molecule, but also on 
 some other unknown property of substances. Thus we 
 see sulphurous acid in the form of a gas at the ordinary 
 temperature and pressure of the atmosphere notwith- 
 standing its large molecule, which is almost equal to 
 that of the solid sulphuric radical. Oxygenated muriatic 
 acid gas has a density, and consequently a molecular 
 mass, still more considerable. Mercury, which as we 
 shall see further on, should have an extremely large 
 molecule, is nevertheless gaseous at a temperature 
 infinitely lower than would be necessary to vaporise iron, 
 the molecule of which is smaller. Thus there is nothing 
 to prevent us from regarding carbonic acid to be com- 
 
Relative Masses of Elementary Molecules. 43 
 
 posed in the manner indicated above, and therefore 
 analogous to nitric, sulphuric, and phosphoric acids, 
 and the molecule of carbon to have a mass expressed by 
 11.36. 
 
 Dalton has made the same supposition as we have 
 done regarding the composition of carbonic acid, and 
 has consequently been led to attribute to carbon a mole- 
 cule equal to 4.4, which is almost in the same ratio to his 
 value for that of oxygen as 11.36 is to 15, the mass of 
 the molecule of oxygen according to us. 
 
 Assuming the values indicated for the mass of the 
 molecule of carbon and the density of its gas, carbonic 
 oxide will be formed, according to the experiments of M. 
 Gay-Lussac, of equal parts by volume of carbon gas and 
 oxygen gas ; and its volume will be equal to the sum of 
 the volumes of its constituents : it will accordingly be 
 formed of carbon and oxygen united molecule to mole- 
 cule, with subsequent halving all in perfect analogy to 
 nitrous gas. 
 
 The mass of the molecule of carbonic acid will be 
 ii. 36 + 2 x 15.074 1.5*96 
 
 and that of carbonic oxide will be 
 
 11.36+15.074 0.96782 
 
 - = 1 3.22 = - 
 
 2 0.07321 
 
 V. 
 
 Amongst the simple non-metallic substances there is 
 still one of which we have to speak. This substance, 
 being naturally gaseous, can leave no doubt, on our 
 principles, as to the mass of its molecule ; but the latest 
 experiments of Davy upon it, and even the earlier experi- 
 ments of MM. Gay-Lussac and Thenard, force us to 
 depart from ideas hitherto received, although the last- 
 
44 A vogadro. 
 
 named chemists have attempted to explain them in accord- 
 ance with these ideas. This is the substance hitherto 
 known by the name of oxygenated muriatic acid, or 
 oxymuriatic acid. In the present state of our knowledge 
 we must now regard this substance as still undecom- 
 posed, and muriatic acid as a compound of it with 
 hydrogen. It is in accordance with this theory, there- 
 fore, that we shall apply our principles regarding com- 
 binations to these two substances. 
 
 The density of oxymuriatic acid according to MM. 
 Gay-Lussac and Thenard is 2.470 referred to atmospheric 
 air as unity ; this gives for its molecule referred to that of 
 hydrogen as unity, 33.74, adopting the density of hydrogen 
 determined by MM. Biot and Arago. According to 
 Davy 100 English cubic inches of oxymuriatic gas 
 weigh 74.5 grains, and an equal volume of hydrogen gas 
 2.27. This would give for the molecule of the former 
 
 74-5 
 
 =32.82. These two estimates differ very little from 
 
 the mass that Davy himself, from other considerations, 
 assigns to this substance, viz., 32.9. It follows from the 
 experiments both of Gay-Lussac and Thenard, and of 
 Davy, that muriatic acid gas is formed by the combina- 
 tion of equal volumes of oxymuriatic gas and hydrogen, 
 and that its volume is equal to their sum. This means, 
 according to our hypothesis, that muriatic acid is formed 
 of these two substances united molecule to molecule, 
 with halving of the molecule, of which we have already 
 had so many examples. Accordingly the density of 
 muriatic acid gas, calculating from that given above for 
 oxymuriatic gas, should be 1.272; it is 1.278 according 
 to the experiments of MM. Biot and Gay-Lussac. If we 
 suppose this last determination to be exact, the density 
 of oxymuriatic gas should be 2.483, and the mass of its 
 molecule 33.91. Should we prefer to adopt this value, 
 
Relative Masses of Elementary Molecules. 4 5 
 
 the mass of the molecule of muriatic acid will be 
 I I>27 The determination of the 
 
 2 0.07321 
 
 specific gravity of the muriatic acid gas by Davy, accord- 
 ing to which 100 cubic inches of that gas weigh 39 
 grains, would give numbers only slightly different, viz., 
 33.36 for the mass of the molecule of oxymuriatic acid, 
 and 17.18 for that of muriatic acid. 
 
 VI. 
 
 Let us now apply our hypothesis to some metallic sub- 
 stances. M. Gay-Lussac assumes that mercurous oxide, 
 in the formation of which 100 parts by weight of mercury 
 absorb 4.16 of oxygen, according to Fourcroy and 
 Thenard, is analogous to nitrous oxide, /.*., that the 
 mercury, supposed gaseous, is combined in it with half its 
 volume of oxygen gas, which on our hypothesis is to 
 say that one molecule of oxygen combines with two 
 molecules of mercury. Supposing this to be the case, 
 the density of mercury gas ought to be to that of oxygen 
 as 100 to 8.32, which would give 13.25 as its density 
 taking that of air as unity, and for the mass of the 
 molecule of mercury 1 8 1 taking as unity that of hydrogen. 
 On this supposition mercuric oxide, which contains twice 
 as much oxygen, should be formed of mercury and 
 oxygen united molecule to molecule ; but some reasons 
 lead me to think that it is mercurous oxide which 
 represents this last case, and that in mercuric oxide one 
 molecule of mercury combines with two of oxygen. 
 Then the density of mercury gas, and the mass of its 
 molecule, would be double what they are on the preced- 
 ing hypothesis, viz., 26 \ for the first, and 362 for the 
 second. In this assumption I am supported by analogies 
 drawn from other metals, and particularly from iron. It 
 follows from the experiments of different chemists, care- 
 
46 A vogadro. 
 
 fully discussed by Hassenfratz, that the two best known 
 oxides of iron, the black oxide and the red oxide, are 
 composed respectively of 31.8 and 45 parts by weight of 
 oxygen to 100 of iron. We see that the second of these 
 two quantities of oxygen is nearly half as great again as 
 the first, so that we are naturally led to suppose that in 
 the first oxide one molecule of iron combines with two 
 molecules of oxygen, and in the second with three. If 
 that is so, and if we admit the proportion for the black 
 oxide to be the more exact, the proportion for the red 
 oxide would be 47.7 for 100 of iron, which comes 
 very near the proportion found directly by Proust, viz., 
 48. The mass of a molecule of iron will therefore be to 
 the mass of a molecule of oxygen as TOO to 15.9, which 
 gives about 94 with regard to hydrogen as unity. It 
 would appear from this that there should be another 
 oxide of iron which would contain 15.9 of oxygen to 100 
 of iron, and this is perhaps the white oxide, although the 
 experiments hitherto performed point to this substance 
 containing a greater proportion of oxygen. Now the two 
 oxides of mercury of which we have spoken, one of which 
 contains twice as much oxygen as the other, should 
 apparently be analogous to this last oxide of iron and to 
 the black oxide, the red oxide having no analogue in the 
 case of mercury. 
 
 In the same way the other metals present for the most 
 part two oxides in which the quantities of oxygen are as 
 i to 2, so that from the proportions of their elements by 
 weight, we may determine in the same manner the mass 
 of their molecules. I find for example, 206 for the 
 molecule of lead, 198 for that of silver, 123 for copper, 
 etc.* 
 
 * I shall here add a few words regarding the molecule of 
 potassium. Davy, assuming that potash is formed from potassium 
 and oxygen united molecule to molecule, has fixed the mass of the 
 
Relative Masses of Elementary Molecules. 47 
 
 VII. 
 
 We shall now make a few applications of our principles 
 to saline compounds, which will furnish us with the 
 opportunity of examining an important point in the 
 theory of these compounds. M. Gay-Lussac has shown 
 that the neutral carbonate, fluoborate, and muriate of 
 ammonia are composed of equal volumes of ammonia 
 gas and of the respective acids. Let us pause to consider 
 the carbonate. On our hypothesis, this salt is composed 
 of one molecule of carbonic acid with one molecule of 
 ammonia i.e. (according to the values previously given 
 and independently of any division), of one molecule of 
 
 molecule of potassium at 40.5, in accordance with the quantity of 
 oxygen by weight in the substance, and has taken the molecule of 
 oxygen to be 7.5. Assuming, as we have done, this last molecule 
 to be nearly twice as great, the molecule of potassium will also be 
 doubled, viz., about 81, if we adopt the other assumptions of Davy. 
 But it may be that in potash one molecule of potassium takes two of 
 oxygen, in which case we should again have to double the last 
 value and make it 162. It might also be (for the analogy drawn 
 from other metals is not in this case a very safe guide) that two 
 molecules of potassium combine with one of oxygen, which would 
 bring back the molecule of potassium to 40.5. 
 
 It is on the assumption of this last value for the molecule of 
 potassium that Davy finds 32.9 for that of oxymuriatic acid, calculat- 
 ing from the composition of muriate of potash, and assuming that 
 this salt is formed of one molecule of potassium with one of acid. If 
 we suppose the molecule of potassium to have a different mass, we 
 must admit another relative number of molecules in the muriate, 
 since both from our hypothesis and from the density of the gas, 
 32.9 is very nearly the molecule of oxymuriatic acid. On the sup- 
 position that the molecule of potassium is 81, and that in sulphide 
 of potassium the combination is molecule to molecule, the molecule 
 of sulphur will be about 27 instead of 13^ as found by Davy on the 
 latter assumption, which will bring about between this result and 
 that derived from sulphurous acid according to our calculations the 
 agreement which exists between Davy's 
 
 OF THE 
 
 UNIVERSITY 
 
48 A vogadro. 
 
 carbon, two of oxygen, one of nitrogen, and three of 
 hydrogen, which would give 57.75 for the mass of its 
 molecule ; but admitting the division into two which had 
 already taken place in the components, this molecule is 
 reduced to 28.87. It would be brought down again to 
 half this number, if there were another division on the 
 union of the acid with the alkali. 
 
 M. Gay-Lussac has suspected that the equality of 
 volume between a gaseous alkali and acid, which by their 
 union form a neutral salt, may be general. That is as 
 much as to say, on our hypothesis, that neutral salts are 
 composed of acid and alkali united molecule to mole- 
 cule ; but certain considerations appear to be opposed to 
 the admission of this principle in all its generality. The 
 idea of acidity, alkalinity, and neutrality, which still seems 
 to me the most conformable to the phenomena, is that 
 which I have given in my Memoir on this subject 
 (Journal de Physique, tome Ixix.). According to it, all 
 substances form amongst themselves a series, in which 
 they play the part of acid or alkali with respect to one 
 another; and this series is the same as that on which 
 depends the positive or negative electricity they develop 
 on mutual contact. I express by the term oxygenicity the 
 property in virtue of which substances are ranked in this 
 scale, placing first those which play the part of an acid 
 with respect to the others. In this scale there is a point 
 about which are placed the substances we term neutral, 
 above it are those which are absolutely acid, below it are 
 those which are alkaline, when their state of aggregation 
 permits them to exhibit these qualities. Lastly, com- 
 posite substances occupy in this scale a place intermediate 
 between those of which they are composed, having regard 
 to the degree of oxygenicity and to the proportion by 
 weight of these constituent substances ; so that a neutral 
 substance results from the combination of two substances, 
 
Relative Masses of Elementary Molecules. 49 
 
 one acid, the other alkaline, in a certain proportion (see 
 the Memoir referred to).* The recognition of the simple 
 ratios observed on combination, and in particular in 
 cases where neutral substances are the result, leads us 
 now to a more exact manner of conceiving the state of 
 neutrality. The oxygenicity in two bodies which com- 
 bine, cannot be supposed to have such a relation to the 
 masses of their molecules, that from the union of certain 
 definite numbers of these molecules there should result a 
 certain definite degree of oxygenicity which would be 
 that of neutrality, and would only depend, as we have 
 already assumed for oxygenicity in general, on the pro- 
 portion by weight and the degree of oxygenicity of the 
 components. It appears, then, that we must admit that 
 the degree of oxygenicity which corresponds to neutrality 
 is not quite fixed, although approximating more or less to 
 a fixed limit, and that this state depends on the excess of 
 mass of one of the components (from which the acid or 
 alkaline quality might result) being prevented from exer- 
 cising these qualities by the simple combination with the 
 contrary principle which retains it by its attraction, 
 although the compound otherwise might have a state of 
 aggregation permitting it to act as an acid or an alkali, if 
 it were endowed with these qualities. The excess of 
 mass thus held back is that which is necessary to com- 
 plete a certain simple relation between the number of 
 combining molecules. Thus amongst the different simple 
 ratios in which molecules can combine, there is one 
 which gives neutrality ; that, namely, which gives the 
 
 * The properties of oxymuriatic acid, as Davy conceives them, 
 being analogous to those of oxygen, are not at all extraordinary from 
 this point of view ; they simply show that this substance is very 
 oxygenic. I had already remarked in my Memoir that the pro- 
 perties of the alkalies, supposed to be oxides, are easily explained 
 according to these ideas. 
 
 D 
 
50 Avogadro. 
 
 compound approximating most closely to the definite 
 point of oxygenicity mentioned above, so that if in the 
 compound formed according to this ratio, one of the 
 component principles let one molecule of the other 
 escape, or took up one in addition, the compound would 
 diverge further from this precise point, about which there 
 oscillate, as it were, the oxygenicities of the various 
 neutral compounds ; and it is this point which would 
 give the neutral state in the combination of two sub- 
 stances which could combine in all proportions, or in 
 ratios expressible by any number of molecules whatever. 
 It is evident that this way of regarding the neutrality of 
 compound substances reconciles the theory given in the 
 Memoir quoted with the ideas put forward by M. de 
 Laplace on this point, and expounded by M. Haiiy in 
 his Traite* de Physique. 
 
 According to this theory it is evident that if the 
 oxygenicity of two acids and two alkalies which combine 
 respectively in pairs, is not extremely different, and if at 
 the same time the mass of the molecule of one of the 
 acids is not in a different ratio to its alkali from that of 
 the other acid with regard to its own alkali, the ratio 
 between the numbers of molecules which gives neutrality 
 may be the same in both compounds ; but in the contrary 
 case, the ratio may vary in such a way that instead of the 
 equality of volumes, or of combination molecule to mole- 
 cule which we see between carbonic and a few other acids 
 on the one hand and ammonia on the other, there may 
 be other simple ratios such as i to 2, &c., which give the 
 neutral state. Nevertheless, the simplicity which will 
 always exist amongst these ratios, in conjunction with the 
 information we may obtain from other sources as to the 
 mass of the molecules and the degree of oxygenicity of 
 the components, will sometimes put us in a position to 
 determine, or at least conjecture, what are the simple 
 
Relative Masses of Elementary Molecules. 5 1 
 
 ratios which may occur in a given case ; but it is the task 
 of experiment to confirm or correct these theoretical 
 estimates. 
 
 VIII. 
 
 It will have been in general remarked on reading this 
 Memoir that there are many points of agreement between 
 our special results and those of Dalton, although we set 
 out from a general principle, and Dalton has only been 
 guided by considerations of detail. This agreement is an 
 argument in favour of our hypothesis, which is at bottom 
 merely Dalton's system furnished with a new means of 
 precision from the connection we have found between it 
 and the general fact established by M. Gay-Lussac. 
 Dalton's system supposes that compounds are made in 
 general in fixed proportions, and this is what experiment 
 shows with regard to the more stable compounds and 
 those most interesting to the chemist. It would appear 
 that it is only combinations of this sort that can take place 
 amongst gases, on account of the enormous size of the 
 molecules which would result from ratios expressed by 
 larger numbers, in spite of the division of the molecules, 
 which is in all probability confined within narrow limits. 
 We perceive that the close packing of the molecules in 
 solids and liquids, which only leaves between the integral 
 molecules distances of the same order as those between 
 the elementary molecules, can give rise to more compli- 
 cated ratios, and even to combinations in all proportions : 
 but these compounds will be so to speak of a different 
 type from those with which we have been concerned, 
 and this distinction may serve to reconcile M. Berthollet's 
 ideas as to compounds with the theory of fixed propor- 
 tions. 
 
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