Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/821

D A L T N 785 yellow, making what I should call different shades of yellow." On March 1, 1799, Dalton read to the Manchester Society a paper on rain and dew, and the origin of springs, which was subsequently followed by various disquisitions on heat, the colour of the sky, steam, the auxiliary verbs and participles of the English language, and the reflectibility and re- frangibility of light. In May 1800 he was elected to the secretaryship of the society, an office which he held until 1808, when he became vice-president in the place of Dr Roget. In 1817 he became president, and remained so till the time of his death. On July 31, 1801, was read the first of four important essays by Dalton, " On the Constitution of Mixed Gases ;" " On the Force of Steam or Vapour from Water and other Liquids in different Temperatures, both in a Torricellian Vacuum and in Air:" "On Evaporation;" and " On the Expansion of Gases by Heat." In the second of these he makes the striking remark, "There can scarcely be a doubt entertained respecting the reducibility of all elastic fluids, of whatever kind, into liquids ; and we ought not to despair of effecting it in low temperatures, and by strong pressures exerted upon the unmixsd gases ; " further, he describes experiments to ascertain the tension of aqueous vapour at different points between 32 and 212 Fahr., and concludes, from observations on the behaviour of the vapour of six different fluids, " that the variation of the force of vapour from all liquids is the same for the same variation of temperature, reckoning from vapour of any given force." In the fourth essay he observes " I see no sufficient reason why we may not conclude that all elastic fluids under the same pressure expand equally by heat, and that for any given expansion of mercury, the corresponding expansion of air is proportionally something less, the higher ths temperature It seems, therefore, that general laws respecting the absolute quantity and the nature of heat are more likely to be derived from elastic fluids than from other substances." Dalton thus both enunciated the law of the expansion of gases, stated six months later by Gay-Lussac, and indicated the future employment of the air-thermometer. But the most important of Dalton s numerous investi gations are those concerned with the atomic theory of chemistry. The subject of chemisty seems to have first occupied his attention about the year 1796. In 1802 he had already arrived at some conception of the law of the multiple combining proportions of the ele ments, which was afterwards developed by him. Thus, in a paper " On the Proportion of the Several Gases or Elastic Fluids constituting the Atmosphere," read on the 29th of October in that year, he says though, as it happened, his conclusions were based upon the incorrect supposition that the size of the vessels he employed affected the nature of the chemical union of the gases they con tained " The elements of oxygen may combine with a certain portion of nitrous gas, or with twice that portion, but with no intermediate quantity. In the former case nitric acid is the result, in the latter nitrous acid." Dr Thomson states (History of Chemistry, vol. ii.) " Mr Dalton informed me that the atomic theory first occurred to him during his investigations of olefiant gas and carburetted hydrogen gas." In 1850, however, in a notice of Wolkston, read before the Glasgow Philosophical Society, he remarks, " Mr Dalton founded his theory on the analysis of two gases, namely, protoxide and deutoxide of azote The first of these he considered as a com pound of one atom of azote with one atom of oxygen, and the second of one atom of azote united with two atoms of oxygen." Inasmuch as from the recognition of the law of definite and multiple combining proportions of the elements originated the establishment of that of their relative, Dalton may be said to have received assistance in the foundation of his atomic theory from the researches here alluded to by Thomson ; but the latter part of the statement is manifestly erroneous, for the two gases referred to were invariably represented by Dalton as compounds respectively of two atoms and one atom of azote (nitrogen) with a single atom of oxygen. It is doubtless the earlier of Thomson s observa tions that is to be regarded as correct, more especially as Dalton himself says, in 1810, in his Few System of Chemical Philosophy, with respect to carburetted hydrogen, " No correct notion of the constitution of the gas about to be described seems to have been formed till the atomic theory was introduced and applied in the investigation. It was in the summer of 1804 that I collected, at various times and in various places, the inflammable gas obtained from ponds." As a matter of fact, the first germs of the atomic theory were Dalton s views of the separate existence of aqueous vapour in the atmosphere, which necessitated the assumption that gases were constituted of independent atoms ; indeed they are represented as such, each atom having its distinguishing symbol, in the plate accompany ing the paper " On the Constitution of Mixed Gases." Dalton appears already in 1803 to have pictured to himself the form of atoms, for in a paper " On the Absorption of Gases by Water " we read " A particle of gas pressing on the surface of water is analogous to a single shot press ing on a square pile of them ; " and five years later, he writes in his New System, "Whatever, therefore, may be the shape or figure of the solid atom abstractedly, when surrounded by such an atmosphere [of heat] it must be globular ; but as all the globules in any small given volume are subject to the same pressure, they must be equal in bulk, and will, therefore, be arranged in horizontal strata like a pile of shot." At the end of the paper on " Absorption " just alluded to, Dalton gives the following first table of the relative weights of the ultimate particles of gaseous and other bodies, which was constructed, he tells us, in order to test whether the solubility of gases in water was dependent upon the weight of their particles : Hydrogen 1 Azot 4-2 Carbone 4 - 3 Ammonia 5 2 Oxygen 5 - 5 Water 6 5 Phosphorus 7 2 Phosphuretted hydrogen. 8 2 Nitrous gas 1 9 3 Ether 9 6 Gaseous oxide of carbone. 9 8 Nitrous oxide 1 1 3 7 Sulphur 14-4 Nitric acid 15 2 Sulphuretted hydrogen.. 16 4 Carbonic acid ln 3 Alcohol 15-1 Sulphureous aeid 19 9 Sulphuric acid 25 4 Carburetted hydrogen from stagnant water... 6 3 Olefiant gas 5 3 As this table contains the results of the analyses of olefiant gas and carburetted hydrogen made in the summer of 1804, it must have been completed after that date, and possibly was not added to the paper containing it till shortly before the printing of the latter in November 1805. It was in 1803, as we are informed in the preface to the Neiv System, that Dalton " was gradually led to those primary laws which seem to obtain in regard to heat and to chemical combination;" and in a letter to his brother in that year he writes that he has been fully engaged in all his leisure hours in the pursuit of chemical and philo sophical inquiries, having got into a track that has not been much trod in before." Dr Bryan Higgins, in a little pamphlet composed about the year 1775, had treated of " atoms " which united with one another ; but the fixity of the constitution of chemical substances had apparently formed no part of his ideas. " The matter of fire," accord ing to him, " limits the quantity in which aeriform fluids, and bodies containing it, can combine chemically/ and it ~i~The figures for nitrous gas (nitric oxide) and nitrous oxi.de should have beenV? and 13 "9, i.e., 5 5 -I- 4 "2 and 5 5 + 4 2x2.

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