CHAP. VI., 3.] 



HEAT (ATOMIC CHEMISTRY). DALTON. 



137 



(615.) 



iwe i of 



(616.) 

 alton's 

 .echamcal 



thin glass with cold spring water, or by adding, if 

 necessary, the solid nitrate of ammonia, until the 

 temperature fell so far that dew began to be deposited 

 on the surface. This excellent method constitutes 

 in fact the dew-point hygrometer. It has received 

 various forms from Mr Daniell and others, but the 

 original one is probably the best. 



These important laws and deductions being fully 

 understood, the theory of the mixed dry gases follows 

 as a simple corollary. For it merely asserts of them 

 what has been admitted in the case of steam, that 

 they may diffuse themselves, and exert their elastic 

 forces quite independently of one another; so that, for 

 example, in our atmosphere the total pressure is made 

 up of the partial elasticities of the oxygen, nitrogen, 

 and carbonic acid which compose it, each acting to 

 the same amount as if it alone had existed in the 

 space which it occupies. In his essay of 1801, Dai- 

 ton stated his view of these facts thus : " The par- 

 ticles of one elastic fluid may possess no repulsion or 

 attractive power, or be perfectly inelastic with regard 

 to the particles of another." 



Discoveries so practical could not fail to excite 

 immediate attention, especially at a time when the 

 researches of chemists were earnestly directed towards 

 ^ ie g ases - " The facts and experiments," Dalton 

 tells us in his Chemical Philosophy, " were highly 

 valued ; some of the latter were repeated and found 

 correct, and none of the results controverted ; but 

 the theory was almost universally misunderstood." 

 Its opponents were Berthollet, Thomson, Henry, and 

 others, and the replies of the author are contained in 

 the work just cited. But he appears to have felt the 

 force of the objection, " Can it be conceived that an 

 elastic substance exists which adds its volume to that 

 of another, and which, nevertheless, does not act on 

 it by its expansive force V for in his chemical phi- 

 losophy he abandons the comparison of gaseous par- 

 ticles to similar magnetic poles which repel each 

 other, but are inert towards non-magnetic matter, and 

 allowing that heat is the primary cause of repulsion 

 in all gases, ascribes their diffusion contrary to gravity 

 to the dissimilar size of their spherical molecules. 

 " The particles of one kind being from their size un- 

 able to apply properly to the other, no equilibrium 

 can ever take place amongst the heterogeneous mole- 

 cules. The intestine motion," he adds, " must there- 

 fore continue till the particles arrive at the opposite 

 surface of the vessel against any point of which they 

 can rest with stability, and the equilibrium at length 

 is acquired, when each gas is uniformly diffused 

 through the other." It may be seriously doubted 

 whether this theory of the facts will bear examina- 

 tion, at least no attempt has been made to demon- 

 strate it on mechanical principles. The subject has 

 been allowed to remain during more than forty years of 

 unequalled activity in such speculations without ma- 

 terial light .being thrown upon the proximate causes 



of these wonderfully general and simple truths. Yet 

 it can hardly be doubted that the mechanical theory 

 of the gases and vapours is capable of a great exten- 

 sion, and even of being illustrated by simple experi- 

 ments. But the attention of chemists has been with- 

 drawn from the physical bearings of their science by 

 the prodigious increase in the number of compounds 

 which they have had to analyze and classify. The 

 most important sequel to Dalton' s discoveries has 

 probably been that of Professor Graham, whose ex- Mr Gra- 

 periments prove that gases when separated by a ham ' slawr 

 porous partition permeate it in both directions, until . f dlffu " 

 they have mixed in proportions which are inversely 

 as the square root of their density. This law clearly 

 shows the purely mechanical causes by which the 

 diffusion is effected. 



A discovery of Dalton, which has scarcely been (617.) 

 considered second in importance to those we have Dalton on 

 mentioned, is that the rate of expansion of all gases gio^of the 

 by heat is the same. Thus thermometers of air, hydro- gases by 

 gen, and carbonic acid would all mark the same degree heat - 

 when plunged in the same medium ; whilst the mer- 

 curial thermometer shows a more rapid expansion, at 

 higher temperatures, if the air thermometer be taken 

 as the standard. This important fact was soon 

 after but independently announced by Gay-Lussac of 

 Paris. Dalton's publication dates from 1801. In 

 his Chemical Philosophy, he gives his view of the 

 difficult subject of a true thermometric scale ; which 

 he does not suppose to be correctly represented by 

 the simple expansion of any known substance ; but 

 that the gases expand with true increments of heat 

 in a very slow geometrical progression, whilst li- 

 quids expand as the squares of the true temperatures 

 from their freezing points. These and other laws 

 equally arbitrary have not received support from 

 later and more precise experiments ; and in the se- 

 cond volume of his work (1827) he freely acknow- 

 ledges the correction of his hypothesis by the more 

 recent French experiments. 



II. I now proceed to the other part of Dalton's ( 618 ;) 

 labours on which his reputation is principally based ; Atomic 

 namely, the clear assertion and experimental esta- Theory. 

 blishment of the Atomic Theory or doctrine of .chemi- 

 cal equivalents. I introduce it here on account of 

 its important bearing on all physical questions in 

 which the constitution of matter and the forces act- 

 ing at minute distances are involved. 



Early opinions on the constitution of matter and (619.; 

 chemical combination. Two opinions have prevailed Early opi- 

 from the very earliest times respecting the constitu- ^"coMti- 

 tion of body : that which supposes its entire homo- tution of 

 geneousness, and that which allows that it consists matter. 

 of material parts or atoms separated by void spaces ; 

 these parts or atoms being indivisible. This last is 

 the doctrine of Democritus and Epicurus, and in 

 modern times of Bacon, Newton, and Dalton. 1 The 

 former opinion has been held by Leibnitz, and many 



The reader will find ample details on the opinions of the older philosophers in Daubeny's Introduction to the Atomic Theory, 



