6o; 



HISTORY OF SCIENCE. 



of the particle signifies the bulk of the supposed impenetrable nucleus, 

 together with that of the surrounding repulsive atmosphere of heat. 

 At the time I formed the theory of mixed gases I had a confused idea, 

 as many have I suppose at this time, that the particles of elastic fluids 

 are all of one 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 reasoning .... I became convinced .... that the following 

 may be adopted as a maxim till some reason appears to the contrary, 

 namely : Every particle of pure elastic fluid has the particles globular 

 and all of a size ; but no two species (of gases) agree in the size of 

 their particles, the pressure and temperature being the same." 



The difficulty which thus presented itself to Dalton was the case of 

 the combination of one volume of one gas to one of another to pro- 

 duce a compound gas occupying two volumes. Each volume of the 

 compound could be said to have at most only half the former number 

 of particles. Let us suppose that one cubic inch of hydrogen gas 

 contains 1,000 particles, and that one cubic inch of chlorine gas also 

 contains 1,000 particles. By their union these gases would produce 

 two cubic inches of hydrochloric acid gas. Therefore Dalton would 

 say, each cubic inch of the compound gas can contain only 500 par- 

 ticles. But Avogadro's hypothesis is saved by the consideration that the 

 particles of which it speaks may include several atoms. We have only 

 to suppose that each particle of hydrogen contains two atoms of hydro- 

 gen, and that each particle of chlorine contains two atoms of chlorine. 

 Then the combination will consist in the exchange of atoms among 

 the particles. This may best be shown to the eye as in Fig. 307, where 



Hydrogen Gas. 



Chlorine Gas. 



FIG. 307. 



Hydrochloric Acid Gas. 



the squares represent the volumes of the gases, each containing four 

 particles. After combination we see that each volume still contains 

 four particles of the compound gas. This view is in perfect accord- 

 ance with other chemical phenomena. In this way Gay-Lussac's laws 

 (see page 530), which, it will be remembered, are not theories but 

 facts, will be seen really to lend a powerful support to the atomic 

 theory. 



Dalton's life extended over more than three-quarters of a century, 



