xv THE MOLECULAR THEORY 335 



the ratio at oC to be 2-00269. * n tne same way it has 

 been shown that the volume of hydrogen contained in two 

 volumes of hydrogen chloride is not i, but 1*00790 (Gray 

 and Burt, Trans. Chem. Soc., 1909, 95, 1656), whilst the 

 combining ratio of hydrogen and nitrogen in ammonia is 

 3-00172 (Guye and Pintza, Comptes rendus, 1908, 147, 928). 



These deviations from exact whole numbers probably 

 depend on the fact that gases differ slightly in their com- 

 pressibility, so that equal volumes under a pressure of i 

 atmosphere would become unequal under a pressure of 2 

 atmospheres or \ atmosphere. It is, however, believed 

 that Gay-Lussac's law would" hold good accurately if the 

 gases could be examined under very low pressures. 



Gay-Lussac's law of volumes explained by the atomic 

 theory. Gay-Lussac's observations provided excellent ex- 

 amples of the laws of chemical combination described in 

 the preceding chapter. Thus, no better illustration could 

 be desired of the law of " Multiple Proportions " than the 

 three oxides of nitrogen, the two oxides of carbon and the 

 two oxides of sulphur, in which the proportions of oxygen 

 to the other element were shown by Gay-Lussac to be in 

 the ratios i : 2 : 4, i : 2 and 2 : 3 respectively. It is there- 

 fore not surprising that Gay-Lussac was ready to interpret 

 his observations by means of the atomic theory which 

 Dalton had suggested a few years previously. 



" According to Dalton's ingenious idea, that combinations 

 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." 



"The numerous results I have brought forward in this 

 Memoir are also very favourable to the theory" (A.C.R. 

 IV. 23). 



The law of volumes rejected by Dalton. It is, however, 

 remarkable that Dalton, instead of welcoming the Law of 



