A CENTURY OF CHEMISTRY. 89 



in inverse ratio. In 1802, Gay-Lussac, whose work 

 touched almost every department of chemistry with 

 important results, stated what had been foreseen 

 (as he says) by Charles fifteen years earlier, that 

 equal volumes of different gases change their volumes 

 equally with equal rise of temperature. Dalton also 

 had perceived this conclusion (the law of Charles) 

 that all gases expand in the same proportion for the 

 same increase of temperature. It should be noted 

 that both these laws (Boyle's and Charles') are ideal 

 formula? which only approximately fit the facts. 



In 1805, along with Alexander von Humboldt, 

 Gay-Lussac observed that exactly two volumes of 

 hydrogen unite with one volume of oxygen to form 

 water. From this starting-point he went on to show 

 (1808) that similarly simple volumetric relations 

 hold true in regard to all gases which combine 

 chemically Avith one another, and that the volumes 

 of the gaseous products formed always have a simple 

 relation to the volumes of their components (all be- 

 ing measured, of course, at the same pressure and 

 temperature). "Having concluded from their simi- 

 lar behaviour with regard to changes of pressure 

 and temperature that all gases possess a like molec- 

 ular constitution, Gay-Lussac deduced from his re- 

 searches (above referred to) the following impor- 

 tant law: — The weights of equal volumes of both 

 simple and compound gases, and therefore their den- 

 sities, are proportional to their empirically found 

 combining weight, or to rational multiples of the lat- 

 ter." * In other words, if gases, like other bodies, 

 combine according to definite proportions of their 

 weights (Dalton's law) ; and if gases (under the 

 same pressure and at equal temperatures) combine 



* E. von Meyer, History of Chemistry, trans. 1891, p. 202. 



