612 HISTORY OF SCIENCE. 



in the meantime being discovered by the genius of a young philoso- 

 pher, who, like many other brilliant men of science of his country at 

 this epoch, had been trained at the Ecole Polytechnique. This was 

 JOSEPH Louis GAY-LUSSAC, who was born at St. Leonard, 1778, and 

 died 1850. The facts which exact experiments enabled him to announce 

 not only confirmed the atomic doctrine, but gave a new extension to 

 chemistry. Indeed, it is not too much to say that Gay-Lussac's laws, 

 interpreted by aid of a certain hypothesis (Avogadro's), are the foun- 

 dation of the whole fabric of the theoretical chemistry of the present 

 day. 



Before Gay-Lussac various experimenters had sought to determine 

 the proportion by measure in which oxygen and hydrogen gases com- 

 bine to form water. The results had been somewhat discrepant. The 

 volumes of hydrogen which combine with 100 volumes of oxygen were 

 variously given as 190, 198, 205, etc. Gay-Lussac, working in con- 

 junction with Humboldt, found that 100 volumes of oxygen required 

 exactly 200 volumes of hydrogen ; that is, the gases united in the 

 exact proportion of i to 2. This result, obtained in 1805, was suffici- 

 ently striking to induce Gay-Lussac to carefully examine other cases 

 of gaseous combination. He found that hydrogen unites with chlorine 

 i volume to i volume ; that to form nitrogen protoxide 2 volumes of 

 nitrogen gas are united to i of oxygen ; and so on. In every case 

 examined the gases were combined in exact integral volumes having 

 a simple ratio to each other. Hence the law : 



" There is a simple ratio between the volumes of two gases which e?iter 

 into combination" 



Gay-Lussac also found that the volume of the product formed by 

 the combination of gases always had itself, when in the gaseous state, 

 a simple ratio to the volume of its constituents. At the temperature of 

 212 R, i volume of oxygen would unite with 2 volumes of hydrogen, 

 and the product would be exactly 2 volumes of steam, i.e., of the gas 

 of water. One volume of hydrogen united to i of chlorine produces 

 exactly 2 volumes of hydrochloric acid gas, and so on in other cases. 

 Hence the second of Gay-Lussac's laws : 



" Thtre is a simple relation between the volumes of gas which enter 

 into combination and the volume of the product taken in the gaseous 

 state" 



It is a curious circumstance that Gay-Lussac himself should suppose 

 as he did that these laws were reconcilable with the opinions of Ber- 

 thollet as to the variability of combining proportions. And, again, it is 

 equally curious that Dalton, whose atomic theory these facts so strongly 

 supported, should have doubted of their correctness. Gay-Lussac's 

 laws, taken in connection with Dalton's theory, indeed, placed in the 

 hands of chemists a means of controlling their determinations of the 

 atomic weights. Thus, if according to the atomic theory i atom of 

 hydrogen combines with i atom of chlorine, and if equal volumes ot 



