LECTURE IV.] HISTORY OF CHEMISTRY. 59 



Mariotte, which was discovered by Boyle, and which states the 

 relation between pressure and volume ; in short, he possessed 

 all the data for reducing the results directly obtained to like 

 pressure and temperature a basis upon which to execute the 

 experiments he had in view. His investigation is a model of 

 accuracy, and, in this respect, it is very markedly distinguished 

 from other experimental investigations of the period. The re- 

 sults obtained are extremely simple ; and he states them some- 

 what after this manner. Two gases always combine in simple 

 proportions by volume, and the contraction which they under- 

 go, and, therefore, also the volume of the product formed, 

 stand in the simplest relation to the volumes of the con- 

 stituents. 



Thus Gay-Lussac found, for example, that 2 volumes of 

 carbonic anhydride are produced from 2 volumes of carbonic 

 oxide and i of oxygen ; that 2 volumes of nitrous oxide are 

 composed of 2 volumes of nitrogen and i of oxygen ; that 

 equal volumes of nitrogen and of oxygen are combined in nitric 

 oxide, while the product has the same volume as the two con- 

 stituent gases separately ; and finally, that i volume of nitrogen 

 and 3 of hydrogen are condensed to 2 volumes in ammonia. 



Gay-Lussac, who was well acquainted with Dalton's theory, 

 shows at the end of his paper that the facts ascertained by him 

 are in harmony with the theory ; that, by the assumption of a 

 similar molecular condition in all gases, it will explain their 

 analogous behaviour towards changes of pressure and of tem- 

 perature ; and that his law of gaseous volumes is an important 

 support of Dalton's view. 



It might be supposed that the latter would have been highly 

 pleased by so unexpectedly brilliant a confirmation of his views. 

 This was not so, however. In the second part of his "New 

 System of Chemical Philosophy," which appeared in 1810, he 

 practically regards Gay-Lussac's experiments as erroneous. I 

 shall endeavour to explain the reasons that prompted him to do 

 this, especially as it has been stated that Dalton wished, from 

 jealousy or want of judgment, to contest Gay-Lussac's merits. 



In the first part of his book, Dalton had already speculated 



