78 HISTORY OF CHEMISTRY. [LECTURE V. 



substance obtained upon burning potassium is different from 

 potash, in so far that it contains more oxygen than the latter ; 40 

 and they point out that the fact of potassium containing 

 hydrogen would necessitate that the new oxide should contain 

 water, since the production of uncombined water is not observed 

 during the combustion. Further, as the oxide is decomposed 

 by dry carbonic anhydride with the formation of oxygen and 

 potassium carbonate, their hypothesis as to the composition of 

 potassium leads to the assumption of the presence of water in 

 salts, in cases where analysis does not reveal any. 



From this period onwards the non-decomposability of the 

 metals was no longer seriously doubted. At the same time the 

 elementary nature of phosphorus and of sulphur was estab- 

 lished anew by decisive experiments of Gay-Lussac and 

 Thenard, 41 and the view that ammonia contained oxygen was 

 recognised as an error by the younger Berthollet. 42 



To the foregoing, I may add an account of another scien- 

 tific discussion, between Davy on the one hand, and Gay- 

 Lussac and Thenard on the other, which is also of importance 

 inasmuch as it led to the overthrow of Lavoisier's theory of 

 acids. The discussion ii\ question touches the composition of 

 hydrochloric acid. In this acid, as in all others, Lavoisier had 

 assumed the existence of oxygen. Its presence there had never 

 been established, it is true, but the general theory required it ; 

 and as this theory was almost universally adopted, the existence 

 of oxygen in hydrochloric acid seemed to be unquestionable. 

 I say it was almost universally adopted, -because Berthollet, for 

 instance, was of a different opinion. In 1787 the latter had 

 examined hydrocyanic acid, discovered some time previously 

 by Scheele, 43 and had found in it carbon, hydrogen, and 

 nitrogen only. It was likewise known, from other investiga- 

 tions by Scheele, that sulphuretted hydrogen contains hydrogen 

 and sulphur only ; and consequently Berthollet felt justified in 

 regarding other elements as acid-producing, as well as oxygen. 44 



40 Rech. phys. chim. I, 125. 41 Ann. Chim. 73, 229. 42 Mem. 

 d'Arcueil. 2, 268. 43 Ann. Chim. I, 30. 44 Stat. Chim. 2, 8 ; E. 2, 8. 



