302 HISTORY OF CHEMISTRY. [LECTURE XV. 



for oxygen could not come into play in Regnault's experiment 

 (since silver oxide breaks up into its constituents at much lower 

 temperatures, and the same thing must certainly take place in 

 presence of hydrogen), Deville shows that the decomposition of 

 water by means of strongly heated lead oxide (at 1200 to 

 1300) is also observed. He succeeds in effecting the same 

 decomposition by means of ingeniously contrived apparatus, 

 without the action of a foreign substance ; and in this way his 

 opinion that the decomposition is a result of the high tem- 

 perature is confirmed in an elegant manner. 18 



The difficulty in these investigations arises from the fact 

 that the constituents separated during the decomposition, com- 

 bine again at lower temperatures, so that the decomposition which 

 has occurred is not recognisable under ordinary circumstances. 

 The proof that decomposition has occurred may be furnished, 

 as Deville shows, (i) by diluting the products of the decom- 

 position by means of a rapid current of an indifferent gas, so 

 that complete recombination is prevented ; (2) by diffusion, 

 whereby the composition of the gaseous mixture is altered ; or 

 (3) by means of the so-called tube chaud et frbid , i.e., by sudden 

 cooling of the products of decomposition. 



In the forms of ap'paratus constructed to carry out these 

 methods, Deville succeeded in proving not only the decom- 

 position of water into hydrogen and oxygen, but also that of 

 carbonic anhydride into carbonic oxide and oxygen, of carbonic 

 oxide into carbon and carbonic anhydride, of hydrochloric acid 

 into chlorine and hydrogen, of sulphurous anhydride into 

 sulphuric anhydride and sulphur, etc. 



Supported by these experiments, Deville compares the 

 formation of compounds with the condensation of vapours. 

 According to him, both changes begin at definite temperatures 

 and both proceed gradually. Certain quantities of heat are 

 given out during the condensation of vapours, and the same 

 thing takes place (frequently to a much greater extent) in the 



18 Comptes Rendus. 56, 195, 322, 729 ; compare also Deville, Lecons 

 sur la dissociation, 1864. 



