536 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



but by cooling quickly to a temperature below that at which the 

 gases recombine it is possible to fix the decomposition products 

 as they are formed at higher temperatures. 



St. Claire Deville, in 1857, described the "spontaneous 

 decomposition " of substances by heat as dissociation, but in 

 1863 limited the term to reversible decompositions. By using a 

 " hot-cold tube " he was able in 1865 to prove the dissociation of : 



(1) Water 2H 2 O '^ 2H 2 + O 2 



(2) Carbon dioxide 2CO 2 ^ 2CO + O 2 



(3) Carbonic oxide 2CO i C + CO 2 



(4) Sulphur dioxide 3SO 2 '4i S + 2SO 3 



(5) Hydrogen chloride 2HC1 ^ C1 2 + H 2 (the chlorine 



being fixed in the form of metallic chlorides on the 



cold tube). 



He brought about similar decompositions by sparking, but 

 showed that sulphur dioxide and oxygen could be recombined 

 by sparking over sulphuric acid and hydrogen and nitrogen by 

 sparking in presence of hydrogen chloride : 



2SO 2 + O 2 '^ 2SO 3 -> (fixed as H 2 S 2 O 7 ) 

 3 H 2 + N 2 ^ 2NH 3 -> (fixed as NH 4 C1). 

 Pebal, in 1862, separated by diffusion the products of dissocia- 

 tion of ammonium chloride : 



High temp. 



NH 4 C1 -^ NH 3 + HC1, 



Low temp. 



He thus proved the correctness of the view suggested by 

 Cannizzaro (1857) and by Kopp (1858) that the abnormalities 

 detected by Bineau (1838) in the vapour densities of the 

 ammonium and phosphonium salts were due to the dissociation 

 of the salts. Deville, however, rejected all cases of dissociation 

 in which the products could not be fixed by suddenly chilling 

 the vapour. 



Cahours,in 1844 and 1847, discovered that the vapour-densities 

 of acetic acid and of phosphorus pentachloride diminished as the 

 temperature rose. Playfair and Wanklyn, in 1862, observed a 

 similar phenomenon in nitrogen peroxide and attributed it to 

 dissociation : 



C 4 H 8 O 4 ^ 2C 2 H 4 O 2 



