CHAPTER XX 



DISSOCIATION 

 A. DISSOCIATION AND ASSOCIATION 



St. Claire Deville (1857) on dissociation. The name 

 DISSOCIATION was introduced in 1 85 7 by H. St. Claire Deville l 

 (Comptes rendus, 1857, 45, 857) to describe the "spontane- 

 ous decomposition " of substances by heat, without the 

 intervention of chemical agencies. This definition (which 

 would include thermal decompositions such as the " coking " 

 of coal and the "charring" of sugar) was afterwards limited 

 (Comptes rendus, 1863, 56, 730) so as to include only 

 cases " in which the decomposition takes place partially, 

 and at a temperature inferior to that which corresponds to 

 the absolute destruction of the compound," i.e. to cases of 

 REVERSIBLE DECOMPOSITION, in which theproducts recombine 

 when the decomposing forces are removed. These reversible 

 decompositions are very difficult to detect because, after 

 heating and cooling, the original substance reappears un- 

 altered, bearing no trace of the changes which it has under- 

 gone. 



1 St. Claire Deville's experiments were summarised in his Lessons on 

 Dissociation delivered to the Paris Academy of Sciences in 1864. 

 These were published as a pamphlet, which includes (almost verbatim) 

 the Comptes rendus papers of 1864 and 1865, together with the 

 diagrams, which were not reproduced in the Comptes rendus. 



