LECTURE III.] HISTORY OF CHEMISTRY. 39 



acid. The barium sulphate, in consequence of its insolubility, 

 is removed from the reaction ; a constantly progressing parti- 

 tion taking place until the whole of the barium sulphate is 

 precipitated. 17 



A similar thing occurs in the cases of volatile acids or 

 bases. In this case also a partition takes place, in the ratio of 

 the masse chimique ; but as one product (carbonic anhydride, 

 for example) escapes, the decomposition proceeds to the end. 18 



Nevertheless, it is only in cases of extreme preponderance 

 of cohesion (insolubility) or of elasticity (volatility) that com- 

 plete decompositions are observed. Cases of partial decompo- 

 sition are far more frequent. Thus, according to Berthollet, 

 calcium salts cannot be completely precipitated by means of 

 oxalic acid. 19 



His views regarding double decomposition are similar. 

 Four salts are, in general, produced in these cases, and the 

 formation of two only is confined to cases where the cohesion, 

 or solubility, is totally different. 



This furnishes the explanation of the so-called reversible 

 reactions. Amongst these reactions there are, in the first 

 place, the various crystallisations which may be obtained at 

 different temperatures from the same mixture of salts, if 

 these salts possess solubilities which vary greatly from one 

 another with changes in temperature. Berthollet adduces 

 several examples of this kind, 20 and I shall mention one of 

 them. If a solution contains soda, magnesia, sulphuric acid, 

 and hydrochloric acid, Glauber's salt crystallises out from it at 

 a very low temperature (o C.), whereas sodium chloride is ob- 

 tained on evaporation. Hence, at o, magnesium sulphate and 

 sodium chloride must change into sodium sulphate and mag- 

 nesium chloride, whilst at high temperatures the reverse takes 

 place. 



In the same way Berthollet is also able to explain the phe- 



17 Stat. Chim. I, 78 ; E. I, 51. 18 Ann. Chim. 36, 314. 19 Stat. 

 Chim. i, 78-79; E. i, 51-52. 2 Ibid. I, 101-102 and 129-130; E. I, 

 7 1 -72 and 395-39 6 - 



