350 Researches respecting 



This precipitation is not the result of elective affinity, and it 

 is not complete, but its quantity is determined by the ratio 

 of the action of the Hquid to the cohesive force of the preci- 

 pitate : hence it happens that the precipitate is often re-dis- 

 solved on augmenting the quantity of the substance opposed 

 to it. 



9th, Though Bergman has explained very clearly the 

 changes which heat may produce on chemical action when 

 the substances have a disposition to volatilisation; and though 

 he even recommends to avoid too strong heat in evaporation^ 

 the extent of the influence which it may have in the opera- 

 tions by which salts are separated, in order to form a judge- 

 ment of their affinities, has not yet been fully discovered. 

 . It ought not to have been concluded that the sulphuric 

 acid has more affinity for fixed alkalies than the nitric or 

 muriatic acids, merely because that by a strong heat it expels 

 these acids from their combinations. Chemists should havc^ 

 observed that, even by the heat employed to produce eva- 

 poration, and to cause salts to crystallize, the proportions of 

 the volatile acids may be considerably changed in regard to 

 the sulphuric acid which remains opposed to them, and that 

 the latter may at length entirely expel them, by means of 

 the difference which exists between its fixity and that of these 

 acids. (Art. VII. No. 5.) 



10th, We are indebted to Bergman for useful observations 

 on the errors which may arise from the solubility of one sub- 

 stance, which is eliminated, and of which the separation is 

 not observed. He remarks that potash and soda do not dis- 

 turb the transparency of the solution of a salt with a base of 

 lime, if this solution be diluted with fifty times as much water, 

 because the lime separated, being soluble, remains in the 

 water ; but he did not reflect that, if the lime, in that case, 

 had no more than its natural solubility, it would be a very 

 weak obstacle to its precipitation, for it requires nearly seven 

 hundred parts of water to dissolve it : what adds greatly to its 

 natural solubility is, that it continues to be in combination 

 with the acid (Art. V. No. 5.), and that it cannot be sepa-"* 

 rated but by retaining a part (Art. III. No. 9.), which in- 

 creases its solubility. 



Uth, 



