13s. Chemistry. [Lecture 29. 



It was formerly common in medicine to talk 

 of the pointed particles of salts, and to say that 

 they cut and stimulate in consequence of their 

 mechanical forms when taken into the body. 

 But what greater reason have we to suppose that 

 the particles of salt have any more these forms 

 than the particles of water ? Salts probably pro- 

 duce their effects in consequence of their attrac- 

 tion for the animal fluids. The solvent power 

 of every fluid is in proportion to the different de- 

 grees of attraction it has for different substances. 

 All the various kinds of salt might be equally 

 mixed with water, if their attraction for water was 

 equal. In many cases, water being saturated 

 with one salt will take up a portion of a dif- 

 ferent kind. If to water saturated with nitrat 

 of potass (saltpetre) I add common salt, the wa- 

 ter will dissolve more of the saltpetre. This has 

 been thought an uncommon phenomenon : but 

 from this fact I infer that these salts have a 

 small degree of attraction for each other, though 

 this does not appear on any other occasion. 

 Many of the saline substances have a strong at- 

 traction for each other, and afterwards unite 

 very closely ; others do not act strongly on each 

 other, and may be easily separated after union. 



If we desire to separate salts in solution, we 

 make use of three means, according to their dif- 

 ference of volatility^ solubility^ and ctystalliza- 

 tion. If the salts differ in volatility, we put 

 them in a retort, and obtain the more volatile. 



