Jdirtvre, and, its Effects. 91 



quantity of acid along with it. When marble 

 has been mixed with the muriatic acid, the latter 

 loses its volatility, and becomes capable of en- 

 during a violent heat : if heat is applied, the 

 watery particles rise in vapour, and leave the 

 marble and acid so united that no heat can sepa- 

 rate them. In this case, therefore, it is impos- 

 sible by heat alone to separate the marble from 

 the acid, but by the addition of a third body. 

 In doing this, the third body always joins itself 

 to one of the other two ; this is called 'elective 

 attraction, or affinity. 



To give a clear notion of this, by separating 

 the mixtures we just now made, I shall first 

 separate the water and sulphuric acid by the 

 addition of another saline liquor; this liquor 

 acts with great violence on the mixture: the 

 mixture grows muddy as soon as a third fluid is 

 added, and as it cools the salt is deposited at the 

 bottom. 



2. In separating the ammonia from the sul- 

 phuric acid, the mixture is at present destitute of 

 smell ; but on adding a little carbonate of potass, 

 destitute likewise of smell, we may perceive that 

 the ammonia is separated, by the pungent smell. 



3. So spirit of wine is separated from camphor 

 by mixing water with the spirit. 



In this attraction there is a sort of gradation. 

 Thus silver dissolved in nitric acid may be sepa- 

 rated by quicksilver, which takes its place. If 

 we add copper, the mercury will be separated. 



