CHEMICAL AFFINITY. 31 



S-f-30, or SO^, or SO3; hyposulphuric acid, 2S + 50, or S^O^, or 

 S^Og, &c. Sometimes abbreviations are made use of: thus, two 

 equivalents of a substance are indicated by the symbol with a short 

 line drawn through or below it ; an equivalent of oxygen is signified 

 by a dot, and one of sulphur by a comma, thus sesquioxide of iron, 



Fe. ; bisulphuret of carbon, C, instead of CS^. 



Chemical affinity. — The most simple case of chemical affinity is 

 where two bodies unite together to form a third body ; as copper 

 and zinc forming brass ; sulphuric acid and soda to form sulphate 

 of soda. 



The second case of affinity, called single elective attraction, is 

 where two heterogeneous bodies having united together to form a 

 compound, another body, being blended with them in solution, 

 unites with one of the former ; for example, potash being added to 

 a solution of sulphate of magnesia, unites with the sulphuric acid, 

 and precipitates the magnesia ; water on being added to the tincture of 

 camphor, (camphor and alcohol,) will unite with the alcohol and 

 precipitate the camphor. 



The third case of affinity, called double elective attraction, is 

 where two compound bodies, on being intimately mingled, undergo 

 a mutual decomposition, the four components interchanging places : 

 thus, a solution of acetate of lead and a solution of sulphate of 

 zinc, on being mixed, give rise to an acetate of zinc and a sulphate 

 of lead. 



The fourth case of affinity is where two bodies being in combina- 

 tion, a third, on being added in excess, combines with both the others ; 

 thus, ammonia being added to the solution of sulphate of copper, at 

 first throws down the oxide of copper ; but on continuing to add the 

 ammonia, the excess combines with the precipitated oxide, which is 

 then redissolved. 



Circumstances modifijing chemical affinity. — One of these is 

 heat ; thus, by heating mercury in the air, it will combine with 

 oxygen ; and by simply increasing the temperature, it will separate 

 again from the oxygen. Another modifying agent is solution : many 

 substances which, when in the dry state, evince no tendency to unite, 

 when moistened or dissolved, exhibit a powerful affinity ; thus tar- 

 taric acid and a carbonated alkali may be kept together, if dry, 

 without any union ; but if moistened, effervescence takes place. 

 Mezhanical division also greatly promotes chemical action, as may 

 be seen by the action of nitric acid on a brass ball being far less 

 violent than when the metal is presented to it in the form of filings 

 or leaves. The nascejit state is also peculiarly favourable to che- 

 mical combination. 



Tables of affinity consist of a series of substances placed in a 



