SOLUBILITY OF SALTS. 183 



plmret of copper is produced by their union, v\ ith the evolution 

 of much heat. The affinity of bodies is, therefore, promoted 

 by everything which tends to their close approximation; in 

 solids, by their pulverization and intermixture, this attraction 

 residing in the ultimate particles of bodies ; in gases, by their 

 spontaneous diffusion through each other, which occasions a 

 more complete intermixture than is attainable by mechanical 

 means ; and between liquids, or between a liquid and solid by 

 the adhesive attraction which liquids possess, which must lead 

 to perfect contact, and also by a disposition of liquid bodies. to 

 intermix, of the same physical character as gaseous diffusion. 

 Elevation of temperature has certainly often a specific action 

 in increasing the affinity of two bodies, but it also often acts 

 by producing a perfect contact between them, from the fusion 

 or vaporization of one or both bodies. Hence, no practice is 

 more general to promote the combination of bodies than to 

 heat them together. 



If the affinity between two gases is sufficiently great to begin 

 combination, the process is never interrupted, but is continued 

 from the diffusion of the gases through each other till complete, 

 or at least till one of the gases is entirely consumed. Thus 

 when hydrochloric acid and ammoniacal gases, in equal mea- 

 sures, are introduced into a jar containing at the same time a 

 large quantity of air, the formation of hydrochlorate of am- 

 monia proceeds, the gases appearing to search out each other, 

 till no portion of uncombined gas remains. The combination 

 of two liquids, or of a liquid and a solid, is also facilitated in 

 the same manner by the mobility of the fluid, and proceeds 

 without interruption, unless, perhaps, the product of the com- 

 bination be solid, and by its formation interpose an obstacle 

 to the contact of the combining bodies. But the affinities of 

 two solids which are not volatile are rarely developed at all, 

 owing to the imperfection of contact. Even the action of very 

 powerful affinities between a solid and a liquid or a gas, is 

 often arrested in the outset from the physical condition of the 

 former. Thus, the affinity between oxygen and lead is cer- 

 tainly considerable, for the metal is rapidly converted into n, 

 white oxide, when ground to powder and agitated with water 

 in its usual aerated condition; and in the state of extreme division 

 in which lead is obtained by calcining its tartrate in a glass 

 tube, the metal is a pyrophorus, and combines with oxygen 



