180 CHEMICAL AFFINITY. 



sometimes cannot again be separated. The action of glue, wax, 

 mortar arid other cements in attaching bodies together, depends 

 entirely upon the same force. In detaching glue from the sur- 

 face of glass, the latter is sometimes injured, and portions of it 

 are torn off by the glue, the adhesive attraction of the two 

 bodies being greater than the cohesion of the glass. The pro- 

 perty of water to adhere to solid surfaces arid wet them, its 

 imbibition by a sponge, the ascent of liquids in narrow tubes, 

 and other phenomena of capillary attraction, and the rapid 

 diffusion of a drop of oil over the surface of water are illustra- 

 tions of the same attraction between a liquid and a solid, and 

 between different liquids. But this kind of attraction is defi- 

 cient in a character which is never absent in true chemical 

 affinity it effects no change in the properties of bodies. It 

 may bind different kinds of matter together, but it does not 

 alter their nature. 



The tendency of different gases to diffuse through each other 

 till a uniform mixture is formed, is another property of matter, 

 the effect of a force wholly independent of chemical affinity. It 

 is certain that this physical property is not lost in liquids, and 

 that it contributes to that equable diffusion of a salt through a 

 menstruum which occurs spontaneously, and without agitation 

 to promote it.* 



Solution. The attraction between salt and water, which 

 occasions the solution of the former, differs in several circum- 

 stances from the affinity which leads to the production of de- 

 finite chemical compounds. In solution, combination takes place 

 in indefinite proportions, a certain quantity of common salt dis- 

 solving in, or combining with any quantity of water however large; 

 while a certain quantity of water, such as 100 parts can dissolve 

 any quantity of that salt less than 37 parts, the proportion 

 which saturates it. Water has a constant solvent power for every 

 other soluble salt, but the maximum proportion of salt dissolved, 

 or the saturating quantity, has no relation to the atomic weight 

 of the salt, and indeed varies exceedingly with the temperature 

 of the solvent. The limit to the solubility of a salt seems to be 

 immediately occasioned by its cohesion. Water, in proportion 

 as it takes up salt, has its power to disintegrate and dissolve 

 more of the soluble body gradually diminished, it dissolves the 



* Jerichau in Poggendorff's Annalen, 34, 613; or Dove and Moser's Reperto- 

 rium der Physik, 1, 96. 1837. 



