538 DISSOLUTION. 



grated state, that much motion must be given to it to 

 cause resumption of the disintegrated state; and an im- 

 mense time may elapse before there occur in the environ- 

 ment, changes great enough to communicate to it the requi- 

 site quantity of motion. AVe will look first at those excep- 

 tional inorganic aggregates which retain much motion, and 

 therefore readily undergo dissolution. 



Among these are the liquids and volatile solids which 

 dissipate under ordinary conditions — water that evaporates, 

 carbonate of ammonia that wastes away by the dispersion of 

 its molecules. In all such cases motion is absorbed; and 

 always the dissolution is rapid in proportion as the quantity 

 of heat or motion which the aggregated mass receives from 

 its environment is great. Xext come the cases in 



which the molecules of a highly integrated or solid aggre- 

 gate, are dispersed among the molecules of a less integrated 

 or liquid aggregate ; as in aqueous solutions. One evidence 

 that this disintegration of matter has for its concomitant 

 the absorption of motion, is that soluble substances dissolve 

 the more quickly the hotter the water: supposing always 

 that no elective affinity comes into play. Another and still 

 more conclusive evidence is, that when crystals of a given 

 temperature are placed in water of the same temperature, 

 the process of solution is accompanied by a fall of tempera- 

 ture — often a very great one. Omitting instances in which 

 some chemical action takes place between the salt and the 

 water, it is a uniform law that the motion which disperses 

 the molecules of the salt through the water, is at the expense 

 of the molecular motion possessed by the water. 



Masses of sediment accumulated into strata, afterwards 

 compressed by many thousands of feet of superincumbent 

 strata, and reduced in course of time to a solid state, 

 may remain for millions of years unchanged; but in sub- 

 sequent millions of years they are inevitably exposed to 

 disintegrating actions. Raised along with other such masses 

 into a continent, denuded and exposed to rain, frost, and 



