Water will dissolve many substances, but in varying degrees, _i.e._, of some it will dissolve much, of others, little, and some not at all. Further, different liquids have different solvent powers. Alcohol will dissolve resin and shellac, but it will not dissolve gum arabic, which is soluble in water. Benzine dissolves grease. Beeswax is not dissolved by water, alcohol or benzine, but is soluble in turpentine. It is found that the _temperature_ of the liquid has a marked effect upon the amount of substance that will dissolve. This is an indication that the _motions_ of the molecules are effective in solution. It appears that dissolving a solid is in some respects similar to evaporation, and just as at higher temperatures more of the liquid evaporates, because more of the molecules will escape from the liquid into the air above, so at higher temperatures, more molecules of a solid will detach themselves through greater vibration and will move into the liquid. Further, just as an evaporating liquid may saturate the space above it so that any escape of molecules is balanced by those returning, so with a dissolving solid, the liquid may become _saturated_ so that the solution of more of the solid is balanced by the return of the molecules from the liquid to the solid condition. =27. Crystals and Crystallization.=--This return from the liquid to the solid state, of molecules that are in solution, is especially noticeable when the solution is cooling or evaporating and hence is losing its capacity to hold so much of the solid. On returning to the solid, the molecules attach themselves in a definite manner to the solid portion, building up regular solid forms. These regular forms are _crystals_. The action that forms them is called _crystallization_. Each substance seems to have its own _peculiar form of crystal_ due to the manner in which the molecules attach themselves to those previously in place. The largest and most _symmetrical crystals are_ those in which the molecules are deposited slowly with no disturbance of the liquid. Beautiful crystals of alum may be obtained by dissolving 25 g. of alum in 50 ccm. of hot water, hanging two or three threads in the solution and letting it stand over night. The thread fibers provide a foundation upon which crystals grow. When a _solution_ of a solid evaporates, the molecules of the _liquid_ escape as a gas, the molecules of the _solid_ remain accumulating as crystals. This principle has many uses: (a) sea water is purified by _evaporating the water and condensing the vapor_, which of course forms pure water. (b) water is forced down to _salt_ beds where it dissolves the salt. The brine is then raised and evaporated, leaving the salt in the evaporating pans.