Solutions in alcohol, if coloured, are termed tinctures, and in vinegar or wine, medicated vinegars or wines. The solu tion of metals in mercury is termed amal gamation. The combinations of Mlle] metals with each other form alloys.
Absorption is the condensation of a gas into a fluid or solid form, in consequence of its combination with a fluid or solid It is facilitated by increase of surface and agitation ; and the power of absorption ir fluids is much increased by compression and diminution of temperature, although in every instance it be limited and deter. urinate. Dr. Nooth invented an ingeni ous apparatus for combining gases with fluids, and Messrs. Schweppe, Paul, and Cuthbertson have very advantageous!) employed compression.
Fluids often become solid by enterinE into combination with solids, and thi. change is always accompanied by consi derable increase of temperature, as in the slaking of lime.
Chemical Decomposition is the separa tion of the elementary parts of which were chemically combined : sac can only be effected by the agency substances possessing a stronger affinit) for one or more of the constituents of the compound, than these possess for eact other.
Decomposition has acquired various appellations, according to the phenome na which accompany it.
Dissolution differs from solution in be ing accompanied by a decomposition, Os change in the nature of the substance dissolved. Thus, we correctly say, a so lution of lime in muriatic acid, and a dis. solution of chalk in muriatic acid.
Sometimes a gas is separated durinE the action of bodies on each other. Wher this escapes with considerable violence and agitation of the fluid it is termed of fervescence. The gas is very frequently allowed to escape into the atmosphere, but at other times is either collected in a pneumatic apparatus, or made to enter into some new combination. The vessels in which an effervescing mixture is made, should be high and sufficiently large, to prevent any loss of the materials from their running over, and in some cases the mixture must be made slowly and gradu ally.
Precipitation is the reverse of solution. It comprehends all those processes in which a solid is obtained by the decom position of a solution. The substance se
parated is termed a precipitate, if it sink to the bottom of the fluid ; or a cream, if it swim above it. Precipitation, like so lution, is performed either viol latinida', or yid sica ; and is effected by lessening the quantity of the solvent by evaporation ; by diminishing its powers, as by reduc tion of temperature, or dilution ; or by the addition of some chemical agent, which, from its more powerful affinities, either combines with the solvent, and precipitates the solvend, or forms itself an insoluble compound with some con stituent of the solution.
The objects of precipitation are, the separation of substances from solutions in which they are contained ; the purifica tion of solutions from precipitable impu rities ; or the formation of new combina tions.
The two first means of precipitation have been already noticed. In perform ing it in the last manner we may observe the following rules :—The solution and precipitant must possess the requisite de gree of purity. The solution should be perfectly saturated, to avoid unnecessary expenditure of the solvent or precipitant. The one is to be added slowly and gradu ally to the other. After each addition, they are to be thoroughly mixed by agi tation. We must allow the mixture to settle after we think that enough of the precipitant has been added, and try a lit tle of the clear solution, by adding to it some of the precipitant ; if any precipita tion takes place, we have not added enough of the precipitant. This is ne cessary, not only to avoid loss, but in ma ny instances the precipitant, if added in excess, re-dissolves or combines with the precipitate.
After the precipitation is completed, the precipitate is to be separated from the supernatant fluid by some of the means already noticed.
When the precipitate is the chief oh ject of our process, and when it is not soluble in water, it is often advisable to dilute to a considerable degree both the solution and precipitant before perform ing the operation. When it is only diffi cultly soluble, we must content ourselves with washing the precipitate after it is separated by filtration. In some cases the separation of the precipitate is much assisted by a gentle heat.