Thus immerse a cubical crystal of potassium bromide 1 in. in dimension in water, and its 6 sq. in. of surface will be in contact with 6 sq. in. of water surface; immediately the 2 surfaces act together, resulting in the disintegration of the surface of the salt, which assumes the liquid form and blends with the surface of the water in the most intimate manner. This change takes place to a fixed extent, dependent upon the temperature and the saturation of the solvent. If the crystal be at the bottom of a vessel of water, it commences most rapidly to diminish in size from the top until finally it disappears. In observing closely the process we notice streams of liquid circulating about the crystal. These currents, colourless and transparent like the surrounding medium, are clearly visible from the fact that they refract the rays of light differently, an optical result caused by the portions of liquids of different densities, for the particles which form the surfaces of the salt unite with those of the water surface, resulting in a compound that has a greater specific gravity than pure water, consequently, as soon as united, this fluid flows over the crystal and down its sides in obedience to the laws of gravitation.
It strikes upon the bottom of the vessel and, in response to the law that fluids of different densities seek their own level, spreads out, and in doing so displaces its bulk of water, which rises and replaces the solution about the crystal, and thus continuous currents flow over and down the sides of the crystal, and fresher menstruum is constantly taking the place of that more saturated. We might liken the foregoing to a surface of liquid resolving against a solid, each movement of which wears away the solid and decreases the wearing force of the liquid. At last, if the amount of water be sufficient, the crystal will have disappeared, and at the bottom of the vessel will be found a dense solution at rest surmounted by a lighter one. Again cautiously introduce a crystal of the same salt and the afore-named phenomenon will take place, though in a less marked degree. The circulation of the medium becomes gradually less and less distinct, and finally, if the salt be in excess, disappears. There remains now a remnant of potassium bromide surrounded by a dense solution, while overlying we find almost pure water. In obedience to what is generally considered another force, which, it is thought, produces the diffusion of liquids, the solution and overlying water continually but slowly intermingle.
At last they are homogeneous, preceding which, however, the remnant of crystal at the bottom of the vessel will have disappeared. The foregoing exemplifies the changes which take place, under like conditions, when the crystal is broken, excepting that the increased amount of surface contact, before considered, hastens the operation.
Thus we find that nature's laws constantly produce circulation while solution is progressing. Arguing therefrom we should be able to hasten the operation at certain stages and assist nature by frequently stirring the entire liquid, thus mixing the solutions. Recognising the theoretical value of circulation and extent of surface, when we wish to dissolve substances we should powder them, and stir the liquids at short intervals.
Temperature is most important. With a few exceptions substances dissolve to a greater extent in warm than in cold liquids, and even though the material be scarcely more soluble in the hot menstruum it dissolves more rapidly. This results from the fact that liquids while rapidly changing temperature are in a more rapid state of circulation, and heat also decreases the cohesive attraction of solids, their molecules being more easily detached from the mass, and therefore more readily unite with those of the liquid. Few operators have failed to notice the benefit of a warm room when dissolving substances. Careful manufacturers cannot allow the process of percolation to be conducted at winter temperature, even though so doing results in great saving of alcohol by lessening evaporation. Time is a consideration of importance. An appreciable amount of contact must be allowed between solvent and solid. That solutions require time for action is a principle well recognised and scarcely necessary to mention.
Having now briefly noticed the influences which govern solution, let us consider the relation between maceration and percolation, as these processes are called, bearing in mind the fact that the direct object is the solution of certain substances. Place 2 oz. of powdered buchu in a-vessel and saturate thoroughly with alcohol. Then fit closely on the powder a sheet of blotting-paper, and add alcohol so that the entire amount used is 16 fl. oz.; then very carefully remove the paper so as not to disturb the powder. Now we shall have the principles of solution exemplified exactly as in the previous example, excepting instead of one crystal we have a number of very small fragments, and instead of a perfectly soluble material the substance is only partially soluble, and in addition to other forces we have capillary attraction.
Solutions of different densities quickly form throughout the interstices of the powder. These solutions are in constant motion. They are subject to the forces before mentioned, but by the predominating influence of gravitation the constant tendency of the heaviest solutions is downward, and the densest part of the solution constantly seeks the lowest point. Thus we have new surfaces presented between solvent and material, attended in the first place with a handing downward of the dissolved matter. Apparently, the liquid and the powder are at rest; actually, there is constant motion, and so long as the act of solution progresses the circulation of the menstruum continues. However, these forces cannot extend their influence above the surface of the powder. It may be suggested here that diffusion can effect the mixture. Consequently the liquid within the interstices of the powder may be strongly saturated with dissolved matters, while that just overlying is scarcely contaminated, and that near the surface of the vessel is for some time perfectly pure.