Every one is familiar with the phenomenon of solution, but few except scientific men really know what a remarkable thing it is. We toss a handful of common salt into some water; in a little while it has entirely disappeared. So far as our sight can determine it has ceased to exist. We can still detect its presence by taste, and by its effects upon other bodies, but until, by the aid of heat or some chemical reagent, we wrench it from the strong grasp of its transparent men-strum we cease to see it. So alcohol absorbs into itself camphor, and other gums or oils, and retains them. Add a little water to these solutions and you will immediately see the dissolved substances reappear like spectres, to again vanish upon the addition of more alcohol. The analytical chemist knows well how to make such appearances and disappearances answer his inquiries, both as to quality and quantity, of any substanca contained in a given mass which he examines. In fact the deportment of substances in solutions in the presence of certain reagents forms the basis of one method of analysis. One of the most conspicuous characteristics of a solution is transparency. This is a test as to whether a solid contained in a fluid is perfectly dissolved. Very concentrated solutions may intercept to a great degree the transmission of light molasses is an example; but if the solution be perfect, thin layers will prove to be transparent. Any opacity or cloudiness is an index that either solid or vesicular matter is present. Solid substances when dissolved are changed into fluids. What is the agent by which the intense cohesion existing between the atoms of the most solid bodies can be so overcome 1 To this question science has, we think, yet given no satisfactory answer. The only means known to us other than solution by which solid bodies can be made fluid is heat. It is a well ascertained fact that heat and cohesion are opposing forces, but in the phenomenon of solution sensible heat does not appear except in such quantity as may be accounted for by the increased density of the entire mass of the solvent and the substance dissolved. In cases where solids placed in contact become liquefied we have decrease of temperature and absorption of heat; an example of this kind of action is the liquefaction of mixed ice and salt. The books account for the phenomenon of solution by classing it among the manifestations of adhesive force. Cohesion is the attraction existing between molecules of the same kind at insensible distances; adhesion is the attraction existing between molecules of different kinds at insensible distances. A very slight consideration of the nature of these attractive forces, and their effects upon the condition of material bodies, will show that solution involves something more than disruption of the particles of a solid by the superior adhesive force of a liquid. A solid body is solid by virtue of the great cohesive force by ' which its particles are held together. When cohesive attraction is nearly or quite in equilibrium with repulsive molecular force, bodies assume the liquid form. Liquids may there-i fore be considered as practically without cohesive attraction, that attraction being neutralized by repulsion. Suppose now , the cohesive force in a solid body to be represented by 4, the j superior adhesive attraction of some liquid for that solid to be j 6, and the cohesive force in the liquid as neutralized by the repulsive force to be nothing. What ought to take place upon the immersion of the solid into the liquid as the result of cohesion and adhesion ? Tha particles of the liquid adjacent to the solid ought to adhere to the solid so strongly that thev could not be removed by an external force without rupturing the solid. If either body be acted upon by an external force, the rupture ought to take place in that body having the least cohesive power, i. e., the liquid. A stick thrust into treacle is a good illustration of this action. When the stick is withdrawn it carries a portion of the treacle with it; the stick is not broken nor any of its particles removed. But it may be said in this case the cohesive force acting between the particles of the wood is greater than the adhesive force of the treacle. Let us then suppose the adhesion of the treacle to the wood to be so powerful that the treacle can not I be removed from the stick except by scraping down into the; body of the wood itself. If solution depends solely upon the fact that adhesion in the liquid is greater that cohesion in the solid, the stick ought in this cage to dissolve. But in order that a substance may dissolve, its particles must not only be seized upon by the particles of the solvent but conveyed away from their position in the solid to new positions in the liquid. We submit that adhesion accounts sufficiently for the seizure but it does not account for the convection. Standing i in a boat by the side of a wharf, a man may clutch a timber attached to the wharf with great force; he may, however, tug in vain to remove it, so long as the want of cohesion in the water upon which his boat is floating affords a resistance less than that which holds the timber to its place. There must be some other principle involved in this matter. j Something perhaps analogous to electrical attraction and repulsion, at least some force acting independently of adhesion which overcomes the cohesion of the solid.
This article was originally published with the title "Solution" in Scientific American 20, 12, 185 (March 1869)