By WILLIAM ACKROYD, F.I.C.
It is customary to regard cohesion as the force which binds together molecules of the same substance, and in virtue of which the particles of solids and liquids are kept together, and also to speak of the attraction exerted between particles of two different bodies as adhesion. The distinction between cohesion and adhesion is a conventional one. The similarity, if not identity, of the two forces is demonstrated by the fact that while cohesion is exerted between particles of the same body, adhesion is exerted with most force between particles of allied bodies. Generally speaking, organic bodies require organic solvents; inorganic bodies, inorganic solvents. For example, common salt is highly soluble in water, but not in ether, and many fats are soluble in ether, but not in water. So many cases like these will suggest themselves to the chemist that I am justified in making the following generalization: A body will dissolve in a solvent to which it is allied more readily than in one to which it in highly dissimilar. Exceptions to the law undoubtedly exist, but none so striking as the following in support of it, viz., that the metal mercury is the only known true solvent for many metals at the normal temperature.
From this standpoint the whole subject of solution is deserving of fresh attention, as it appears highly probable that, just as Prof. Carnelley has shown by the use of my meta-chromatic scale, the colors of chemical compounds come under definite laws, which he has discovered and formulated in connection with Mendeleeff and Newlaud's periodic law,2 so, likewise, may the solubility of an allied group of compounds, in regard to any given solvent under constant conditions of temperature, conform to similar laws; that, e.g., the chlorides of H, Na, Cu, and Ag, in Mendeleeff's Group I., may vary in their solubility in water from an extreme of high solubility in the case of hydrogen chloride to the opposite extreme of comparative insolubility in the case of silver chloride. In this natural series of compounds, hydrogen chloride is the body nearest akin to water, and silver chloride the most remote in kinship.
It is in virtue of cohesion that a freely suspended drop of liquid assumes the spherical form. If such a sphere be dropped on to the surface of a liquid of higher specific gravity at rest, one obtains what is called the cohesion figure of the substance of the drop. A drop of oil, e.g., spreads out on the surface of water until it is a circular thin film of concentric rings of different degrees of thickness, each displaying the characteristic colors of thin plates. The tenuity of the film increases; its cohesion is overcome; lakelets are formed, and they merge into each other. The disintegrated portions of the film now thicken, the colors vanish, and only islets of oil remain. Some liquid drops of the same or higher sp. gr. than water do not spread out in this fashion, but descend below the surface of the liquid, and, in descending, assume a ring shape, which gradually spreads out and breaks up into lesser rings. Such figures have been termed submergence cohesion figures; they are vortex rings. I have solidified such vortex rings in their first stage of formation. If drops of melted sulphur, at a temperature above that of the viscous state, be let fall into water, the drops will be solidified in the effort to form the ring, and the circular button, thick in the rim and thin in the center, may be regarded as a solidified vortex ring of plastic sulphur.
It may be shown that the conditions of the formation of a submergence cohesion figure are those which exist in the formation of an aerial vortex. Those conditions in their greatest perfection are (1) a spherical envelope of a different nature from the medium in which the rings are produced; (2) a circular orifice opening into the medium; and (3) a percussive impact on the part of the sphere opposite the orifice. In the production of vortex rings of phosphorus pentoxide in the making of phosphoreted hydrogen, the spherical envelope is water, the orifice the portion of the bubble which opens into the air immediately it rises to the surface, and the impact is furnished by gravity. So, also, in the case of a submergence cohesion figure, the spherical envelope is the air surrounding the drop, the orifice the portion of it which first comes in contact with the liquid at rest; and here again the impact is due to gravity more directly than in the former case. These conditions are somewhat imperfectly copied in the ordinary vortex box, which is usually cubical in form, with a circular orifice in one side, and a covering of canvas on the opposite one, which is hit with the fist.
Notes from a lecture given to the Halifax Scientific Society, July 19, 1886.
Philosophical Magazine, August, 1884.