COHESION-FIGURES, a remarkable class of figures produced in liquids by the action of their natural cohesive attraction for the surfaces of other liquids or solids on which they are deposited, or by induced cohesive attraction effected by the means of electricity. They may be described under four heads: (1) The surface-cohesion figures of Tomlin son; (2) The submersion-figures of Tomlinson; (3) The breath-cohesion figures of Stret hill Wright; (4) The electric-cohesion figures of Strethill Wright.
1. The Surface-cohesion Figures of Tomlinson. —This class of figures was submitted to the chemical section of the British association by .11r. Charles Tomlinson of King's col lege, London, in 1851. He showed that a drop of an " independent liquid," such as an oil, alcohol, or ether, when gently placed upon chemically clean water, spreads itself out into a definite figure as it enters into solution or diffuses itself over the surface. lie stated that each figure is characteristic of the fluid employed, and that any change in the chemical or molecular state of the fluid is attended with a corresponding change in its " cohesion-figure." Hence he recommended that these figures should be applied to the qualitative analysis of various liquids whose ordinary methods of testing were inoperative or inadequate. Cohesion analysis, performed as he directs, has been applied with signal success in the verification of oils and balsams, and as a ready means of indi cating the changes which take place in those bodies by age or oxidation. The cohesion figures of Tomlinson, from their great beauty and variety, combined with the exquisite harmony of coloring displayed by many of them, have been employed, like those of the kaleidoscope of Brewster, to suggest forms for the pattern-designer.
In the production of cohesion-figures, water was the receiving surface generally employed by their discoverer; but in certain cases he also employed other fluids, such as mercury, acetic acid, cocoanut-oil and castor-oil in the cold state; and spermaceti, white wax, lard, and sulphur in a state of fusion. On each of these substances, the liquid to be tested formed a different and characteristic figure, and hence additional means of comparison and verification were afforded.
2. the Submersion-figures of Tomlinson.—In the Philosophical _Magazine for June, 1864, this author brought forward a new series of cohesion-figures of liquids, in which the drop, being of great specific gravity, instead of forming a figure on the surface, sank beneath it, and formed a figip•e as it slowly made its way to the bottom of the vessel. In order to exhibit these phendmena, lie employed a column of liquid in a cylindrical vessel. He states that a solution of cochineal dropped into water formed a figure typi cal of a large class of these figures. A drop laid on the surface sank down, opened into a ring, which became depressed at two opposite points; from these points, lines of fluid deseended,which terminated in secondary rings; the secondary rings. in like man ner, drooped down into lines carrying tertiary rings, and so on, until the lower part of the vessel became crowded with a complicated system of drooping rings and lines. Oil of lavender in a column of alcohol, fusel oil in paraffin oil, in benzoic, in ether, etc., gave various and distinctive figures. He found that similar figures were obtained by the use of oils dropped into columns of hot spermaceti, lard, wax, etc.; and that these figures underwent considerable variation under the influence of change of temperature. We have already stated that each of these figures, whether surface-figures or submer sion-figures, is characteristic of the fluid which forms it; and Tomlinson considers, with regard to the that it is a function of the cohesive force and diffusibility of the liquid, and the adhesion of the surface on which it is deposited; he also considered that it might be a function of the solubility and the diffusibility of the fluid in question, or of the solubility, the density, and the molecular attraction; while with regard to the submersion figure, he thought each figure to be a function of the solubility, the density, and the molecular attraction.