ADHESION, or ADHERENCE, from ad and lucre°, to stick, is properly the condition of two separate bodies, when they are held together by the mutual attraction of their particles. It is generally employed to denote the force by which the surfaces of solids adhere to the surfaces of fluids, in consequence of their mutual at traction.
Our countryman Dr B. Taylor seems to have been the first who made this branch of physics a subject of in vestigation. He observed so early as 1713, the ascent of water between two glass plates, and from his expe riments on the adhesion of surfaces, he concluded, that the adhesive force might be measured by the weight which was necessary to separate the contiguous sur faces. He found, that a square inch of wood required fifty grains to raise it, and that the adhesive force was always in the direct ratio of the surface in contact with the fluid. It was the opinion of some other philoso phers, that this adhesion was owing to atmospherical pressure, and therefore no new experiments were in stituted till 177S, when M. Guyton Morveau investigat ed the subject with the greatest ardour and success. He suspended a polished disc of glass, thirty lines in dia meter, to the arm of a balance, and found that a weight of nine gross, and a few grains, was necessary to separate it from the surface of mercury. The apparatus was then placed in the exhausted receiver of an air-pump, and the same counterpoise, as before, was necessary to dis unite the mercury and glass. This experiment, which clearly showed that the pressure of the air was not the cause of adhesion, was followed by a variety of others, from which Morveau obtained very interesting results. The metals which he employed, were in the greatest purity; they were each an inch in diameter, well polish ed, and of the same thickness, and had a small ring in their mitre, by which they were suspended at the arm of an assay balance. With this apparatus, he obtained the following results.
To the preceding 'fable, we have added the specific gravities of the metals, in order to show that the force of adhesion is in no respects connected with the densi ties of the solids. The adhesive force of each metal
seems to be proportional to its solubility in mercury ; and therefore the numbers which express that force in the different metals, may be regarded as an approxima tion towards the ratio of the relative affinities of thc me tals for mercury.
Dr Taylor's method of ascertaining the force of adhe sion was also employed by Mr Achard of Berlin, who has made an immense number of interesting experi ments upon this subject. By varying the atmospherical pressure, he found no change in the adhesive force of glass and water ; but when fluids of different tempera tures were employed, he found that their adhesion to so lids was uniformly in the inverse ratio of the temperature. The relation between the adhesion of glass, and tho temperature of water, he expresses by an equation similar 16 3 to the following : t = 53O—a, or a = 3 16 and the table which Mr Achard has computed from these formula for different degrees of Sulzer's thermometer, accords wonderfully with the experimental results. From these results, it appears that for every additional degree of heat of Sulzer's thermometer, (equal to 1.1565 of Fahrenheit,) the adhesive force diminishes 0.1858 grains according to experiment, or a grain for every degree of Fahrenheit ; and 0.1876 according to calculation. This diminution in the force of adhesion is ascribed by Morveau to the diminution of the fluid's density in consequence of its rarefaction by heat, from which it happens that the disc of glass comes in con tact with fewer points of the fluid.
In order to ascertain the relation between the adhesive force and the area of the surface of the solid that is brought into contact with the fluid, Mr Achard em ployed discs of glass, from 1.5 to 7 inches in diameter, and found that the force was in the direct ratio of the squares of the surfaces.