Adhesion denotes a union, to a certain degree, between two distinct substances, and differs from cohesion, (with which the former word is often confounded,) inasmuch, as the latter term is alone properly applicable to the retaining together of the component particles of the same mass. Adhesion is, however, of two kinds; the one, a species of natural attraction, which takes place between the surfaces of bodies, whether similar or dissimilar, and which, in a certain degree, connects them together; the other, the joining or fastening together or two or more bodies, by the application of external force. With respect to the first-mentioned, it has been proved, that the power of adhesion is proportionate to the number of touching points; and this, in solid bodies, depends upon the degree in which their surfaces are polished and compressed. The effects of this power are extremely curious, and in many instances astonishing. It is stated by Musschenbroek, that two cylinders of glass, of rather less than 2 inches diameter, being heated to the temperature of boiling water, and brought into contact, with melted tallow between their surfaces, required a force of 130 lbs. to separate them; pieces of lead, of the same area of surface, required 275 lbs.; and soft iron, 300 lbs.
The experiments made and described by Mr. Martin, in the Philosophia Britannica, make the force of this kind of adhesion much greater than Musschenbroek. He took two leaden balls, and having carefully scraped off, with the edge of a sharp pen-knife, so much of their spherical surfaces as to form two planes of one-thirtieth of an inch in area, he pressed them together forcibly, and with a gentle turn of the hand. The adhesion of these small surfaces was such, that he lifted, with the balls so united, above 150 lbs. weight. The adhesion between two brass planes 41/4 inches in diameter, with grease smeared over their surfaces, was such, that he could never meet with two men strong enough to separate them by pulling against each other. The editor of this work had put into his hand many years ago two brass plates, of about 2 inches diameter, having their surfaces so perfectly flat, that, without any interposing matter, he could only separate them by sliding them edgeways. With respect to the second-mentioned kind of adhesion, some useful experiments were made by Mr. B. Bevan, on the adhesive force of iron nails, screws, and pins; also of the common cements, glue, and sealing-wax, which that gentleman communicated to the editor of the London Mechanics' Magazine.
The following is a condensed account of them: -