ADHESION denotes an union, to a certain degree, between two distinct substances, and differs from cohesion, (with which the former word is often con founded,) 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 of 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 Musschenbroilk, 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 pounds to separate them ; pieces of lead, of the same area of surface, required 275 pounds ; and soft iron, 300 pounds. The experiments made and described by Mr. Martin, in the Philosophia Britannica, make the force of this kind of adhesion much greater than Musschenbrak. He took two leaden balls, and having careftilly 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 pounds weight. The adhesion between two brass planes 4.1 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 per fectly 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 Mecha nics' Magazine. The following is a condensed account of them :— Annum( or IRON NAILS, in which Mr. Bevan's object was to determine, first, the adhesive force of different kinds of nails, when driven into wood of different species : second, the actual weight, without impulse, necessary to force a nail a given depth : third, the force requisite to extract a nail when so driven.
Mr. Bevan observes, that the theoretical investigation points out an inequality of resistance to the entrance and extraction of a nail, supposing the thickness to be invariable ; but as the general shape of nails is tapering towards their points, the resistance of entrance necessarily becomes greater than that of extraction ; in some experiments he found the ratio to be about 6 to 5. The following Table exhibits the relative adhesion of nails of various kinds when driven into dry Christiana deal, at right angles to the grain of the wood.
The percussive force required to drive the common sixpenny nail to the depth of 14 inch into dry Christiana deal, with a cast-iron weight of 6.275 lbs. was four blows or strokes falling freely the space of 12 inches; and the steady pressure to produce the same effect, was 400 lbs. A sixpenny nail driven into dry elm, to the depth of one inch across the grain, required a pressure of 327 lbs. to extract it; and the same nail driven end-ways or longitudinally into the same wood, was extracted with a force of 257 lbs. The same nail driven 2 inches end-ways into dry Christiana deal, was drawn by a force of 257 lbs.; and to draw out 1 inch, under like circumstances, took 87 lbs. only. The relative adhesion, therefore, in the same wood, when driven transversely and longitudinally, is 100 to 78, or about 4 to 3 in dry elm, and 100 to 46, or about 2 to 1 in deal ; and in like circumstances, the relative adhesion to elm and deal is as 2 or 3 to 1. The progressive depths of a sixpenny nail into dry Christiana deal, by simple pressure, were us follows:— One quarter of an inch, a pressure of 24 lbs.
Half an inch 76 —• One inch 235 One inch and a-half 400 Two inches 610 In the above experiments, great care was taken by Mr Bevan to apply the weights steadily, and towards the conclusion of each experiment, the additions did not exceed 10 lbs. at one time, with a moderate interval between, generally about one minute, sometimes ten or twenty minutes. In other species of wood, the requisite force to extract the nail was different. Thus, to extract a common sixpenny nail, from a depth of one inch, out of Dry oak, required 507 lbs.
Dry beech 667 Green sycamore . 312 From these experiments, we may infer that a common sixpenny nail driven two inches into dryoak, would require a force of more than half a ton to extract it by a steady force.