COHESION
, one of the four species of attraction, denoting that force by which the parts of bodies adhere or stick together.
This power was first considered by Newton as one of the properties essential to all matter, and the cause of all that variety observed in the texture of different terrestrial bodies. He did not, however, absolutely determine that the power of cohesion was an immaterial one; but that it might possibly arise, as well as that of gravitation, from the action of another. His doctrine of cohesion Newton delivers in these words: “The particles of all hard homogeneous bodies, which touch one another, cohere with a great force; to account for which, some philosophers have recourse to a kind of hooked atoms, which in effect is nothing else but to beg the thing in question. Others imagine that the particles of bodies are connected by rest, i. e. in effect by nothing at all; and others by conspiring motions, i. e. by a relative rest among themselves. For myself, it rather appears to me, that the particles of bodies cohere by an attractive force, whereby they tend mutually toward each other: which force, in the very point of contact, is very great; at little distances is less, and at a little farther distance is quite insensible.”
It is uncertain in what proportion this force decreases as the distance increases: Desaguliers conjectures, from some phenomena, that it decreases as the biquadratic or 4th power of the distance, so that at twice the distance it acts 16 times more weakly, &c.
“Now if compound bodies be so hard, as by experience we find some of them to be, and yet have a great many hidden pores within them, and consist of parts only laid together; no doubt those simple particles which have no pores within them, and which were never divided into parts, must be vastly harder. For such hard particles, gathered into a mass, cannot possibly touch in more than a few points: and therefore much less force is required to sever them, than to break a solid particle, whose parts touch throughout all their surfaces, without any intermediate pores or interstices. But how such hard particles, only laid together, and touching only in a few points, should come to cohere so firmly, as in fact we find they do, is inconceivable; unless there be some cause, whereby they are attracted and pressed together. Now the smallest particles of matter may cohere by the strongest attractions, and constitute larger, whose attracting force is feebler: and again, many of these larger particles cohering, may constitute others still larger, whose attractive force is still weaker; and so on for several successions, till the progression end in the biggest particle, on which the operations in chemistry, and the colours of natural bodies, do depend; and which by cohering compose bodies of a sensible magnitude.”
Again, the opinion maintained by many is that which is so strongly defended by J. Bernoulli, De Gravitate Ætheris; who attributes the cohesion of the parts of matter to the uniform pressure of the atmosphere; confirming this opinion by the known experiment of two polished marble planes, which cohere very strongly | in the open air, but easily drop asunder in an exhausted receiver. However, if two plates of this kind be smeared with oil, to fill up the pores in their surfaces, and prevent the lodgment of air, and one of them be gently rubbed upon the other, they will adhere so strongly, even when suspended in an exhausted receiver, that the weight of the lower plate will not be able to separate it from the upper one. But although this theory might serve tolerably well to explain the cohesion of compositions, or greater collections of matter; yet it falls far short of accounting for that first cohesion of the atoms, or primitive corpuscles, of which the particles of hard bodies are composed.
Again, some philosophers have positively asserted, that the powers, or means, are immaterial, by which matter coheres; and, in consequence of this supposition, they have so refined upon attractions and repulsions, that their systems seem but little short of scepticism, or denying the existence of matter altogether. A system of this kind is adopted by Dr. Priestley, from Messrs. Boscovich and Michell, to solve some difficulties concerning the Newtonian doctrine of light. See his History of Vision, vol. 1. pa. 392. “The easiest method,” says he, “of solving all difficulties, is to adopt the hypothesis of Mr. Boscovich, who supposes that matter is not impenetrable, as has been perhaps universally taken for granted; but that it consists of physical points only, endued with powers of attraction and repulsion in the same manner as solid matter is generally supposed to be: provided therefore that any body move with a sufficient degree of velocity, or have a sufficient momentum to overcome any powers of repulsion that it may meet with, it will sind no difficulty in making its way through any body whatever; for nothing else will penetrate one another but powers, such as we know do in fact exist in the same place, and counterbalance or over-rule one another. The most obvious difficulty, and indeed almost the only one, that attends this hypothesis, as it supposes the mutual penetrability of matter, arises from the idea of the nature of matter, and the difficulty we meet with in attempting to force two bodies into the same place. But it is demonstrable, that the first obstruction arises from no actual contact of matter, but from mere powers of repulsion. This difficulty we can overcome; and having got within one sphere of repulsion, we fancy that we are now impeded by the solid matter itself. But the very same is the opinion of the generality of mankind with respect to the first obstruction. Why, therefore, may not the next be only another sphere of repulsion, which may only require a greater force than we can apply to overcome it, without disordering the arrangement of the constituent particles; but which may be overcome by a body moving with the amazing velocity of light?”
Other philosophers have supposed that the powers both of gravitation and cohesion are material; and that they are only different actions of the etherial fluid, or elementary sire. In proof of this doctrine, they allege the experiment with the Magdeburg hemispheres, as they are called. The pressure of the asmosphere we see is, in this case, capable of producing a very strong cohesion; and if there be in nature any fluid more penetrating, as well as more powerful in its effects, than the air we breath, it is possible that what is called the attraction of cohesion may in some measure be an effect of the action of that fluid. Such a fluid as this is the element of fire. Its activity is such as to penetrate all bodies whatever; and in the state in which it is commonly called fire, it acts according to the quantity of solid matter contained in the body. In this state, it is capable of dissolving the strongest cohesions observed in nature. Fire, therefore, being able to dissolve cohesions, must also be capable of causing them, provided its power be exerted for that purpose, which possibly it may be, when we consider its various modes or appearances, viz, as fire or heat, in which state it consumes, destroys, and dissolves; or as light, when it seems deprived of that destructive power; and as the electric fluid, when it attracts, repels, and moves bodies, in a great variety of ways. In the Philos. Trans. for 1777 this hypothesis is noticed, and in some measure adopted by Mr. Henly. “Some gentlemen (says he) have supposed that the electric matter is the cause of the cohesion of the particles of bodies. If the electric matter be, as I suspect, a real elementary sire inherent in all bodies, that opinion may probably be well founded; and perhaps the soldering of metals, and the cementation of iron, by fire, may be considered as strong proofs of the truth of their hypothesis.”
But whatever the cause of cohesion may be, its effects are evident and certain. The different degrees of it constitute bodies of different forms and properties. Thus, Newton observes, the particles of fluids, which do not cohere too strongly, and are small enough to render them susceptible of those agitations which keep liquors in a fluor, are most easily separated and rarefied into vapour, and make what the chemists call volatile bodies; being rarefied with an easy heat, and again condensed with a moderate cold. Those that have grosser particles, and so are less susceptible of agitation, or cohere by a stronger attraction, are not separable without a greater degree of heat; and some of them not without fermentation: and these make what the chemists call fixt bodies.
Air, in its fixed state, possesses the interstices of solid substances, and probably serves as a bond of union to their constituent parts; for when these parts are separated, the air is discharged, and recovers its elasticity. And this kind of attraction is evinced by a variety of familiar experiments; as, by the union of two contiguous drops of mercury; by the mutual approach of two pieces of cork, floating near each other in a bason of water; by the adhesion of two leaden balls, whose surfaces are scraped and joined together with a gentle twist, which is so considerable, that, if the surfaces are about a quarter of an inch in diameter, they will not be separated by a weight of 100 lb; by the ascent of oil or water between two glass planes, so as to form the hyperbolic curve, when they are made to touch on one side, and kept separate at a small distance on the other; by the depression of mercury, and by the rise of water in capillary tubes, and on the sides of glass vessels; also in sugar, sponge, and all porous substances. And where this cohesive attraction ends, a power of repulsion begins.
To determine the force of cohesion, in a variety of different substances, many experiments have been made, and particularly by professor Muschenbroek. The ad- | hesion of polished planes, about two inches in diameter, heated in boiling water, and smeared with grease, required the following weights to separate them:
| Cold grease | Hot grease | |
| Planes of Glass | 130 lb | 300 lb |
| Brass | 150 | 800 |
| Copper | 200 | 850 |
| Marble | 225 | 600 |
| Silver | 150 | 250 |
| Iron | 300 | 950 |
But when the Brass planes were made to adhere by other sorts of matter, the results were as in the following table:
| With | Water | 12 | oz |
| Oil | 18 | ||
| Venice Turpentine | 24 | ||
| Tallow Candle | 800 | ||
| Rosin | 850 | ||
| Pitch | 1400 |
In estimating the Absolute Cohesion of solid pieces of bodies, he applied weights to separate them according to their length: his pieces of wood were long square parallelopipedons, each side of which was .26 of an inch, and they were drawn asunder by the following weights:
| Fir | 600 | lb |
| Elm | 950 | |
| Alder | 1000 | |
| Linden tree | 1000 | |
| Oak | 1150 | |
| Beech | 1250 | |
| Ash | 1250 |
He tried also wires of metal, 1-10th of a Rhinland inch in diameter: the metals and weights were as follow:
| Of | Lead | 29 1/4 | lb |
| Tin | 40 1/4 | ||
| Copper | 299 1/4 | ||
| Yellow Brass | 360 | ||
| Silver | 370 | ||
| Iron | 450 | ||
| Gold | 500 |
He then tried the Relative Cohesion, or the force with which bodies resist an action applied to them in a direction perpendicular to their length. For this purpose he fixed pieces of wood by one end into a square hole in a metal plate, and hung weights towards the other end, till they broke at the hole: the weights and distances from the hole are exhibited in the following table.
| Distance | Weight | ||
| Pine | 9 1/2 inc | 36 1/2 | oz |
| Fir | 9 | 40 | |
| Beech | 7 | 56 1/2 | |
| Elm | 9 | 44 | |
| Oak | 8 1/2 | 48 | |
| Alder | 9 1/4 | 48 |
