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 M. Morveau ascertained likewise, that the adhesive force is not diminished by removing the pressure of the air.

A great number of experiments on the same subject were made at a still later period, by Mr Achard of Berlin. He measured the force of adhesion between various substances and water. He found, that when the temperature was increased, the adhesion proportionally diminished. He even attempted to determine the diminution occasioned by the elevation of temperature, amounting to a single degree of the thermometer; and gives us a formula denoting that diminution. But it is not necessary to enter into any details respecting his experiments here, for a reason that will appear immediately.

Besides these three philosophers, many others have examined the rise of liquids in capillary tubes, a phenomenon which is nothing else than a peculiar case of adhesion; though we cannot with propriety treat of it here. Laplace published a dissertation on the subject in the year 1805, in which he has given us an historical detail, which however is far from accurate. His reasoning appears to us in general correct; though several very plain propositions are rather obscured than elucidated by his mathematical demonstrations.

When we make experiments on the adhesion of solids to liquids, and endeavour to ascertain the force requisite to separate them from each other, two cases may occur. The solid body may separate from the liquid, dry; or its surface may be covered with a thin coating of the liquid which it retains. If a surface of tallow be placed in contact with water, and separated from it by weights successively introduced into the opposite scale, we shall find, after the separation has taken place, that the surface of the tallow is dry, or that it has not carried along with it a thin coating of the water. But when we employ a fir-board as Dr Brook Taylor did in his experiments, and as Mr Achard did in many of his, the case is very different. We shall find the whole surface of the board thoroughly wetted; that is to say, a thin film of the liquid remains adhering to the wood. Now, it is only the first of these two cases that can be considered as exhibiting the true force of adhesion. In the second case, it is not the solid which separates from the liquid; but one portion of the liquid which separates from the other. Such experiments therefore really shew the force of cohesion between the particles of the liquid; not the force of adhesion between the solid and the liquid. Now, as the experiments of Brook Taylor and Achard belong all to this last case, it is obvious that they cannot be considered as experiments on adhesion. We must therefore leave them out of our consideration at present. The cohesion of the particles of liquids is well known to diminish as the temperature increases; till at a certain temperature this cohesion disappears altogether, and the liquids assume the state of elastic fluids, the particles of which repel each other. Hence, the reason why, in Achard’s experiments, the adhesive force diminished as the temperature increased.

Adhesion is obviously an attractive force, by which the two surfaces are kept in contact. It must evidently increase as the surfaces adhering, because the number of adhering particles increase in the same ratio. This force is insensible, when the two surfaces are at any perceptible distance from one another; so that it acts only at insensible distances. From Morveau’s experiments it appears, that it differs very much in intensity when different solids are made to adhere to the same liquid. Thus gold adheres to mercury, with a force more than twice as great as zinc does, and almost fifty-six times as great as cobalt does. Now these two properties, namely, acting only at insensible distances, and varying in intensity in different bodies, characterize that peculiar force known by the name of chemical affinity. But there is one particular in which chemical affinity appears at first sight to differ from adhesion. Chemical affinity is confined to the ultimate particles or atoms of bodies; whereas, adhesion takes place between surfaces of any size whatever.

But, if we consider that these surfaces consist each of a congeries of atoms united into a large mass by the force of cohesion; that adhesion is not sensible at any perceptible distance, however great the extent of surfaces may be; and that its strength increases in proportion to the surfaces—if we consider these phenomena, we shall find reason to conclude, that adhesion is a force which acts only between the atoms or integrant particles of bodies. It is therefore merely a case of chemical affinity.

The phenomena of adhesion depend upon the strength of affinity between the adhering bodies. If the affinity be weak, the two surfaces will separate by a small force applied, and the solid will retain no impression of the liquid whatever. This happens when cobalt is brought in contact with mercury, or tallow with water. If the affinity be strong, a considerable force will be requisite to separate the two surfaces. This is the case when gold or silver is brought in contact with mercury. So great is the affinity indeed in these cases, that if the adhesion continue for a short time, a combination actually takes place between the two metals. in that case the gold comes away white, or coated over with a film of mercury; the experiment is no longer an example of the force of adhesion between mercury and gold, but exhibits the cohesive force of the particles of mercury to each other. We have even found that this holds with platinum, though it be a metal which has 4amuch weaker affinity for mercury than gold has. If a clean surface of platinum be kept for some time in contact with that of mercury, a very evident amalgamation takes place.

When a surface of wood, marble or metal, comes in contact with water, on removing it we find that surface moist; that is to say, it has carried with it a thin film of water. This shews us, that the adhesive force af water, or the affinity of water to these different bodies, is greater than the cohesive force of the particles of water for each other. Yet this force is not sufficiently strong to produce a chemical combination between the respective bodies. When a surface of sugar or common salt comes in contact with water, this surface is not merely wetted. If the contact be continued for a sufficient time the solid loses its cohesion, and is dissolved by the liquid. This is a complicated case. The water by capillary attraction insinuates itself through the pores of the sugar. The 1em