Adhesion (Lat. ad, to, and haerere, to stick), the force by which the particles of different bodies stick together, distinguished from cohesion, which is the force that holds the molecules of the same body together. There are six kinds of adhesion: solids to solids, liquids to solids, liquids to liquids, gases to solids, gases to liquids, and gases to gases. 1. Solids to solids. Two glass or metal plates with well ground surfaces, when pressed together, will adhere with such force that the upper one will not only support the lower, but an additional weight will be required to separate them. The amount of this adhesive force has been measured by recording the weights necessary for their separation. The records of the old experimenters on this subject are worthless, because they placed a lubricating fluid, oil or fat, between the plates; they found thus the cohesion of the oil or fat, and not the adhesion of the plates. In later times Prechtl in Germany has made the most careful experiments in this line; he took polished metal plates of 1 1/2 inch diameter, suspended the upper one to a balance, brought it to an equilibrium in a horizontal position, and attached the lower plate to a support underneath it.
Both plates were then brought in contact, so that the flat polished surfaces covered one another perfectly, and the weights required in the scale at the other end of the balance beam to separate the plates were the measures of the adhesion.
Prechtrs Adhesion Balance.
He found thus the following remarkable law: The adhesion between two plates of the same material is the same as that between one of the plates and any material which possesses a less adhesive force. For instance, to separate two copper plates required a weight of 21 grains; but the same weight was required to separate one of the copper plates from a plate of bismuth, zinc, tin, lead, etc, notwithstanding the adhesive force of bismuth to bismuth, zinc to zinc, etc, was found to be smaller than that of copper to copper. Prechtl found also that an attraction of the plates manifested itself at an appreciable distance before actual contact, and he even measured the amount of this attraction at the distance of 1/24 of an inch by means of weights in fractions of grains. The suspended plate when brought within this distance was attracted with an accelerated motion till the contact took place with a slight concussion. The idea that the pressure of the air was the chief cause of the adhesion of two such plates, as it is in the case of the well known experiment with the Magdeburg hemispheres, was set at rest by Boyle, who suspended the adhesive plates charged with weight in the vacuum of an air pump: the plates were not separated, while the hemispheres held together by the vacuum alone fell apart.
The adhesion of solids to solids is also seen in the dust, which will not only adhere to perpendicular but even to inverted surfaces. Granite consists of feldspar, quartz, and mica, kept together by adhesion. A portion of such apparently adhesive force is, however, cohesion. For instance, brick and mortar adhere chiefly by the cohesion of the mortar, which penetrates the pores of the brick; stones without sensible pores do not adhere so well to mortar. 2. Liquids to solids. Taylor was the first who investigated this subject in a scientific manner. He suspended a polished plate on the balance as above described, and brought it carefully down on the surface of a liquid, when it adhered, and the adhesion was measured by the weight required to separate the plate. After this method Guyton de Morveau in Paris found that plates of a French inch in diameter had the following adhesive power to mercury: gold, 446 grains; silver, 429; tin, 418; lead, 397; bismuth, 372; zinc, 204; copper, 142; antimony, 126; iron, 115; cobalt, 8; cold platinum, 108; red-hot platinum, 10 grains. Taylor also believed that the pressure of the air was the main cause, but Guyton found nearly the same results in the vacuum of the air pump.
Link took a polished plate of agate of nearly one inch diameter, and tested its adhesion to different liquids; he found for water, 25 grains; sulphuric acid, 29; hydrochloric acid, 25; solution of saltpetre, 23; of lime, 21; almond oil, 10; petroleum, 10; turpentine and alcohol, 15; ether, 10. Where in many of these experiments drops of the liquid adhere to the plate used, it proves that the adhesion of the liquid to the solid is stronger than the cohesion of the liquid itself, and that the numbers obtained express rather the cohesion of particles of the liquid which were separated by the weight, than the adhesion of the plate to the liquid. The ascent of liquids in capillary tubes is also a result of adhesion, as well as the spreading out of liquids between two surfaces kept in close proximity. The chain pump, in which the water is carried up by a simple chain in a tube, is a practical application of adhesion. Prevost made interesting experiments on elective adhesion, showing how one fluid will drive another away from a surface for which it has more adhesion. He found that they displace one another in the following order: ether, alcohol, oil of bergamot, poppy oil, olive oil, nut oil, and other oils, water. Pure water displaces in its turn solutions of salts and alkaline earths.
Camphor drives a film of water away from a surface, and pieces of camphor placed on water will show a peculiar motion; the same is seen with camphor or phosphorus placed on pure mercury. These phenomena are due to the evaporation of the solid and the cohesion of its vapor. We see practical applications of the adhesion of liquids to solids in writing, painting, printing, dyeing, washing, and elutriation, or separation of coarse from fine powders by suspension and settling in a large quantity of water. 3. Liquids to liquids. If a drop of water is placed on mercury, or a drop of oil placed on water, it does not keep its round form, but spreads out at once, because its adhesion to the liquid surface is greater than the cohesion of its particles. A drop of water on an oily surface, however, will not spread out, as the cohesion of its particles is greater than its adhesion to the oil. The manner of displacement of one liquid by another having greater adhesive force to the liquid they float on, gives rise to a series of phenomena, for the study and exhibition of which Prof. Morton of the Stevens institute at Hoboken has recently contrived an apparatus in the style of a magic lantern. 4. Gases to solids.