Many solids have the property of condensing gases on their surface (see Absorption of Gases), and polished metallic surfaces, even when long exposed to the air, will be covered with such a gaseous film, which is the first manifestation of chemical affinity. In the process of daguerreotyping, the polished silver plate will be inert unless this film of air has been removed by a polishing process just before the operation. Such removal of air may be made visible on the surface of a glass mirror which has not been rubbed for some time, by drawing a few figures or letters on it with a clean finger; the invisible change of surface will become visible by breathing on the glass, when the appearance of the deposit of watery vapor will show where the air film has been removed by friction. The adhesion of gases to solids is further illustrated by the small air bubbles which are often visible in mineral waters and effervescent drinks, sticking to the sides of the glass vessel in which they are contained, and not rising to the surface notwithstanding they are some 6OO times lighter than the liquid.
This adhesion is also illustrated by heavy powders and even sewing needles floating on water; the air adhering around the needle prevents the adhesion of the water, and the latter by its own cohesion forms a hollow depression in which the needle floats. The same adhesion of air around a piece of solid iron causes it to float on melted iron, notwithstanding it has not a less specific gravity than the fluid material. The mutual adhesion of solids and gases is also illustrated by the floating of particles of dust in the air; subdivision of matter increasing the surface, a continued subdivision will at last cause a point to be reached where the surface adhesion overcomes gravitation. In the vacuum of the air pump the dust falls down like a heavy body. It is the same with smoke, as this consists of solid particles carried upward by a current of heated air; the white smoke evolved by the burning of magnesium, zinc, or phosphorus illustrates this point very plainly. 5. Gases to liquids. The adhesion of gases to the surface of liquids is stronger in proportion to their solubility or absorption by the liquid. So carbonic acid adheres to water with greater force than air; but air possesses very strong adhesion to water, as shown by the currents of air carried down by any considerable cataract.
Use is made of this adhesion in the so-called water bellows, in which a stream of water falling through a wide tube carries air downward and produces a blast so strong that this principle was used for driving the drills during the boring of the Mont Cenis tunnel. In the so-called atomizer a current of air is used to divide water into a fine spray. In the Giffard injector a blast of steam is used to carry water by its adhesion to it into the boiler against its own pressure. The adhesion of air to water is further illustrated by the friction of a strong wind on its surface, which not only pushes it forward, but creates the waves. If oil is spread over the water, the air finds a surface for which it has little adhesion, and glides easily over it. This is the cause of the quieting influence of oil upon ocean waves, of which advantage has occasionally been taken in a storm by vessels having oil on board; the oil will spread at once over a large surface. The peculiar motions of camphor on water, phosphorus on mercury, etc, belong to the same class of phenomena; it is the elective affinity of the vapors of these volatile substances for the liquid on which they float, which is the cause of a strong and unequal evaporation at the points in contact, the evolving gas or vapor pushing the floating solid onward by its mechanical reaction.
When the water is touched with a substance containing the merest trace of oil or grease, the motion of the camphor stops at once, as the water becomes then at once covered with a very thin but strongly adhering oil film, which has no affinity for the vapor of the camphor. A similar action is seen as soon as mercury is covered with a film of phosphorus; removing this film with the edge of a knife, the motion recommences at once, and is visible in the dark. The mutual adhesion of liquids and gases is also illustrated by the floating of watery particles in the atmosphere, as is seen in clouds and fogs. Watery vapor, present in the air in an invisible condition, becomes visible as soon as condensation commences, when a kind of fine water dust is formed, identical with the spray of large cataracts where it originates by mechanical means. These watery particles are kept floating simply by their adhesion to the air, the total surface being very large compared with the total weight; but when the particles, by contact and mutual adhesion, form larger bodies, the total surface diminishes in proportion to the weight, while finally their gravitation becomes greater than their adhesion, and they fall down like rain.
This falling down of water dust not only takes place in a vacuum, but even a trifling diminution in the atmospheric pressure will cause it when the amount of watery vapor in the air is large, as seen in the fact that rain is usually preceded by a descent of the mercurial column of the barometer. A descent of temperature is also a cause of this condensation of watery vapor, as it diminishes the capacity of the air for holding it. This adhesion theory makes the hypothesis of De Saussure quite unnecessary. This savant imagined that the particles of watery vapor were supported in the clouds by being hollow, with a vacuum inside, and thus, being lighter than the same volume of air, they were supported like a balloon. He was strengthened in this notion by the microscopic illusion which often causes solid small spheres to appear as if hollow. Still such hollow spheres are occasionally seen like microscopic soap bubbles, but they have air inside, are heavier than the air, and are only supported by adhesion. 6. Gases to gases. The interpenetration of gases being very great, it is impossible to keep their surfaces distinct; there must consequently be much adhesion and friction between them.
Direct experiment with two gases cannot well be made, but observation demonstrates this great adhesion and friction. So one small jet of air in a wide tube will cause a rush of air to follow; the exhaust steam blown upward in the locomotive flue causes the air to rush out with it, and so creates the draft necessary to keep up a sufficient heat. This adhesion of gases plays no doubt a most important part in the actions of the atmosphere in the economy of nature.