FLUIDS, ELEVATION OF.
WHEN a solid body is partially plunged in a When a solid body is partially plunged in a fluid, the level surface near it is disturbed, and the fluid is observed either to ascend or descend, so as to form a ring round the part immersed. If a tube of glass be inserted in a vessel containing water, the liquid will rise in a concave ring both on the outside and the inside; and if the tube be small enough, the cylinder of water within it will be elevated above the general level, and the elevation will be greater nearly in the same proportion that the bore is less. On the other hand, if the tube be plunged in mer cury, the fluid in contact with the glass will be de pressed, forming a hollow ring with the convexity upward ; and when the diameter is very small, the cylinder of mercury in the inside will sink below the level on the outside. In all these appearances the physical cause is the same, and it has received the name of CAPILLARY Amos, because its effects are most remarkable in the case of tubes with ex tremely minute diameters.
No part of Natural Philosophy has been the sub ject of a greater variety of researches than Capillary Action. It has been viewed in almost every pos sible light, and it would be difficult to suggest a new principle that has not been proposed by some phi losopher in order to account for the observed ap pearances. One advantage has resulted from re peated discussion ; for by this means the true cause of the phenomena is no longer doubtful, although there is still considerable difference of opinion with regard to the manner in which the effects are pro duced. It is now universally allowed, that the sus pension of fluids in capillary tubes is to be ascribed to the attraction observed to take place between the elementary particles of which bodies are composed.
We shall not stop to detail the different experi ments which prove the reality of this attractive force, and we shall at once assume that the two fol lowing facts, which are the fundamental principles of this theory, are fully established ; namely, that glass and other solid bodies attract the particles of fluids with which they are in contact, and, that the particles of fluids attract one another. Admitting
these two kinds of attraction, it remains to investi gate the consequences that flow from them.
2. Corpuscular attraction acts with great intensity in contact, or at the nearest distances, but it de creases very rapidly as the distance increases, and, on the whole, is confined within a very small range. Clairaut supposed that the sides of a capillary tube extend their action to the central parts of the con tained cylinder of fluid. But in this opinion lie is singular. All other philosophers confine the sphere of attraction within much narrower limits. They suppose that the corpuscular force has produced its full effect, and has become evanescent, at a distance so small that it cannot be appreciated by the senses. But from this, we are not to conclude that a particle attracts those only which are quite contiguous to it ; its action, although confined within a sphere of a very small radius, nevertheless extends to some dis tance, and reaches to the particles beyond the nearest.
As corpuscular attraction extends its influence to a distance, it must vary, within the sphere of its ac tion, according to some law, which is unknown, and in all probability will never be discovered. But a knowledge of this law is not necessary to explain the capillary phenomena ; for these are caused by the accumulated action of the force in its whole range, and are independent of the intermediate va riations of intensity which it may undergo. In this respect, capillary action resembles the attraction by which transparent bodies refract the rays of light. In both cases, what we observe is the total effect of the attractive energy, which may remain the same, although the intermediate degrees of intensity be in finitely varied.