It has been said that the mixture of certain different fluids produces a diminution of volume; but it must be observed that a contrary effect frequently takes place. Some of the metals, when mixed together in a melted state, produce a volume greater than the sum of the component volumes; and melted metals, on becoming solid, like water on being frozen, expand in volume. The latter effect may arise from the crystals, on being formed, placing themselves across one another so as to leave comparatively large intervals ; but the other can only be caused either by a diminution of the attractive power which the particles exert on one another, or by its being changed into a power of repulsion. One of these latter circumstances must also be the cause of the great augmentation of volume which takes place when the components of some bodies are disengaged from each other. It is said that if the parts of olefiant gas were separated, the sum of the separate volumes would be four times as great as the volume of the compound ; that is, two volumes of hydrogen and two of carbon vapour are condensed in olefaant gas into the space of onevolume.
It is right to observe that the word repulsion is often applied to phenomena which are in reality the results of attraction. A small quantity of quicksilver being laid on a glass plate assumes a spherical form, instead of spreading over it in a thin surface; and this was once supposed to arise from a repulsive power in the glass, whereas it is owing to the attraction of the particles of quicksilver for one another being greater than the attraction of the glass for the quicksilver. Again, when a small sewing-needle is placed on the surface of water, it remains there without sinking, and the water is depressed about the needle as if it were repelled by the steel; in fact, however, the trough is caused by the weight of the needle, which displaces the particles of water, but is not great enough to overcome their attraction for each other. Also, when two balls, one of them of glass, which is capable of attracting water, and the other of burnt cork, which is not, or only in a very small degree, are placed near one another in water, the latter seems to be repelled from the former; but the cause of the pheno menon is that the ring of elevated water about the glass assumes on the exterior a conical surface, so that when the cork ball is brought near enough to the other to be partly on the slope, it immediately slides off by its gravity.
The elasticity of bodies is a result either of attractive or repulsive powers, or both. For example, when a steel rod is bent, the particles on one side will be forced towards, and on the opposite they will be drawn from one another ; in recovering itself, a force of attraction will be exerted on the latter side, and of repulsion on the other ; and this may be considered as an evidence that in the insensible spaces between the particles of bodies attractions and repulsions prevail according as the distances between those particles are varied. While the change of
figure in the rod is small, so that the displacement of any two particles is but a small part of their whole distance from one another, the attractions and repulsions exerted by the force applied are proportional to that force; and upon this principle depends the observed isochro nisin in the oscillations of a watch-balance, whatever be the extent of the arcs of vibration. The expansions of solids and fluids by heat, and the elastic powers of gas at different temperatures, are consequences of the repulsions residing in the particles of caloric, or induced by the latter in those of the bodies with which they are combined. [Etna TICITY ; GAS; HEAT.] The repulsive power existing in the air which ie condensed in nitre, produces, on being combined with heat, a velocity of expansion equal to about 7000 feet per second; and the force of pressure resulting from it is thought to be equal to 2000 times the pressure of the atmosphere. (Hutton, ' Tracts?) The repulsive force which produces some of the electric explosions in the atmosphere is supposed to be much greater. But the forces both of attraction and repulsion by which the particles of light are deflected from their course 'When they impinge on a refracting or reflecting surface are enormous ; and Sir John Herschel computes that they exceed the force of gravity in the ratio of 2 x 10•• to 1. This is on the hypothesis of radiation; and that philosopher observes that on the undulatory hypothesis the numbers are equally high.
The circumstances of electrical attractions and repulsions are shown in the article ELECTRICITY; and the results of experiments prove that the intensities of these forces in the electric, galvanic, and magnetic fluids, like that of general attraction, vary'inversely as the squares of the distances of the bodies.
Boacovich has ingeniously represented the series of alternate attractions and repulsions supposed to be experienced by a particle of matter within the very small 'distances between that particle and another, by a curve consisting of several bends crossing and recrossing an axis in points at various distances from the origin, which may be supposed to be the place of the second particle above mentioned. The ordinates of this curve on one side of the axis represent attractions, and those on the other side repulsions ; the places of crossing being sup posed to be those at which the first particle would be at rest. Beyond the small distance above mentioned, this axis becomes an asymptote to the curve, and the ordinates of the curve here represent the general law of attraction (the inverse square of the distances). Near the origin of the axis the ordinates represent repulsions ; and these ordinates constantly increase till they become infinite, so that a right line drawn through the place of the second particle, perpendicular to the axis, is an asymptote to this branch of the curve.