REPULSION, in physics, that property in bodies, whereby, if they are placed just beyond the sphere of each other's attraction of cohesion, they mutually fly from each other. Thus, if an oily sub. stance, lighter than water, be placed on the surface thereof, or if a piece of iron be laid upon mercury, the surface of the fluid will be depressed about the body laid on it : this depression is manifestly occasioned by a repelling power in the bodies, which hinders the approach of the fluid towards them. But it is pos sible, in some cases, to press or force the repelling bodies into the sphere of one another's attraction ; and then they will mutually tend towards each other, as when we mix oil and water till they in. corporate. Dr. Knight defines repulsion to be that cause which makes bodies mu tually endeavour to recede from each other, with different forces, at different times ; and that such a cause exists in na ture, he thinks evident, for the following reasons. 1. Because all bodies are elec trical, or capable of being made so ; and it is well known, that electrical bodies both attract and repel. 2. Both attrac tion and repulsion are very conspicuous in all magnetical bodies. 3. Sir Isaac Newton has shown from experiments, that the surfaces of two convex glasses repel each other. 4. The same great philosopher has explained the elasticity of the air, by supposing its particles mu tually to repel each other. 5. The par ticles of light are, in part at least, re pelled from the surfaces of all bodies. 6. Lastly, it seems highly probable, that the particles of light mutually repel each other, as well as the particles of air. The same gentleman ascribes the cause of re pulsion, as well as that of attraction, to the immediate effect of God's will ; and as attraction and repulsion are contraries, and consequently cannot, at the same time, belong to the same substance, the doctor supposes there are in nature two kinds of matter, one attracting, the other repelling ; and that those particles of matter which repel each other, are sub ject to the general law of attraction in respect of other matter. A repellent matter being thus supposed, equally dis persed through the whole universe, the doctor attempts to account for many na tural phenomena by means thereof Ile thinks light is nothing but this repellent matter put into violent vibrations, by the repellent corpuscles which compose the atmosphere of the sun and stars : and that, therefore, we have no reason to be lieve they are gulphs of fire, but, like the rest of the heavenly bodies, inhabita ble worlds. From the same principles, he attempts to explain the nature of fire and beat, the various phenomena of the magnet, and the cause of the variation of the needle : and, indeed, it is difficult, if not impossible, by the doctrine of attrac tion alone, to account for all the pheno mena observable in experiments made with magnets, which may now be solved by admitting this doctrine of a repellent fluid ; but whether it will be sufficient to account for all the particular phenomena of nature, which are the proper tests of an hypothesis, time and experience alone must determine. The doctor also endea
vours to show, that the attractions of co hesion, gravity, and magnetism, are the same, and that by these two active prin ciples, viz. attraction and repulsion, all the phenomena of nature may be explain ed ; but as his ingenious treatise on this subject is laid down in a series of propo sitions, all connected together, it would be impossible to do justice to his argu ments without transcribing the whole : we shall therefore refer the curious to the book itself According to 'sGravesande and others, when light is reflected from a polished spherical surface, the particles of light do not strike upon the solid parts, and so rebound from them ; but are repelled from the surface, at a small distance before they touch it, by a power extended all over the said polished surface. And Sir Isaac Newton observes, that the rays of light are also expelled by the edges of bodies, as they pass near them ; so as to make their shadows, in some cases, lar ger than they would otherwise be.
Ran:LS.10N, in chemistry. Sir Isaac Newton demonstrated, that if this law be correct, then the force, by which the particles of air recede from each other, increases or diminishes at the same rate thai the distance between the centres of the particles, or atoms, of which it is composed, diminishes or increases ; or, which is the same thing, that the repul sion between the particles of gaseous bo dies is always inversely as the distance of their centres from each other. Now the distance between the centres of the atoms of elastic fluids always varies as the cube root of their density, taking the word in its common acceptation. Thus, if the density of air, under the mean pres sure of the atmosphere, be supposed 1; if it be forced into ph of its hulk, its density becomes 8. In these two cases we have the distance between the atoms of air inversely as the cube root of 1 to the cube root of 8, or as 1 to 2. So that if air be compressed into of its bulk, the distance between its particles is re duced to one half, and of course the re pulsion between them is doubled. If air be rarified 300 times, we have its density reduced to _th of that of common air.
Here we have the distance between the atoms of common and the rarified air, as 3 3 : 300, or nearly as 1 : 7. So that when air is rarified 300 times, the dis tance between its particles becomes al most seven times greater, and of course their repulsion is diminished almost se venfold