Attraction denotes the tendency which is observed in bodies to approach and adhere to each other; it is also sometimes employed to signify the unknown cause of this tendency. It assumes various names, according to the circumstances in which it acts. Thus we have the attraction of gravitation - the attraction of electricity, or electrical attraction - the attraction of magnetism - cohesion, and chemical affinity. The first three of these act at sensible distances, but the range of the other two is too limited to be appreciated by the unassisted senses.
The Attraction of Gravitation is the power that upholds the whole planetary system, and retains bodies on the surface of our earth; its action is perceptible at the remotest part of the solar system, and is even manifest among the fixed stars and nebulae. The laws to which the manifestations of this power are subject will be stated under the article on Gravity: at present, we shall confine ourselves to a statement of two experiments connected with this subject, which will clearly evince the existence of this power as inherent, not only in the earth, as a distinct body, but in small, and even detached portions of its surface. The first of these experiments was made by Dr. Maskeline, on the mountain Shebralien, in Scotland. A long plumb-line, attached to a telescope, was suspended successively on the north and south sides of the mountain, when it was found, on each occasion, that the plumb-line did not hang perpendicularly, but was deflected towards the mountain. The other experiment was made by Mr. Cavendish, who, by means of a long and fine silver wire, suspended a slender deal rod, so that it might, by twisting the wire, vibrate freely in a horizontal plane.
Small balls of lead were attached to the ends of the deal rod, and, by means of mechanism, larger balls of that metal were carefully brought near to the smaller ones. At each approach of the larger balls, the small ones were sensibly attracted, made to vibrate, and finally arrange themselves in a new position.
Attraction of Electricity is that which occurs between two bodies, one of which is electrified positively, and the other negatively; or one of which is positive or negative, and the other in its natural state. It is probable, however, that this last case is merely apparent, since every body, when brought near to an electrized body, becomes itself electrical by induction. See Electricity.
Attraction of Magnetism is that which takes place when a loadstone or an artificial magnet is approximated to a piece of iron or steel. It also occurs when the south pole of one magnet is made to approach the north pole of another.
Cohesive Attraction is that which subsists among the minutest particles or atoms of a body, forming them into a solid mass. In gases, this power appears to be wholly deficient, or at least more than counteracted by the effects of an antagonist power - repulsion. In liquids there are but small manifestations of its existence, but in solids it evinces itself in the most decided and conspicuous manner. The intensity of the cohesive force varies in different bodies, whether solid or liquid, - a circumstance which renders a knowledge of its laws of the greatest importance to mechanics, engineers, and other practical men. For the elucidation of these laws, and the facts on which they are founded, see Cohesion.
chemical Attraction is that which connects the particles of different bodies, and forms them into compound substances. Thus, the power that unites the particles of a mass of copper is called cohesion; but that which causes the union of copper with sulphuric acid, or nitric acid, so as to form sulphate or nitrate of copper, is denominated chemical attraction, affinity, elective attraction, or attraction of composition. Hence we see that chemical attraction and cohesion coexist in the same body. Thus, in the sulphate of copper, one particle of sulphate of copper is connected with another particle by cohesion; while, at the same time, the particles of sulphuric acid are connected with those of copper by chemical affinity. See Chemistry.