to explain, because we can scarcely find any thing else with which they can be compared. If a mechanic removes a prodigious rock by the force of one man, it will be cause of wonder to one unacquainted with the mechanical principles; but on examining the instruments by which this effect is wrought, we shall find that the whole depends upon certain fixed laws of mechanism and motion. If the chemist removes a rock of the same magnitude by a small quantity of nitre, sulphur, and charcoal, we shall not so easily understand the manner in which the effect is produced. It is more easy to increase the number of facts than to explain them.

After Lord Bacon, chemistry became more known, and attempts were made for a more rational and popular theory. As mechanical powers are easily understood, we are apt to suppose that the action of bodies depends upon mechanical principles.

The most popular opinions were, that the powers of bodies depended upon their particles having particular figures and motions. Boyle was one of the first who published this opinion, but he did it in a moderate and diffident manner. Many after him, however, have extended this application so far as to show the full absurdity of it, especially the older French chemists. If we were to consult them upon the first experiment we made, they would tell us that the sulphuric acid (or, as they would call it, the oil of

vitriol) consists of heavy inert particles, which contain a quantity of fire, and that upon application of the water the particles are dissolved, and the particles of fire break loose and become active; but these reasoners would be puzzled to explain the phænomenon of the same sulphuric acid, or oil of vitriol, with ice. In explaining the dissolution of marble, they say, the acid contains innumerable wedges or needles, which get into the pores of the marble, and throw off its particles, which are suspended in the fluid like a boat by its oars*. But supposing the particles sharp and pointed, they would require a force to make them act. In answer to this, they tell you that the particles of fluids are agitated in a constant and irregular manner; they refer you to the beams of the sun let into a chamber, in which you will see the particles of dust, moving in all directions. It is true, there is a motion of this kind in the air, which is the most elastic and tremulous of all fluids, every motion throwing it into agitation; but this is not so rapid as to throw the particles against each other with sufficient force to produce the effects in question. There are many instances of solid bodies acting with violence upon one another, as corrosive muriate of mercury, when powdered, and mixed with pure antimony in powder. There are many facts also relating to fluids,

• This is the very expression of Lemery.

which render this theory very unsatisfactory. Nitric acid, for example, dissolves silver, and not gold; aqua regia, or nitro-muriatic acid, gold, and not silver; the gold is the denser of the two.

In the dissolution of solid bodies in fluids, they tell you, the reason why a solid body continues equally suspended without subsiding from the fluid, is, that the division of the bodies is so minute, that the weight of the particles is not sufficient to overcome the resistance of the fluid. This, however, does not explain the dissolution, nor the dispersion of the solid particles through the fluids.

The last theory I shall mention is that of Sir Isaac Newton, which is now generally received, and for which I refer to the thirty-first query at the end of his Optics.

Sir Isaac Newton supposes that the phænomena of chemical solution arise from a disposition in the particles of one body to unite with the particles of another. Thus, when two bodies on being mixed unite with effervescence, &c. the cause of this phænomenon he supposes to be a powerful attraction between them, which disposes them to unite very strongly with a rapid and accelerated motion; whence the visible effects in the attraction which obtains between the particles of the fluid and solid; this attraction is more powerful than that which binds the particles of the solid together. In consequence of

this attraction, the mixture is much more difficult to evaporate than either the solid or fluid in their separate state. Heat produces vapour by lessening the attraction between the particles of a body; it enlarges the dimensions of a solid till it becomes fluid; if continued, it enlarges the dimensions of a fluid to vapour. According to Sir Isaac Newton, this attraction is equal at equal distances, and at unequal distances unequal.

When the volatility of a body is repressed by mixture, and the two cannot be separated by heat, the separation is produced by the addition of a third body, which has a stronger attraction for one of the two bodies than they have for one another. This kind of attraction, from the circumstance of certain bodies uniting with a kind of preference to certain other bodies, was at first named elective attraction; and afterwards, for a similar reason, affinity, which is the term at present in most general use.

Many of the hardest bodies are very quickly separated by mixture. Metals require a very great force to divide them mechanically, but this division is easily effected by chemical solvents. But the following observations will serve more fully to explain the nature of elective attraction or affinity.

1. This sort of attraction differs from that of gravitation, electricity, and magnetism, in not obtaining between large and sensible parts of

matter, but only between its minute and invisible particles. Thus, if we place a piece of salt as near as possible to water, it will not unite with it, unless it is in contact.

2. The minuteness of the particles into which bodies are divided by mixture is beyond the power of imagination. We may form some sort of notion of this minuteness, by dissolving a small grain of the size of a pin's head of nitrat of silver in a quart of river water. The water is tinged with a milky colour; every particle of water has therefore a sensible quantity of silver in it. If to this mixture we add the solution of muriate of soda, or common sea salt (a transparent fluid), we shall see the little grain of silver, which was dispersed through the whole of the water, separating in a subtile powder, and falling to the bottom. The saline liquor added has no disposition to disturb the transparency of the water, when it contains none of the silver; yet it is no sooner added to this solution of silver than the whole of the liquor becomes muddy, and every drop appears to have contained particles of the metal. The nitrat of silver is a combination of the nitric acid and silver. The particles of silver were actually divided from each other by the nitric acid before put into water. That the whiteness is occasioned by the common salt, of which there is a little in most river water, is plain by adding

VOL. II.

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