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pure distilled water, which produces no whiteness at all.

3. The influence or energy of this attraction reaches to a distance so small as to be imperceptible to our senses, as it does not appear to act till the bodies are in contact. Of this there is an instance in the difficulty of applying the particles of sulphur and mercury sufficiently near to make them unite. In this respect chemical attraction is analogous to the attraction of cohesion. For if two pieces of lead are pressed strongly together with a twisting motion, so as to bring them into violent contact, they take hold of one another almost as firmly as if united by fusion.

From these observations we shall understand some of the principal laws of mixture and solution.

I. When we desire to make two bodies act upon one another, it is necessary that one or both of them should be fluid, or rendered fluid, or disposed to fluidity, immediately after mixing. This rule is applied to salts in particular, "which do not act unless in solution." To illustrate this by experiment:

First. To crude muriate of ammonia in powder (which is a compound of ammonia or volatile alkali and muriatic acid, which has a strong attraction for it, and depresses its volatility) I add another salt in powder, which has a greater

attraction for the fixed salt than the alkali; but though I take great pains to mix them, they will not act; but when dissolved they immediately act on one another.

Second. Salt of amber mixed with potass while kept dry will continue separate, and the brown and white particles may be distinguished by a microscope; but if water is thrown on them, they immediately act upon each other.

The reason why fluids promote their action upon one another seems to be, that they bring their particles into closer contact than it was possible to bring them by any other means. The surfaces of solids are rough, and there seems to be an atmosphere over them which prevents them from uniting. Sir Isaac Newton calls it a tenacious atmosphere, and extremely subtile, upon which depends a variety of electrical facts. When a solid is applied to another solid, the particles of each are united by the attraction of cohesion; therefore they will not act upon each other, but they will act readily when this attraction of cohesion is removed by solution. But whatever the cause, the law is founded in experience. The necessary fluidity is obtained by dissolving the body in a fluid that produces no change in its qualities: thus, salt is dissolved in water; or by melting it, as metals; or by converting it into vapour, as in bodies which assume the form of vapour before they become

fluid. The terms for these operations are solu tion, fusion, and evaporation.

II. When we would dissolve a solid in a fluid, divide the solid mechanically, or by other means increase its surface. If a brittle body, by pulverizing; if a metallic, by beating it into leaves. The reason is plain, viz. that the fluid only acts on the surface with which it is in contact.

III. The action of one body on another is promoted by moderate heat, which performs the solution sooner in the action of a fluid on a solid; for, on removing the heat, the additional quantity which the heat had enabled it to dissolve will be deposited. This is particularly the case in salts and water. In general, the action of bodies on each other is more languid as the heat is less, and the contrary. From these facts it appears that the attraction the particles of one body have for those of another is increased by heat; and, on the contrary, the attraction between the particles of the same body is diminished by heat, as is evident in the formation of vapour.

Digestion is the continued action of bodies on each other, one or all of which are in a fluid state. This operation may be performed with or without the employment of additional heat, and in an ordinary vessel when the bodies are not volatile; when they are, a reflecting vessel is

used to condense the vapour which is returned back to the first vessel: this is called circulation. In this process the condensing vessel is placed above. Cohobation is an absurd term; it, however, means the collecting of the vapour into the second vessel, and pouring it back into the first, as in circulation; but instead of placing the condensing vessel over the first, it is placed on one side.

cane of the

By a knowledge of chemical attraction a chemist obtains the means of separating compound bodies, viz. by applying a third substance, in order to decompound thein by superior elective attraction which one of the bodies may have for that third substance. Hence it appears that Sir Isaac Newton's theory is extremely probable, yet it is not altogether sufficient; for why may not the three substances unite? This may perhaps arise from a repulsion between the third substance and one of the other two. But I am not inclined to multiply causes.

Geoffroy was the first who formed tables of the elective attractions of bodies, which were greatly extended and essentially corrected by Bergman.

The perfection of chemical science consists in what is called chemical analysis, that is, the resolving of bodies into their constituent parts; and it is surprising to reflect how far these operations have been carried by modern

chemists. The food we eat, the medicines we apply as remedies for diseases, have all been, if I may repeat the figure employed in the first of these chemical lectures, anatomized and dissected.-Hence we are the better enabled to judge of the salutary nature of each. All the productions of art and nature have, in short, been investigated, and their composition fully explained.—This is the true practical, I might say the really philosophical use, of chemistry; and to this every student ought, in the first instance, and as the first object, to apply.

The analysis of bodies is effected, as was formerly stated, partly by the agency of heat, still more, perhaps, by that of mixture, and still more by the united effects of both judiciously applied. It is of the utmost importance, therefore, that the student should render himself master of the affinities of different substances, and know with what bodies they will unite in solution, &c., and what bodies they will dislodge. The acids are the most powerful agents as menstrua, or in the liquid state, and with their affinities for the different solid matters it is necessary to be well acquainted; for by their means alone many of the most important facts in natural philosophy may be investigated and explained. But in a state of fusion, which may commonly be effected by the means of heat, most bodies may be brought to act upon each other.

In the subsequent lectures the action of the