the combustion, which is equal in weight to the substance which produced it.

The theory of combustion is now indeed as well understood as most facts in chemistry. According to the second canon I have laid down, combustible bodies are such as will readily combine with oxygen gas; consequently, when any such body is raised to a certain temperature, it begins to be decomposed, and to combine with the oxygen of the atmosphere, and this oxygen during its combination lets go the caloric with which in the state of air or gas it was combined. Hence combustion consists of two things, a decomposition, and a combination; and the substance which has undergone combustion is essentially altered; it is, in fact, a compound of the body which has been subjected to combustion. and oxygen. It is of course incombustible, because its base being already saturated with oxygen, cannot combine with any more.

Upon the same principles, if by any process the oxygen is taken away, the substance will again be rendered combustible. To illustrate this whole theory, take a familiar instance. Sulphur, we have just seen, is reduced to a corrosive acid by burning in the open air; but by charcoal applied in a particular manner with the assistance of fire, this acid may be again converted into inflammable sulphur. It is the same thing whether we use charcoal made of blood, flesh, or bone, or whether the charcoal from any

vegetable matter. Nor is it necessary to have recourse to charcoal, as pit-coal and other combustible bodies have the same effect. The fact is easily explained upon the principles just laid down. The sulphur, when burned, extracts the oxygen, or acidifying principle, from the air, and is converted into sulphuric acid *. The charcoal when applied again to this acid draws off the oxygen, with which it unites, and forms carbonic acid gas, and leaves the sulphur in its former state.

* Formerly called vitriolic acid,.

LECTURE XXVII.

CHEMISTRY.

MIXTURE, AND ITS EFFECTS.

THE following principles may be laid down with respect to mixture.

1. We find that some bodies cannot be made to unite with others, as oil and water, and water and quicksilver: however shaken together, they will still separate again.

2. We find others unite in the most intimate manner, and form a compound in appearance perfectly homogeneous. Of these some unite slowly and gently, as salt and water.

3. There are other cases, where the union is attended with perturbation and commotion, the production of heat, smoke, and sometimes flame. If I pour into a flask a quantity of water quite cold, and I add an equal quantity of sulphuric acid, which has a great tendency to mix with water, and is likewise cold, the liquid becomes hot, so as actually to boil, emit steam, &c.

Again-If I drop a little carbonate of ammonia, which is a volatile salt, (in a fluid state,) into sulphuric acid, (this experiment requires caution,) the agitation is greater than if

a red-hot iron had been dropped into it. This intestine commotion is called effervescence.

I shall now exhibit some instances of the mixture of solids with fluids. In their union the solid body is often divided into atoms so minute, that they make with the fluids a homogeneous liquor, and will remain dissolved as long as the quality or quantity of the fluid is not altered.

Thus-Camphor thrown into spirit of wine sinks at first; but, dissolving and uniting by degrees, the fluid remains transparent as before.

Again-Drop a piece of marble into muriatic acid, and bubbles will arise, a violent effervescence is produced, the marble is dissolved into atoms so perfectly minute, that they become invisible, and are equally mixed with the fluid and diffused in it. This is an instance of effervescence between a solid and a fluid; the operation is called solution, and the fluid a solvent or menstruum. In solution a solid must not only be so mixed with the fluid, as to be equally dispersed and never to subside, but the mixture must be perfectly transparent. In some mixtures, though the matter is entirely mixed, yet for want of transparency it is not called solution, but diffusion (and in pharmacy an emulsion).

Saturation, I formerly observed, is used to signify that some bodies are capable of being united only in a certain proportion, and when a menstruum or fluid has taken up exactly as much

of any matter as it is capable of holding in solution, it is said to be saturated. Saturation is sometimes single, sometimes double: camphor in spirit of wine is an instance of the first; volatile salts and the sulphuric acid are an instance of the second, for both may be saturated; the salts by having a proportion of the acid lose their pungency, and the acid its sourness. If more than this portion is added of either, the overplus retains its separate qualities.

Chemists have a power of separating, by the application of heat, such bodies as differ in volatility, and are not too strongly united; but in some instances of solution we find it very difficult. If we endeavour to separate them by heat, we find them capable of enduring a great deal more than any of the bodies, when separate, could have done; and if we apply a still greater heat, the vapour rises without any separation of parts, the volatile carrying the fixed along with them. A mixture of the sulphuric acid and water is an instance: the water in its separate state would be converted into vapour at 212, the acid at 600; but mixed, the water will not evaporate at 212; the heat must be greatly increased, and then the vapour of the water rises mixed with the acid. The alkaline salt known by the name of ammonia is very volatile, but when mixed with sulphuric acid, its volatility will be depressed by the acid, so that it has not the least smell; and if heat is applied, the alkali cannot