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 not 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 be raised without a

quantity of acid along with it.

When marble has been mixed with the muriatic acid, the latter loses its volatility, and becomes capable of enduring a violent heat: if heat is applied, the watery particles rise in vapour, and leave the marble and acid so united that no heat can separate them. In this case, therefore, it is impossible by heat alone to separate the marble from the acid, but by the addition of a third body. In doing this, the third body always joins itself to one of the other two; this is called elective attraction, or affinity.

To give a clear notion of this, by separating the mixtures we just now made, I shall first separate the water and sulphuric acid by the addition of another saline liquor; this liquor acts with great violence on the mixture: the mixture grows muddy as soon as a third fluid is added, and as it cools the salt is deposited at the bottom.

2. In separating the ammonia from the sulphuric acid, the mixture is at present destitute of smell; but on adding a little carbonate of potass, destitute likewise of smell, we may perceive that the ammonia is separated, by the pungent smell. 3. So spirit of wine is separated from camphor by mixing water with the spirit.

In this attraction there is a sort of gradation. Thus silver dissolved in nitric acid may be separated by quicksilver, which takes its place. If we add copper, the mercury will be separated,

If we throw in lead, the copper will be deposited -the lead by iron, &c.

Another remarkable phænomenon is, that some bodies are less in bulk, others greater, when mixed. Silver is lighter than quicksilver; yet quicksilver and silver united form a compound heavier than quicksilver of equal bulk.

Water is not increased in bulk by the addition of a small quantity of salt; if the quantity of salt is, however, increased, the liquor will be increased, but not in the proportion of the aggregate weight of the two.

The

M. Reaumur made the following experiment: a vessel was half filled with water, spirit of wine was then gently poured into it, till the mixture rose half way to the neck of the vessel. bottle was then shaken, and a little heat was produced; the liquor consequently rose; but when the heat was gone off, the mixture took up less room than it did before the two fluids were united by shaking.

I shall now consider the several theories and explications of the effects of mixture. From the first æra of chemistry till the time of Lord Verulam, we had no intelligible theory; they seldom went further than to explain one term by another. Thus finding that the alkalies effervesced with acids, whenever they found a substance that effervesced with an acid, the philosophers of the day defined it to be of an alkaline nature.

Most of the chemical phænomena are difficult