Evenings at home; or, the juvenile budget opened: Consisting of a variety of miscellaneous pieces, ... [pt.2]
Aikin, John, 1747-1822., Barbauld, Mrs. (Anna Letitia), 1743-1825.

A TEA LECTURE.

Tutor—Pupil.
Tut.

COME—the tea is ready. Lay by your book, and let us talk a little. You have assisted in tea-making a great many times, and yet I dare say you ne|ver considered what kind of an opera|tion it was.

Pup.

On operation of cookery—is it not?

Page  69
Tut.

You may call it so; but it is properly an operation of chemistry.

Pup.

Of chemistry? I thought that had been a very deep sort of a business.

Tut.

O—there are many things in common life that belong to the deep|est of sciences. Making tea is the che|mical operation called infusion, which is, when a hot liquor is poured upon a substance in order to extract something from it. The water, you see, extracts from the tea-leaves their colour, taste, and flavour.

Pup.

Would not cold water do the same?

Tut.

It would, but more slowly. Heat assists almost all liquors in their power of extracting the virtues of herbs and other substances. Thus good house|wives were formerly used to boil their tea, in order to get all the goodness from it as completely as possible. The greater heat and agitation of boiling makes it act more powerfully. The Page  70 liquor in which a substance has been boiled is called a decoction of that sub|stance.

Pup.

Then we had a decoction of mutton at dinner to-day.

Tut.

We had—broth is a decoction, and so are gruel and barley-water. But when any thing is put to steep in a cold liquor, it is called maceration. The in|gredients of which ink is made are ma|cerated. In all these cases, you see, the whole substance does not mix with the liquor, but only part of it. The rea|son is, that part of it is soluble in the li|quor, and part not.

Pup.

What is the meaning of that?

Tut.

Solution is when a solid put into a fluid entirely disappears in it, leaving the liquor clear. Thus when I throw this lump of sugar into my tea, you see it gradually wastes away till it is all gone; and then I can taste it in every single drop of my tea; but the tea is clear as before.

Page  71
Pup.

Salt would do the same.

Tut.

It would. But if I were to throw in a lump of chalk, it would lie undis|solved at the bottom.

Pup.

But it would make the water white.

Tut.

True, while it was stirred; and then it would be a diffusion. But while the chalk was thus mixed with the li|quor, it would lose its transparency, and not recover it again, till by standing the chalk had all subsided, and left the liquor as it was before.

Pup.

How is the cream mixed with the tea?

Tut.

Why, that is only diffused, for it takes away the transparency of the tea. But the particles of cream being finer and lighter than those of chalk, it remains longer united with the liquor. However, in time the cream would se|parate too, and rise to the top, leaving the tea clear. Now, suppose you had a mixture of sugar, salt, chalk, and tea Page  72 leaves, and were to throw it into water, either hot or cold;—what would be the effect?

Pup.

The sugar and salt would melt and disappear. The tea-leaves would yield their colour and taste. The chalk—I do not know what would become of that.

Tut.

Why, if the mixture were stirred, the chalk would be diffused through it, and make it turbid or muddy; but on standing, it would leave it unchanged.

Pup.

Then there would remain at bottom the chalk and tea-leaves?

Tut.

Yes. The clear liquor would contain in solution salt, sugar, and those particles of the tea, in which its colour and taste consisted: the remainder of the tea, and the chalk, would lie undis|solved.

Pup.

Then I suppose tea-leaves, after the tea is made, are lighter than at first.

Tut.

Undoubtedly. If taken out and dried they would be found to have lost Page  73 part of their weight, and the water would have gained it. Sometimes, however, it is an extremely small portion of a sub|stance which is soluble, but it is that in which its most remarkable qualities re|side. Thus a small piece of spice will communicate a strong flavour to a large quantity of liquid, with very little loss of weight.

Pup.

Will all liquors dissolve the same things?

Tut.

By no means. Many dissolve in water, that will not in spirit of wine; and the contrary. And upon this dif|ference many curious matters in the arts are founded. Thus, spirit varnish is made of a solution of various gums or resins in spirits that will not dissolve in water. Therefore, when it has been laid over any surface with a brush, and is become dry, the rain or moisture of the air will not affect it. This is the case with the beautiful varnish laid upon coaches. On the other hand, the Page  74 varnish lest by gum-water could not be washed off by spirits.

Pup.

I remember when I made gum|water, upon setting the cup in a warm place, it all dried away, and left the gum just as it was before. Would the same happen if I had sugar or salt dissolved in water?

Tut.

Yes—upon exposing the solu|tion to warmth, it would dry away, and you would get back your salt or sugar in a solid state as before.

Pup.

But if I were to do so with a cup of tea, what should I get?

Tut.

Not tea-leaves, certainly! But your question requires a little previous explanation. It is the property of heat to make most things fly off in vapour, which is called evaporation or exhalation. But this it does in very different degrees to different substances. Some are very easily made to evaporate; others very difficultly; and others not at all by the most violent fire we can raise. Fluids, Page  75 in general, are easily evaporable; but not equally so. Spirits of wine fly off in vapour much sooner than water; so that if you had a mixture of the two, by applying a gentle heat you might drive off all the spirits, and leave the water pure. Water, again, is more evaporable than oil. Some solid sub|stances are much disposed to evaporate. Thus, smelling salts by a little heat may be entirely driven away in the air. But in general, solids are more fixed than fluids; and therefore when a solid is dis|solved in a fluid, it may commonly be recovered again by evaporation. By this operation common salt is got from sea-water and salt springs, both artifi|cially, and in hot countries by the na|tural heat of the sun. When the water is no more than is just sufficient to dis|solve the salt, it is called a saturated so|lution, and on evaporating the water fur|ther, the salt begins to separate, form|ing little regular masses called crystals.Page  76 Sugar may be made in like manner to form crystals, and then it is sugar|candy.

Pup.

But what is a syrup?

Tut.

That is, when so much sugar is dissolved as sensibly to thicken the li|quor, but not to separate from it. Well—now to your question about tea. On exposing it to considerable heat, those fine particles in which its flavour con|sists, being as volatile or evaporable as the water, would fly off along with it; and when the liquor came to dryness, there would only be left those particles in which its roughness and colour con|sist. This would make what is called an extract of a plant.

Pup.

What becomes of the water that evaporates?

Tut.

It ascends into the air, and unites with it. But if in its way it be stopped by any cold body, it is condensed, that is, it returns to the state of water again. Lift up the lid of the tea-pot, and you Page  77 will see water collected on the inside of it, which is condensed steam from the hot tea beneath. Hold a spoon or knife in the way of the steam which bursts out from the spout of the tea-kettle, and you will find it immediately covered with drops. This operation of turning a fluid into vapour, and then condens|ing it, is called distillation. For this purpose, the vessel in which the liquor is heated is closely covered with another called the head, into which the steam rises, and is condensed. It is then drawn off by means of a pipe into another ves|sel called the receiver. In this way all sweet scented and aromatic liquors are drawn from fragrant vegetables, by means of water or spirits. The fra|grant part, being very volatile, rises along with the steam of the water or spi|rit, and remains united with it after it is condensed. Rose-water and spirit of lavender are liquors of this kind.

Pup.

Then the water collected on the Page  78 inside of the tea-pot lid should have the fragrance of the tea?

Tut.

It should—But unless the tea were fine, you would scarcely perceive it.

Pup.

I think I have heard of making salt-water fresh by distilling.

Tut.

Yes. That is an old discovery lately revived. The salt in sea-water, being of a fixed nature, does not rise with the steam; and therefore, on con|densing the steam, the water is found to be fresh. And this indeed is the method nature employs in raising water by exhalation from the ocean, which collecting into clouds, is condensed in the cold regions of the air, and falls down in rain.

But our tea is done; so we will now put an end to our chemical lecture.

Pup.

But is this real chemistry?

Tut.

Yes, it is.

Pup.

Why, I understand it all with|out any difficulty.

Tut.

I intended you should.