Conversations on Chemistry Part 100

Let us now proceed to examine the nature of the three princ.i.p.al materials of the animal system.

_Gelatine_, or _jelly_, is the chief ingredient of skin, and of all the membranous parts of animals. It may be obtained from these substances, by means of boiling water, under the forms of glue, size, isingla.s.s, and transparent jelly.

CAROLINE.

But these are of a very different nature; they cannot therefore be all pure gelatine.

MRS. B.

Not entirely, but very nearly so. Glue is extracted from the skin of animals. Size is obtained either from skin in its natural state, or from leather. Isingla.s.s is gelatine procured from a particular species of fish; it is, you know, of this substance that the finest jelly is made, and this is done by merely dissolving the isingla.s.s in boiling water, and allowing the solution to congeal.

EMILY.

The wine, lemon, and spices, are, I suppose, added only to flavour the jelly?

MRS. B.

Exactly so.

CAROLINE.

But jelly is often made of hartshorn shavings, and of calves' feet; do these substances contain gelatine?

MRS. B.

Yes. Gelatine may be obtained from almost any animal substance, as it enters more or less into the composition of all of them. The process for obtaining it is extremely simple, as it consists merely in boiling the substance that contains it with water. The gelatine dissolves in water, and may be attained of any degree of consistence or strength, by evaporating this solution. Bones in particular produce it very plentifully, as they consist of phosphat of lime combined or cemented by gelatine. Horns, which are a species of bone, will yield abundance of gelatine. The horns of the hart are reckoned to produce gelatine of the finest quality; they are reduced to the state of shavings in order that the jelly may be more easily extracted by the water. It is of hartshorn shavings that the jellies for invalids are usually made, as they are of very easy digestion.

CAROLINE.

It appears singular that hartshorn, which yields such a powerful ingredient as ammonia, should at the same time produce so mild and insipid a substance as jelly?

MRS. B.

And (what is more surprising) it is from the gelatine of bones that ammonia is produced. You must observe, however, that the processes by which these two substances are obtained from bones are very different.

By the simple action of water and heat, the gelatine is separated; but in order to procure the ammonia, or what is commonly called hartshorn, the bones must be distilled, by which means the gelatine is decomposed, and hydrogen and nitrogen combined in the form of ammonia. So that the first operation is a mere separation of ingredients, whilst the second requires a chemical decomposition.

CAROLINE.

But when jelly is made from hartshorn shavings, what becomes of the phosphat of lime which const.i.tutes the other part of bones?

MRS. B.

It is easily separated by straining. But the jelly is afterwards more perfectly purified, and rendered transparent, by adding white of egg, which being coagulated by heat, rises to the surface along with any impurities.

EMILY.

I wonder that bones are not used by the common people to make jelly; a great deal of wholesome nourishment, might, I should suppose, be procured from them, though the jelly would perhaps not be quite so good as if made from hartshorn shavings?

MRS. B.

There is a prejudice among the poor against a species of food that is usually thrown to the dogs; and as we cannot expect them to enter into chemical considerations, it is in some degree excusable. Besides, it requires a prodigious quant.i.ty of fuel to dissolve bones and obtain the gelatine from them.

The solution of bones in water is greatly promoted by an acc.u.mulation of heat. This may be effected by means of an extremely strong metallic vessel, called _Papin's digester_, in which the bones and water are enclosed, without any possibility of the steam making its escape. A heat can thus be applied much superior to that of boiling water; and bones, by this means, are completely reduced to a pulp. But the process still consumes too much fuel to be generally adopted among the lower cla.s.ses.

CAROLINE.

And why should not a manufacture be established for grinding or macerating bones, or at least for reducing them to the state of shavings, when I suppose they would dissolve as readily as hartshorn shavings?

MRS. B.

They could not be collected clean for such a purpose, but they are not lost, as they are used for making hartshorn and sal ammoniac; and such is the superior science and industry of this country, that we now send sal ammoniac to the Levant, though it originally came to us from Egypt.

EMILY.

When jelly is made of isingla.s.s, does it leave no sediment?

MRS. B.

No; nor does it so much require clarifying, as it consists almost entirely of pure gelantine, and any foreign matter that is mixed with it, is thrown off during the boiling in the form of sc.u.m. --These are processes which you may see performed in great perfection in the culinary laboratory, by that very able and most useful chemist the cook.

CAROLINE.

To what an immense variety of purposes chemistry is subservient!

EMILY.

It appears, in that respect, to have an advantage over most other arts and sciences; for these, very often, have a tendency to confine the imagination to their own particular object, whilst the pursuit of chemistry is so extensive and diversified, that it inspires a general curiosity, and a desire of enquiring into the nature of every object.

CAROLINE.

I suppose that soup is likewise composed of gelatine; for, when cold, it often a.s.sumes the consistence of jelly?

MRS. B.

Not entirely; for though soups generally contain a quant.i.ty of gelatine, the most essential ingredient is a mucous or extractive matter, a peculiar animal substance, very soluble in water, which has a strong taste, and is more nouris.h.i.+ng than gelatine. The various kinds of portable soup consist of this extractive matter in a dry state, which, in order to be made into soup, requires only to be dissolved in water.

Gelatine, in its solid state, is a semiductile transparent substance, without either taste or smell. --When exposed to heat, in contact with air and water, it first swells, then fuses, and finally burns. You may have seen the first part of this operation performed in the carpenter's glue-pot.

CAROLINE.

But you said that gelatine had no smell, and glue has a very disagreeable one.

MRS. B.

Glue is not pure gelatine; as it is not designed for eating, it is prepared without attending to the state of the ingredients, which are more or less contaminated by particles that have become putrid.

Gelatine may be precipitated from its solution in water by alcohol. --We shall try this experiment with a gla.s.s of warm jelly. --You see that the gelatine subsides by the union of the alcohol and the water.

EMILY.