Conversations on Chemistry Part 102

That defect arises, I believe, in general, from its being badly prepared, which is the case when the iron is not so fully oxydated as to form a red oxyd. For a solution of green oxyd of iron (in which the metal is more slightly oxydated), makes only a pale green, or even a white precipitate, with prussiat of potash; and this gradually changes to blue by being exposed to the air, as I can immediately show you.

CAROLINE.

It already begins to a.s.sume a pale blue colour. But how does the air produce this change?

MRS. B.

By oxydating the iron more perfectly. If we pour some nitrous acid on it, the prussian blue colour will be immediately produced, as the acid will yield its oxygen to the precipitate, and fully saturate it with this principle, as you shall see.

CAROLINE.

It is very curious to see a colour change so instantaneously.

MRS. B.

Hence you perceive that prussian blue cannot be a permanent colour, unless prepared with red oxyd of iron, since by exposure to the atmosphere it gradually darkens, and in a short time is no longer in harmony with the other colours of the painting.

CAROLINE.

But it can never become darker, by exposure to the atmosphere, than the true prussian blue, in which the oxyd is perfectly saturated?

MRS. B.

Certainly not. But in painting, the artist not reckoning upon partial alterations in his colours, gives his blue tints that particular shade which harmonises with the rest of the picture. If, afterwards, those tints become darker, the harmony of the colouring must necessarily be destroyed.

CAROLINE.

Pray, of what nature is the paint called _carmine_?

MRS. B.

It is an animal colour prepared from _cochineal_, an insect, the infusion of which produces a very beautiful red.

CAROLINE.

Whilst we are on the subject of colours, I should like to learn what _ivory black_ is?

MRS. B.

It is a carbonaceous substance obtained by the combustion of ivory.

A more common species of black is obtained from the burning of bone.

CAROLINE.

But during the combustion of ivory or bone, the carbon, I should have imagined, must be converted into carbonic acid gas, instead of this black substance?

MRS. B.

In this, as in most combustions, a considerable part of the carbon is simply volatilised by the heat, and again obtained concrete on cooling.

This colour, therefore, may be called the soot produced by the burning of ivory or bone.

CONVERSATION XXIV.

ON THE ANIMAL ECONOMY.

MRS. B.

We have now acquired some idea of the various materials that compose the animal system; but if you are curious to know in what manner these substances are formed by the animal organs, from vegetable, as well as from animal substances, it will be necessary to have some previous knowledge of the nature and functions of these organs, without which it is impossible to form any distinct idea of the process of _animalisation_ and _nutrition_.

CAROLINE.

I do not exactly understand the meaning of the word animalisation?

MRS. B.

Animalisation is the process by which the food is _a.s.similated_, that is to say, converted into animal matter; and nutrition is that by which the food thus a.s.similated is rendered subservient to the purposes of nourishing and maintaining the animal system.

EMILY.

This, I am sure, must be the most interesting of all the branches of chemistry!

CAROLINE.

So I think; particularly as I expect that we shall hear something of the nature of respiration, and of the circulation of the blood?

MRS. B.

These functions undoubtedly occupy a most important place in the history of the animal economy. --But I must previously give you a very short account of the princ.i.p.al organs by which the various operations of the animal system are performed. These are:

The _Bones_; _Muscles_, _Blood vessels_, _Lymphatic vessels_, _Glands_, and _Nerves_.

The _bones_ are the most solid part of the animal frame, and in a great measure determine its form and dimensions. You recollect, I suppose, what are the ingredients which enter into their composition?

CAROLINE.

Yes; phosphat of lime, cemented by gelatine.

MRS. B.

During the earliest period of animal life, they consist almost entirely of gelatinous membrane having the form of the bones, but of a loose spongy texture, the cells or cavities of which are destined to be filled with phosphat of lime; it is the gradual acquisition of this salt which gives to the bones their subsequent hardness and durability. Infants first receive it from their mother's milk, and afterwards derive it from all animal and from most vegetable food, especially farinaceous substances, such as wheat-flour, which contain it in sensible quant.i.ties. A portion of the phosphat, after the bones of the infant have been sufficiently expanded and solidified, is deposited in the teeth, which consist at first only of a gelatinous membrane or case, fitted for the reception of this salt; and which, after acquiring hardness within the gum, gradually protrude from it.

CAROLINE.

How very curious this is; and how ingeniously nature has first provided for the solidification of such bones as are immediately wanted, and afterwards for the formation of the teeth, which would not only be useless, but detrimental in infancy!