The provision is very beautiful, and is as follows:-The bones of birds are hollow, and being hollow, are always filled with air; moreover, air cavities exist in various parts of the body, the gaseous element penetrating even into the muscular sheaths. Hence the lungs, not being required as vessels of capacity, but merely as instruments of chemical action, are smaller than they would have otherwise been required to exist. But there still remains to be devised a provision for filling the lungs with air, otherwise than by expansion. The muscular exercise of the animal is made subservient to this end. When at rest, and when respiration is but little required, the bird, by means of a slight muscular motion, causes sufficient air for its present necessities to enter the tiny lungs; but when the same bird is exposed to the violent exercise of flying, and when, consequently, the function of respiration is required to assume its highest grade of intensity, then the increased muscular action of the bird is made subservient to the end. Every flapping of the wings against the ribs throws a set of muscles into activity, which, by their contraction, force a largely increased supply of air into the lungs; and thus the performance of a function is accomplished as a necessary result of attendant circumstances.

Lungs of almost the same external character, though different in their internal conformation, are supplied to reptiles, although not for the same reason. In the serpent and lizard tribe the large, heavy, expansive

lungs of mammalia would be evidently incompatible with the conditions of their little, slender, elongated forms; but frogs have the same kind of lungs, and may be frequently seen to inflate them with air by a very simple, yet at the same time very effectual process. A frog may occasionally be seen moving inwards and outwards the sides of his face and the skin of his capacious throat. The animal is performing an act of respiration; not as we perform it, by expanding our chests; but this motion of the face and throat is precisely analogous to the motion of the leather of a pair of bellows-a motion by which the animal forces air into the lungs.

The chemical portion of the respiratory function chiefly involves the removal of carbon from the lungs. According to the experiments of Allen and Pepys, who were the earliest to investigate the chemistry of their function, the removal of carbon in the form of carbonic acid was the sole object; and according to them the carbonic acid corresponded exactly in proportional quantity with the amount of oxygen taken in. Later investigations, however, have proved that the explanation is not quite so simple; have proved that a portion of oxygen is absorbed, that nitrogen is sometimes absorbed, at other times given out. Still the most prominent consequence of the respiratory function in animals is evidently the removal of carbon from the blood, in the form of carbonic acid.

And now a very beautiful provision of Nature is rendered manifest to us. Carbonic acid being con

tinually evolved from animals would soon render the atmosphere unfit for respiration, did there not exist some purifying agency or agencies. Doubtless there are many such with which we are not conversant; but one, and a very powerful one, is evident—namely, the purifying agency of living vegetables. It is their function to assimilate the element of carbonic acid which animals reject it is their object to fix carbon. Hence they absorb carbonic acid by their leaves, fix carbon, and set oxygen free. So admirably is the balance of atmospheric composition maintained that the amount of carbonic acid scarcely varies in any part of the world-constituting one part in a thousand of atmospheric air. Now, the total amount of atmosphere being known with tolerable accuracy, and, consequently, the total amount of atmospheric carbonic acid, a doubt has been mooted by some, whether that amount be sufficient for supplying and keeping up that large amount of carbon which vegetables require. Calculation shows that the amount of atmospheric carbonic acid is sufficient for this end; but pursuing the contemplation further, the question was mooted, whether an atmosphere containing no more carbonic acid than at present could have been reasonably assumed to have furnished that enormous amount of carbon which is stored away in coal-fields. Probably, it was assumed, the atmosphere in that early period of the world, when coal-fields were deposited, contained more carbonic acid than it does at present; but for the greater number of vegetable species, an atmosphere charged with

any considerable amount of carbonic acid over and above that supplied to us is fatal. As regards the fern tribe, however, they have been proved by experiment to be capable of living and thriving in an atmosphere containing an amount of carbonic acid fatal to other species. Now, the fact is well known that coal-fields are chiefly made up of the remains of gigantic ferns; and hence we recognise the exercise of a wise foresight in so adapting the organism of these vegetables, that they could live and flourish in an atmosphere of carbonic acid which would be fatal to most other vegetables and the higher orders of animals.*

* All the animal remains which have been disclosed in older geological formations than coal are of very low organic types, to the organism of which, judging from their existing analogies, carbonic acid would not be injurious.

LECTURE VI.

CARBON.

NOTES OF MATERIALS AND APPARATUS REQUIRED FOR

ILLUSTRATING THE FOLLOWING LECTURE.

Slips of paper dipped in alum or borax.

Specimens of charcoal, diamond, plumbago, coke, &c.
Diamonds turned into coke.

Animal charcoal.

Tincture of litmus.

Materials for illustrating the process of fermentation :—
Jars of carbonic acid.

of carbonic oxide.

Apparatus for burning the diamond in oxygen gas.
Lime-water.

A burning candle to illustrate the property of flame.

A jar of olefiant gas, and another of chlorine, ready to be mixed.

"On

[THIS was the final lecture of the course of six, the Non-Metallic Simple Elements." The theatre was very fully attended, and his Royal Highness Prince Albert honoured the audience by taking the chair.

The lecturer commenced by directing the attention of his audience to the various pieces of carbon present on the table—the diamond, coke, charcoal (animal and vegetable), plumbago, and diamonds partly converted into the black amorphous form.]