« ÎnapoiContinuați »
time, there must be an incessant vibration, or dilatation and contraction, in all bodies. We observe this reciprocation in several instances, particularly in plants, the trachea or air-vessels of which do the office of lungs; for the contained air alternately contracting and expanding, as the heat increases or diminishes, by turns presses the vessels, and eases them again; and thus promotes the ascent and descent of their juices. Hence we find that no vegetation or germination will proceed in vacuo. Beans, indeed, have been observed to grow a little tumid therein; and this has led some to attribute that to vegetation, which was really owing to no other cause than the dilatation of the air within them.
The air is very instrumental in the production and growth of vegetables, not only by invigorating their several juices, while in an elastic active state, but also by greatly contributing, in a fixed state, to the union and firm connection of their several constituent parts.
From this elastic force it is, that the air contained in bubbles of ice, by its continual action, bursts the ice; and thus glasses and other vessels frequently crack, when their contained liquors are frozen. Thus also, entire columns of marble sometimes cleave in the winter time, from the increased elasticity of some little bubble of included air. From the same principle arise all putrefaction and fermentation, neither of which will proceed, even in the best disposed subjects,
Air, moreover, not only acts by its common properties of gravity and elasticity, but there are numerous other effects, arising from the peculiar ingredients of which it consists. It not only dissolves and attenuates bodies by its pressure and attrition, but as a chaos containing all kind of
menstrua, and consequently possessing powers for dissolving all bodies. Iron and copper readily dissolve, and become rusty in air, unless well defended with oil. Boerhaave asserts, that he had seen pillars of iron so reduced by air, that they might be crumbled to dust between the fingers. Copper is converted by the air into a substance much like the verdigris produced by vinegar. Mr. Boyle relates, that, in the southern English colonies, the great guns rust so fast, that, after lying in the air a few years, large cakes of crocus martis may be separated from them. Acosta adds, that in Peru the air dissolves lead, and considerably increases its weight. Yet gold is generally esteemed indissoluble by air; being never found to contract rust, though exposed to it ever so long. Stones, also, undergo the changes incident to metals.
Air, likewise, as I have observed in a former paper, by means of its dissolving power, accelerates evaporation and distillation. It volatilizes fixed bodies; fixes volatile bodies; and brings many quiescent bodies into action, by exciting their latent powers. In a word, in the various operations of chemistry, it is a very necessary and important agent; the result of particular processes depending on its presence or absence, on its being opened or inclosed.
Great part of the effects of air which I have enumerated, were quite unknown, as such, to the antients. They considered it, however (as ap pears from the mottoes at the head of this paper), as an essential part of the world, as connecting heaven and earth, and the only medium by which objects can be visible to us, and sounds become audible. If we consider,' says Dr. Derham,
• No. XII., containing the Theory of Evaporation.
the nature and make of the air, we find it a mass of subtile penetrating matter, fit to pervade other bodies; to penetrate into the inmost recesses of nature; to excite, animate, and spiritualize; and, in short, to be the very soul of this lower world; useful to the life, the health, the comfort, the pleasure, and the business of the whole globe.' It is essential to the existence of all creatures.
Were once the energy of air denied,
The heart would cease to pour its purple tide;
To the motion and conveyance of many animals it is also necessary.
The feathered warbler hence, with rapid wings,
Various sorts of fish, too, are enabled to sustain themselves in any depth of water, and to rise or sink at pleasure, by means of a kind of vesicula, or air-bladder, which is found in their bodies'.
Our atmosphere is of admirable use in the enlightening of the world, by reflecting the heavenly bodies to us, and refracting the sunbeams to our eye; so that the day is protracted throughout the whole globe, the long and dismal nights are shortened in the frigid zones, and the day approaches them sooner. The sun itself rises in appearance, when, in reality, it is absent from them, to the great comfort of those dreary regions.
It is in the atmosphere that are assembled the clouds, which assume such a variety of hues and
If this air-bladder be pricked or broken, the fish sinks to the bottom, unable either to support or raise itself up again. Flat fish, as soles, plaice, &c. which always lie grovelling at the bottom, have no air-bladder.
forms, and which, as they are condensed or rarefied, retain the vapours, or return them to the earth in rain, or hail, or snow.
The streams, their beds forsaking, upward move,
In a former Paper', I have given a philosophical account of the winds, which are another important part of the subject of air. To that paper I refer my readers, and shall conclude my present Essay with a poetical enumeration of their benefits:
Of what important use to human kind,
To what great ends subservient, is the wind!
It drives the clouds, and ventilates the skies;
O'er torrid climes it pours the healthful breeze:
See No. II.
ON THE INFLUENCE OF AIR ON VEGETATION, AND OF VEGETATION ON AIR.
I cannot go
Where Universal Love not smiles around—
1 OBSERVED, in my preceding Paper, that air is very instrumental in the production and growth of vegetables. All vegetable bodies are provided with certain canals, or ducts, whereby a kind of respiration is effected in them. These canals, which are called air-vessels, are distinguished from another wise contrivance of Nature, the sap-vessels. The former answer to the trachea and lungs of animals; the latter to their lacteals and blood-vessels.
An ingenious natural philosopher, in an inquiry into the motion and cause of the air in vegetables, has shown that it enters them various ways, not only by the trunk, leaves, and other parts above ground, but at the root. For the reception as well as expulsion of the air, the pores are so very large in the trunks of some plants (as in the better sort of thick walking canes), that they are visible to a good eye without a glass; but, with a glass, the cane seems as if it were stuck full of large pin-holes, resembling the pores of the skin at the ends of the fingers and ball of the hand. In the leaves of the pine, seen through a glass, they make an elegant appearance, standing almost in rank and file, throughout the length of the leaves.