LETTER XX.

FROM the general glance which we have thus cast upon the principal of the permanently elastic fluids, we have been enabled, at least, to establish a conviction of their very high importance in the general chain of terrestrial nature. But there are yet more familiar lights, in which they are to be placed; I mean the powerful and uniform effects which they have upon the animal and vegetable, as well as upon inanimated nature. Under this view, therefore, we shall again, for a moment, consider them.

Plants and animals, as I have already said, act reciprocally on air, for each other's advantage; the breath of animals corrupts the air; the air so corrupted becomes more nourishing to plants; and the respiration observable in plants, is the reverse of that in animals. The latter take in pure air, and send it out foul and phlogisticated. Plants take in this foul air, and return it pu rified; and this is simply analogous to their fa

culty

*Philosophy of the Elements.

culty of elaborating sweet juices from impure earth and manure. The sun raises a pure air from plants; whence the atmosphere in the daytime is rendered more wholesome than in the night; for plants in the night, or in the shade, have a contrary effect upon the air; but their bad effect in the night is not nearly so great as their good effect in the day. Flowers always hurt it both day and night. Water plants are remarkably vigorous in their faculty of yielding pure air to correct the inflammable air, which is bred by the soil in low marshy grounds. Thus the best remedies are produced, in every region, for its native evils; and thus do all things work together for the general good.

"The effluvia of human bodies," says Arbuthnot," and the same in some degree of other animals, are extremely corruptible; and so is the water in which people bathe, by retaining cadaverous particles. Less than three thousand human creatures, living within the compass of an acre of ground, would make an atmosphere of their own steams, about seventyone feet high, which, if not carried away by the winds, would turn pestiferous. Owing to this circumstance, the air of prisons often produce mortal diseases; and ships crews turn sickly in

bays

bays and harbours, who would be healthy in the open seas.

That the air which passes from the lungs is highly mephitic, is easily proved, by breathing five or six times into a glass vessel, and by then putting a lighted candle into it, the candle will instantly be extinguished; but introduce a fresh plant into the vessel, and then immerse the vessel in water, so as that the newly generated air shall not escape, and it will be so altered, as to be rendered thoroughly pure and wholesome, and, of course, the candle will burn in it with the greatest freedom. The manner in which phlogiston thus taints air, may be in this familiar way exemplified. A lighted taper placed under a glass, or other close cover, will preserve its flame, until the interstices between the particles of the enclosed air shall be saturated with the phlogistic matter, and shall be capable of containing no more. In like manner, if a lighted taper under a glass cover be immersed in water, the flame will continue in vigour, until the surrounding air shall begin to be contaminated by the phlogiston from the taper. It will then gradually die away, and at length go out altogether. And hence the experimentalist, who descends into a diving bell, repeatedly falls the sacrifice to

the

the air which he has breathed. That which passes from himself, he is frequently obliged to inhale, and thus every expiration adds to the mephitic quality of the surrounding fluid, and consequently so loads it with phlogiston, that unless there be a fresh supply of atmospheric or dephlogisticated air, the vital principle will be inevitably destroyed.

Animal effluvia, even from the living body, we thus see, are highly pernicious: when accumulated, and repeatedly respired, they are fatal to the animal economy; they diminish the energy of the brain and nervous system, and weaken the action of the heart and arteries. The Count de Milly, of the Royal Academy of Sciences at Paris, undertook various experiments to ascertain the reality of the animal emanations, and at length very satisfactorily proved, that the animal air which issued from his body in a bath, was different from the air of the atmosphere, and had properties that gave it a striking resemblance to what we call fixed air. The pulmonic air, indeed, sent forth by respiration, is nothing else than a mixture of fixed air, diffused through a large quantity of common air, by which the former is carried off, and the pernicious effects of its accumulation are prevented; and this explains the principle

of suffocation, and the danger of close crowded apartments, in which the introduction of common air being obstructed, the emanations of fixed air, or animal gas, gain the ascendant. Priestley, however, says, "that it is respiration, and not perspiration, which injures the air." But it seems pretty evident, that both one and the other tend to the accumulation of the mephitic fluid.

Odour, the old chymists said, was an indication of that which modern chymists have proved to be phlogiston. The most delicate flower, as I have already observed, considerably injures air. For instance, nothing is sweeter than a rose, and yet the effluvia from a rose are far from being favourable to the air in which they are confined. In a certain quantity of atmospheric air, the air they would yield would be so noxious, that an animal would immediately expire in it. And hence the odours which arise copiously from bodies, without diminishing their weight, may be supposed to be occasioned, not by an actual diffusion of the substance, but by the modification of the more subtile phlogistic spirit which is continually passing through their pores. But many of the discharges, especially from the surface of the body, and from the

lungs,