lungs, are, even in the most healthy persons, in a state not very remote from putrefaction; but in persons labouring under disease, they are still of a more noxious nature. Nor are we to stop even here. Fresh meat, even without the least sign of putrescency, phlogisticates common air to a great degrec, and in a very short time. This inflammable air, or phlogiston, (for, like ice and the vapour of water, they are one and the same substance) is rendered pure and wholesome, by that which, in a state of decay, is equally, if not in a superior degree, deleterious, I mean the vegetable kingdom.

Animal substances have at all times a strong disposition to putrefaction, while the tendency to it in vegetables is slow; and the reason probably is, that the air in animals is mostly inflammable, but that in vegetables fixible. However this may be, it is an indisputable fact, that putrid air is rendered wholesome by the means of vegetation perfectly in health, and the plants growing in situations natural to them. The noxious effluvium, or phlogiston, is in some measure extracted from the air by means of the plants, or the phlogiston of the air unites with their exhalations, and they thereby render the remainder

* Adair. + Priestley.

remainder more fit for respiration. They, in short, imbibe the superabundant phlogiston; for fluid fire, as well as fluid air, is imbibed by plants in their growth.

In the whole œconomy of nature we in this manner see, that one substance purifies another. Thus fire purifies water. It purifies it by distillation, when it raises it in vapour, and lets it fall in rain; and farther still by filtration, when keeping it fluid, it suffers that rain to percolate the earth. * Animal substances, when mixed with earth, and applied as manure, are converted into sweet vegetables; and putrid substances, mixed with air, may in the same manner be supposed to have a similar effect; but flowers and fruits, we have already observed, and even the roots of plants, when kept out of ground, generally yield bad air, and contaminate the atmospheric air, especially in the night. Yet, the leaves of these plants, while growing, struck by the rays of the sun, are sources whence exhale a continual torrent of pure air, destined to renovate the atmosphere.

Plants, as I have already explained, begin to yield dephlogisticated air a few hours after the

Priestley.

sun

in

ge

sun has made his appearance, and cease, neral, with the close of the day. In a clear day, they yield more than when it is cloudy. It is also greater when the plants are more exposed to the sun, than when they are situated in shady places. From all which it is demonstrable, that the damage done by plants in the night-time, is more than counterbalanced by the benefit they afford in the day-time. By a rough calculation, it has been found that the poisonous air, yielded during the whole night by any plant, could not amount to the one-hundred part of the dephlogisticated air, which the same plant yielded in two hours in a fine day. Plants, in themselves, do not generate dephlogisticated air, they merely filtrate the common air, and separate the phlogiston from it; which phlogiston is absorbed by the plants, and incorporated into their nature. In this operation they do just the contrary of what is performed by animals: they in their vegetation absorb phlogiston from the air; whereas animals, by their respiration, separate the phlogiston from the bodies, and give it to the air. Hence it is that, phlogiston being one of the principal nutriments of vegetables, vegetation is so strong in the neighbourhood of large towns; for large towns, from the number of fires, the breathing of multitudes,

and

and various other phlogistic processes, send into the atmosphere a prodigious quantity of phlogiston, which being afterwards precipitated, or caught by the leaves, gives them a vigour and growth greatly superior to those in the country; of this you may easily be convinced by experiment: for, put two vegetables under glass jars, as nearly alike as possible, and serve the one with phlogisticated, and the other with atmospheric air, the former you will find shall be strong, healthy, and considerably grown, while the latter shall have lost its colour, be yellow, sickly, and drooping.

The very weights of phlogisticated and dephlogisticated airs, as we have already seen, are calculated by nature for our benefit; for the former being specifically lighter, tends directly upwards, and consequently continually recedes from us; whereas the latter, being specifically heavier, as soon as it is yielded by the leaves of plants, tends downwards. * In regard to vegetation, observes Priestley, "It is remarkable, that wells have been injured by being cleansed. green matter which sometimes is seen to grow on the top of the water, in many instances," says he, "keeps the water sweet, by imbibing the

*Cavallo.

The

the phlogistic matter which it discharges in its tendency to putrefaction. Take that green substance away, and the water will become foetid, and unfit for use, and will continue so until the sides and top of the reservoir of water shall again grow foul. This was Priestley's idea some years ago, but I do not know that he continues of the same opinion. Many naturalists, as well as he, conceived this substance to be neither animal nor vegetable, but to be merely an aerial or gazous scum. Ingenhouz, however, on the contrary, conceived it to be a vegetable, a species of delicate moss; but they probably all were mistaken as to its real nature. Later experiments prove it to be an animal substance, a mass of scarcely perceptible green animalcula, which have a very short existence, terminated in being clung together by a gluten that seems to exude from their own bodies, and which thus forms the crust on the surface of the water. This production, indeed, is wonderfully to be augmented, by putting any substance into the water which is in a state of putrescency. And in this, we have another striking instance of the wise and stupendous plans of the Creator, who has thus ordained, that in the midst of putrefaction, which has a natural tendency to contaminate the air, a race of beings should be brought

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