brought into existence, destined to counteract those pernicious effects by diffusing the purest air through the atmosphere. This verdure, or chain of animals, observable on the surface of wells and stagnant pools, was first discovered to be what it is by the Abbè Fontana.

Besides what I have already said of the extraordinary properties of vital air, there is one principle, whose applications are very interesting to be known. In most combustions, the oxigenous gas, agreeably to Lavoisier's theory, becomes fixed and concrete. It therefore abandons the caloric which maintained it in the aeriform state; and this caloric being set at liberty, produces heat, and endeavours to combine itself with the substance nearest at hand. The disengagement of heat is, therefore, a constant effect in all the cases wherein vital air is fixed in bodies; and it follows from this principle, 1. That heat is most eminently resident in the oxigenous gas, which maintains combustion. 2. That the more oxigene is absorbed in a given time, the stronger will be the heat. 3. That the only method of producing a violent heat consists in burning bodies in the purest air. 4. That fire and heat must be more intense in proportion as the air is more condensed. 5. That currents of

air are necessary to maintain and expedite combustion. It is upon this principle that the theory of the effects of the cylinder lamps is founded; the current of air, which is renewed through the tube, supplies fresh air every instant; and by continually applying a new quantity of oxigenous gas to the flame, a heat is produced sufficient to ignite and destroy the smoke. On the same principle is likewise explained, the great difference which exists between heat produced by a slow combustion, and that which is afforded by rapid combustion. *

* Chaptal's Chymistry.

S 2

LET

LETTER XXI.

AFTER all that I have advanced on the subject of air, in the style of the old school indeed, but not, I hope, without some degree of accuracy, I still think it necessary to be somewhat more particular on so important a question. Bergman, to whom science is so much indebted, treating of the aeriform fluid, says, "By the term aerial form, I understand such a subtilization of a body, as renders it elastic, pellucid, invisible, light, and permanent, in cold, though not capable of passing through the pores of glass. Vapours, which constitute imperfect kinds of air, are condensed by refrigeration; on the other hand, we have elastic fluids, which may not be improperly stiled ethereal; to such neither the pores of glass, nor of any other known body, are impenetrable. To these belong the matter of heat, and the magnetic fluid. The electric fluid easily penetrates all bodies, except electrics per se. Moreover, light seems to be something intermediate between aerial and ethereal substances; for it passes through the

pores

pores of glass, but not those of metals, and other opaque substances."

We thus see, that the term aerial is not entirely confined to the fluids, which we are endeavouring to investigate. But as these elementary distinctions will more properly occupy our attention at a future moment, we shall still confine our attention to those permanently elastic fluids, generally known by the name of air. In enumerating some of these, we have slightly touched upon the noxious, as well as upon the beneficial airs. There are a few others, however, of the former class, which it is essential to be acquainted with. And in the first instance, let me call to your recollection, that the air in which metals are burned, is at all times highly phlogisticated. For it appears demonstrated, that in the calcination of metals there is nothing more than a combination of the metal with the pure air contained in the atmosphere, which consequently vitiates it. Let me request you likewise, to bear in remembrance, that the air, the most agreeable to animal life; is that whic takes least of sulphur; for sulphur, and a the modifications of it, (at least the aerial) from /hatever sources they may arise, whether from minerals under the earth, from marshy grounds,

ar

or smoaking chimnies, is noxious to the lungs, and will generally have an ill effect on animal respiration. * And hence the reason why charcoal, which, according to Bergman, is nothing but a combination of phlogiston and aerial acid, (99 parts out of 100 being phlogiston) and a species of sulphur, is so greatly pernicious.

"The hepatic air," says Mr. Kirwan," is an actual sulphur kept in an aerial state by union with the matter of heat. It is obtained from combinations of sulphur with alkalis and other substances. Its most obvious characteristics are, a peculiarly fœtid smell; inflammability, when mixed with a certain proportion of common, or nitrous air; miscibility with water in a certain quantity; and a power of discolouring metals, particularly silver and mercury. mercury. This air acts an important part in the economy of nature. It is frequently found in coal-pits; and Bergman has shewn it to be the principle on which the sulphurous properties of mineral waters depend. There is also reason to think, that it is the peculiar product of the putrefaction of animal substances. Rotten eggs and corrupt water are known to emit the smell peculiar to this species of air, and to dissolve metallic substances in the same manner.

Besides

* Philosophy of the Elements.