the impression which the nitric, sulphuric, or muriatic acids make upon the tongue. The causticity of acids appears to arise from their strong tendency to combination: and it is from this property, that Newton has defined them to be bodies which attract and are attracted. It is likewise from this property, that certain chymists have supposed acids to be pointed substances. On account of this decided tendency also to combination, it seldom happens that they are found in a disengaged state. With alkalis acids they generally effervesce, but not always. Thus, the carbonic acid, and almost all weak acids, cannot be distinguished by this property; and the purest alkalis combine with acids, without motion or effervescence. In regard to alkalis, it is usual to call any substance an alkali, which has an acid, burning, urinous taste; the property of converting syrup of violets, green; the virtue of forming glass when fused with quartzose substances; and the faculty of rendering oils miscible; of effervescing with certain acids; and of forming neutral salts with all of them. The alkalis are divided into fixed and volatile. This distinction is established upon the smell of these substances: the former are not volatilized, even in the focus of burning mirror, and emit no characteristic smell; where

as

as the latter are easily reduced into vapour, and emit a very penetrating odour.

But as we have already observed, fixed air enters universally into the composition of all animal substances, and is of so extraordinary a nature, that it may be made the instrument either of health or of disease, of life or of death.* For instance, if an animal be immersed in a sufficient quantity, a total loss of sense and motion immediately ensues; and if the animal be not speedily returned into common air, death is the certain consequence. On the other hand, when properly managed, it promises salutary effects in some instances, and may be variously adapted to the purposes of medicine. It may be externally applied, either by a proper apparatus determining it upon the diseased parts, or by mixing it with the air of the chamber, so as to be freely and constantly respired.

The cohesion and solidity of substances are, in the opinion of Macbride, owing to the fixed air they contain: and, when by any means this is taken away, the mutual adhesion of the several parts will be destroyed, and the body will either run into the putrefactive fermentation, or crumble

* Dobson.

ble into dust, according to the nature of its constituent parts. Hence it necessarily follows, that whatever substance has the power of impeding the separation of fixed air, or of restoring it when separated, will also prevent or correct putrefaction. A piece of flesh, for instance, surrounded by a substance of this kind, is kept sweet, because the fixed air cannot make its escape, and that, probably, on account of its pores being blocked up by the finer particles of the antiseptic matter. If it has already, however, become putrid, it will receive fixed air from the antiseptic body, and hence cease to exhale a fœtid smell, lose its fluidity and flabbiness, and at last recover its sweetness and firmness.

The power of this aerial acid, you thus perceive, is very great; it preserves and it destroys. But, there is one thing very singular regarding it, which is, that if an animal be plunged into a sufficient quantity of it, he will instantaneously expire, and without a convulsion. Now why this should be the case is uncertain, unless the system of an animal, or its vital principle, be of the nature of the electric matter, which is in a similar manner, and with equal expedition, swallowed up and dissipated by it.

Thus we

see

see that not only grottoes and caves, but that even waters themselves are sometimes so strongly impregnated with fixed air, as to kill frogs and fishes of every kind. "Ranas nimirum, cancros, pisces quoscunque, in his aquis incontinenter mori." And hence the probable utility of fixed air in vermicular diseases, as recommended by Dr. Hulme. As it deprives an animal of existence, so it effectually prevents fire from being kindled, and combustible matter from explosion; it even completely and instantly extinguishes a body red hot and flaming; so that to determine the presence of this mephitic fluid by flame, is a common experiment. In some places the poisonous stratum of fixed air has a very considerable depth indeed. In the Grotto del Cane near Naples, men, and even large dogs, as you have seen, suffer nothing while they stand erect; but if the nostrils of any animal be brought near the ground, it is instantly seized by stupefaction. This deadly vapour may be seen, like a very subtile smoke, which in summer rises about a foot from the ground, but in winter not above a few inches.*

in

Feeling the benefits, however, of fixed air, as well as its deleterious qualities, philosophers

* Bergman.

con

conceived hopes, that it might be rendered of the most decided utility to mankind. Sir John Pringle among the rest, and with great probability, supposed, that the great abundance of it contained in vegetables and saccharine substances, and which at this time make up a considerable part of the diet of European nations, was the cause of the prevention of those putrid diseases and plagues, which were in former days so very prevalent. Fixed air most assuredly has the power of sweetening the putrid effluvium. It has indisputably an antiseptic power. It may be said to be demonstratively certain, that it destroys the putrefactive fermentation. Hence, (says Priestley) the use of wort and other substances, which contain fixed air, and yield a great quantity of that fluid in the stomach, are strongly recommended in scorbutic cases. Administered, or taken internally as medicine, they have been found of prodigious service in putrid disorders. Men in the sea scurvy are cured by being buried partially, or up to the necks in fresh earth, and this is owing to the absorption of the fixed air by the pores of the skin. Following the plough is also an old prescription for a consumption. Fresh mould takes off the foetor from meat beginning to putrify.