Fixed-Air, an aerial fluid which is disengaged from allsub-stances liable to undergo the vinous fermentation, as well as by mixing alkaline salts and earths with acids. It is, strictly speaking, a gas which is essentially different in its properties from atmospheric air, as the former is unfit to support either the respiration of animals, or the burning of a candle; being likewise specifically heavier than the common air we breathe. From its acid properties, it has been variously denominated aerial acid, cretacious acid, or carbonic acid: and, from its noxious qualities, it has received the name of mephitic gas. The appel lation of fixed air has been applied to it from its readily losing its elastic property, and fixing itself in various bodies, especially those, which are of a calcareous nature.

Fixed air was first discovered by Dr. Black, who, inconsequence of various experiments, found that chalk, and the other earths reduci-ble to quick-lime by calcination, consist of an alkaline earth, which is soluble by itself in water; but which, when combined with, a large quantity of fixed-air, becomes insoluble; losing the properties of quick-lime, and assuming the apearance those earths naturally have, when not reduced to a calcareous state.

Dr. Black observed the same phenomenon in white magnesia, and in alkalis both fixed and volatile. Their effervescence with acids, arid their mildness, depend on the fixed-air which these bodies contain ; because alkalis and calcareous earths become in a high degree caustic, when divested of that gas. He farther remarked, that fixed-car had differentdegrees of affinity with various substances; being stronger with calcareous earth than with fixed alkali; with the latter than with magnesia ; and with this than with volatile alkali.

This new gas was introduced into the catalogue of medicines, by its .strongly antiseptic properties : it cannot, however, on account of its fatal effe6ts, be inspired in large quantities,though in small portions it may be inhaled without danger.

Dr. PeRcival first administered it on a large scale, and directed his patients, in more than thirty cases of pulmonary consumption, to inspire the steam of effervescing mixtures of chalk and vinegar through the spout of a coffee-pot. By this treatment, the hectic fever was, in several cases, considerably abated, and the matter expectorated became less offensive, and better di-gested. Although Dr. Percival was not so fortunate as to effect a cure in any one instance, yet the late Dr. Withering met with better success ; as one of three patients was thus restored to perfect health ; another received great ben-fit, and was much relieved ; and the third was kept alive by inhaling this gas for more than two months. Fixed-air, however, can only be employed with advantage in those stages of pulmonary consumption, when a purulent expectoration, or a rupture and discharge, of an abscess in the lungs, have taken place : in such cases, this remedy affords a powerful palliative.

Farther, it is equally useful when applied to foul ulcers ; and instances have occurred, in which the sanies, or corrupt matter issuing from can-cers, has been sweetened, the pain alleviated, and a better suppuration produced, even after the carrot-poultice had failed. But, though fixed-air evidently checks the progress of a cancer, there is reason to apprehend that it never will ef-fect a cure.

Considerable benefit has also been received in ulcerated sore-throats, from inhaling the vapours arising from effervescing mixtures. This remedy ought, however, by no means to exclude the use of other antiseptic applications.

In that dreadful disorder, the malignant fever, wines strongly impregnated with fixed-air may be administered, with a view to check the septic ferment, and to neutralize the putrid matter in the stomach and intestines. If the patient's common drink were thus prepared, it might be attended with beneficial effects. - As the latter stages of malignant fevers are generally accompanied with putrid diarrhoeas, this evacuation ought not to be restrained by the use of astringent medicines; because the retention of putrid matter in the body will aggravate the delirium, and increase the vehemence of the fever. And i€ if the disorder be suffered to take its usual course, death is the inevitable consequence. - In cases of this dangerous nature, mephitic or fixed air, produced from a mixture of chalk and oil of vitriol, has been injected into the intestines, by-means of the instrument employed for tobacco clysters ; by which application the violence of the diarrhoea was quickly abated; the heat and fetor of the stools corrected; and every alarming and dangerous 6ymptom in a short time removed.

The last disorder in which the use of fixed air has been attended with success, is the calculus, or stone, of which it is said to be an excellent solvent; but, as the experiments made on this subject have not hitherto accurately determined its efficacy, we cannot speak of it with any degree of confidence.-See Stone.