The evolution of oxygen gas in sunshine, is the chief means with which we are acquainted, by which the chemical equipoise of the atmosphere is maintained, against the operations constantly going on, to which we alluded. These observations relate to the chemical composition of the air, considered as one great whole. There are many situations in which it becomes not only deprived of its oxygen in part, but becomes vitiated by admixture with foreign bodies or vapours, most detrimental to health, in short, most pestiferous. But, before pointing out the manner in which it becomes so tainted, and its unwholesome consequences, we would here point out the use of the atmosphere. By the act of respiration, air is carried into the lungs; it acts upon the blood brought there in large quantities, and spread out in innumerable vessels, forming a sort of network. The blood, upon its arrival at the lungs, is dark, grumous, and unfit for the maintenance of life, and the nutrition of the body; but, under the action of the air, it becomes florid or crimson, has changes wrought upon it, by which it is fitted to perform its various and important functions. This chemical process gives a crimson and florid hue to the old blood of the system, and imparts a colour and other qualities to the fluid brought from the bowels, the result of digestion, which give it the character of blood. It gives to that fluid the last preparation before being converted into blood. The heat of the body, which is above that of the surrounding atmosphere, is maintained by the chemical changes which occur between the mass of blood in the lungs, and the air to which it is there exposed. There is a constant generation of heat, which is diffused along with the blood throughout the whole system,—to supply the place of that which is ever being abstracted by surrounding bodies, among which exists a constant tendency to preserve an equilibrium of temperature. When the atmosphere is vitiated, it is reasonable to suppose, that the changes in the blood passing through the lungs will not take place in their wonted integrity, and that, among other results, a diminution of the vital heat of the body may be experienced. Vitiated air admits of division into different kinds:— _1st_, Into air simply deprived more or less of its oxygen. _2d_, Into air holding in solution, or having mingled with it, effluvia from animal bodies, living and dead. _3d_, Into air holding in solution, or having mingled with it, noxious gases or effluvia arising from decomposing vegetable matter. Vitiated air, of every kind, is unwholesome and favourable to the invasion of disease. Vitiated air has been coexistent with many of the most appalling visitations of disease, which have befallen man since the creation of the world. It delights in the production of the most formidable distempers, such as are marked with extreme debility and proneness to the putrefactive character.