We propose to close this part of the work. Much has been said in order to prove the position, that the doctrine of atmospheric contagion gains no support from the actual character of disease, no countenance from the ungarbled history of its career. Arguments in favour of our views might have been drawn from the fact, that diseases said to be propagated by contagion, do not manifest themselves in all parts of the globe to which the poison would be likely to be taken, as they undoubtedly would do, were they dependent on the operation of one single object, such as contagious matter; and also from the consideration that those diseases, with whose causes we are intimately acquainted, by reason of their immediate operation, or of their being otherwise obvious, such as inflammation and wounds, are never said to be dependent on such an agency; but it is feared in the endeavour to be explicit, we have already been tiresome. CHAPTER V. ON VITIATED AIR. The question of air holding in solution, an animal contagious matter, eliminated in the body of a sick person, and capable of producing the same disease, when inhaled by another, has hitherto occupied our attention. It is now our design to treat of vitiated air, that is, an atmosphere deprived of part of its more essential principle, viz. oxygen gas, or tainted with the admixture of effluvia or gaseous products, from putrefying animal bodies, both living and dead, and from corrupting vegetable matter. It is one of the most common, and most widely spread causes, of the most virulent and widely prevalent diseases, to which humanity is subject. The importance of the atmosphere to the animal economy, is so very great, and its derangements so very hurtful to health, that it appears that a few observations respecting it may be useful to some non-professional readers. It may enable them to understand better the observations that are to follow on its vitiation. The atmosphere is a fluid of an elastic nature, encompassing the globe, occupying the space comprehended from its surface, to the distance of twenty or thirty miles therefrom. It possesses weight, and it is by this property that water rises in pumps, and that mercury is sustained in the barometer. It is in constant motion, going, as it does, with the globe itself, revolving on its axis, and rushing, in counter streams, from the tropics to the poles, and from the poles to the tropics. That portion nearest the sun becomes rarefied and lightened with the heat which it acquires:—it then rushes, by virtue of its comparative lightness, to the poles, and that in temperate regions presses forward to occupy its place. By means of this motion, the temperature of the earth is kept pretty uniform, and it is corrected of any impure taint it may acquire.