If therefore we certainly know that there are some kinds of aerial vapours which when applied to the human body do exert a power directly opposite to the vital principle, there is no reason to doubt that such vapours may be confined among certain soft substances, such as cotton, wool, &c. and remain there for an unknown length of time, again exerting their malignant powers, when a fresh object comes in their way. Besides, as all kinds of air with which we are acquainted consist of a basis united with the ethereal fluid and volatilised by it, there is reason to suppose that contagions themselves are formed in the same manner. Some kinds of air also are very easily decomposed, in which case the basis attaches itself to some terrestrial substance, the ethereal fluid which volatilised it diffusing itself around in an invisible manner, but generally with a perceptible heat. Fixed air affords a notable example of this; for, by exposing it to lime-water, or even dry lime, alkaline salt, volatile alkali, or common water, a decomposition of the air very readily takes place, and its basis is found to be attached to those substances. What happens to fixed air may also happen to _contagion_. The basis of it may have a tendency to unite itself to cotton, or such like substances, and thus may not only infect them, but concentrate itself to such a degree as to produce a disease much more violent than that of the person who gave the infection; and something of this kind has even been observed with regard to infected cotton. But now another question occurs: As fixed air, by being attached to terrestrial substances, loses its aerial property, why should the basis of contagious effluvia still retain its malignant quality though in a state of decomposition? Here we are again helped out by analogy. Fixed air is known to be capable of resuming its aerial properties occasionally, from causes unknown to us, though we cannot suppose them to be any thing else than the invisible action of the ethereal fluid so often mentioned; which, being guided by laws unknown to us, we cannot possibly comprehend. The fact, however, is certain, that the basis of fixed air does very often quit the substances to which it is attached, and assume an aerial state in great quantity, and with very mischievous effects. Thus the old lavas of volcanoes, if chemically tried will be found at _all_ times to contain great quantities of the basis of fixed air, but it is only at _some_ times that the mofetes which are supposed to be the air itself, break forth.[96] In like manner the strata under ground always contain great quantities of the aerial basis, but the _damps_ in mines, which are certainly known to consist mostly of fixed air, do not always appear; neither do they gradually accumulate, but come suddenly, spreading unexpected destruction among those who unfortunately come in their way. The same may take place with contagion. After remaining some time in a