4. The air acts upon the blood by the _latent_ heat it contains. The air itself is composed of something volatilised by heat. In some cases this is evidently a terrestrial substance, as in that of inflammable air, or hydrogen, which is formed of charcoal volatilised by heat, with the addition of a little water. In the case of oxygen, or dephlogisticated air, the combination seems to be the matter of heat (which I shall hereafter distinguish by the name of the _ethereal fluid_) with water deprived of its carbonic principle. This coincides with the opinion of Dr. Priestley, who says that the basis of dephlogisticated air seems to be _dephlogisticated water_. But, let the basis be what it will, the ethereal fluid which volatilises it is the _agent_; the basis is entirely _passive_, and only modifies or restrains the action of the other fluid, so that it does not exert itself except in particular cases. Fixed air, or carbonic acid, is composed of the base of oxygen united with a certain portion of carbon, and the whole volatilised by the ethereal fluid. Phlogisticated air, azote, or septon, according to Dr. Priestley, consists of the basis of dephlogisticated air along with a certain proportion of carbon different from that which produces fixed air, volatilised by the same agent;[90] and so we may determine concerning every other species of air. [90] See Medical Repository, vol. ii, No. iii. 5. In certain cases the ethereal fluid quits those substances with which it is united: the air is then decomposed, the substance into which the other fluid enters is heated, or rendered more fluid than before (perhaps both) while the basis either unites itself to the moisture of the lungs, or is thrown out by the breath. Whether in any case the basis can pervade the membranes, and thus mix itself with the blood, notwithstanding the positive assertions of Dr. Girtanner and others, is very doubtful, and does not admit of any positive proof. 6. The blood, being a _fluid_, must be subject to the same laws with other fluids. A certain quantity of _latent_ heat must be contained in it, in order to give the degree of fluidity naturally belonging to it. If this quantity be augmented, the fluidity will be augmented, and the blood will become thinner; if it be diminished, the contrary will take place; and if we suppose a great proportion of this latent heat to be abstracted, it is not unreasonable to suppose that something like a congelation may take place, and the blood be changed into a solid substance of such a nature as cannot any more be made to resume its former qualities. 7. By augmenting the sensible heat, the blood is affected in the same manner as any other fluid; it suffers expansion, by which the vessels are dilated in proportion, and, if this expansion and dilation be carried to a certain length, a rupture of many of the small vessels, and apoplexy, or some other grievous disease, may ensue.