GAS, (Enyel.) The composition of aerial vapours or gases, is a subject which has drawn the attention of the curious, though little can be known with precision. What can be known with any degree of probability is drawn entirely from the effects of latent heat upon terrestrial bodies. By means of this element almost all substances become for a time aeriform; and if they do not constantly remain so, it is because of their inability to absorb the heat in a permanent manner. From analogy, however, we must be apt to conclude, that those aerial fluids which continue permanently elastic are formed in like manner by the union of a vast quantity of latent heat with some small portion of terrestrial matter, in such a manner that they cannot afterwards be separated, at least so easily as the latent heat of mere aqueous or other vapours is absorbed. This will be in some measure confirmed by the two following considerations. 1. There is no kind of aerial vapour which does not expand by heat to a very great degree. Terrestrial bodies, whose texture is close and compact, expand but little by heat. These, we know, are formed of a great quantity of inactive matter, together with a very fine invisible fluid with which their pores are filled. The expansion probably takes place by means of an agitation in this fluid excited by heat. Air is exceedingly expandible; whence its terrestrial particles are probably few in number, while the quantity of fluid interposed betwixt them is very great. 2. All kinds of air seem to be produced from some terrestrial matter and heat, at least heat in some way or other seems to be concerned in the formation of all that are yet known. In the first process published by Dr Priestley for making dephlogisticated air, a violent heat was employed. Oil of vitriol poured on red-lead was afterwards found to yield dephlogisticated air also: but the red-lead had been previously subjected to a strong heat; and, even in the very act of producing the air, a great degree of heat is spontaneously generated. In the natural processes for producing dephlogisticated air, the light of the sun, and consequently his heat, in some degree is necessary for its formation. Besides, all the different airs or gases we know, fixed air, inflammable, nitrous, marine acid, vitriolic acid, fluor acid, vegetable acid, phlogisticated airs, &c. are every one of them produced from terrestrial materials. In these materials we know that an invisible fluid, the same with elementary heat or fire, is continually acting; whence it is exceedingly probable that all of these vapours are nothing else than combinations of the two.