After these theoretical speculations, “in which,” he says, “I fear I have not communicated much light, though it is as much as I have been able to get,” Priestley proceeds to make some observations relating to _phlogiston_, “the existence of which is at present a great subject of discussion with philosophers; some maintaining that there is no such thing, and others holding the doctrine of Stahl on the subject.” “According to Stahl, phlogiston is a real substance, capable of being transferred from one body to another; its presence or absence making a remarkable difference in the properties of bodies, whether it add to their _weight_ or not. Thus he concluded that oil of vitriol deprived of water, and united to phlogiston, becomes sulphur; and that the calces of metals, by the addition of the same substance, become metals.... What is now contended for is that in the oil of vitriol changing into sulphur something is lost and nothing gained, and also that a calx becomes a metal by the loss of air only. And did facts correspond to this theory it would certainly be preferable to that of Stahl, as being more simple; there being one principle less to take into our account in explaining the changes of bodies. But I do not know of any case in which phlogiston has been supposed to enter into a body, but there is room to suppose that something does enter into it.... “What has been insisted upon, as most favourable to the exclusion of phlogiston, is the revival of mercury without the addition of any other substance from the precipitate _per se_. In this case it is evident that mere _heat_ ... is sufficient to revive the metal. And as what is expelled from this calx is the purest dephlogisticated air, it has been said that mercury is changed into this calx by imbibing pure air, and therefore becomes a metal again, merely in consequence of parting with that air.” The dexterous Mr Kirwan, not long before he himself embraced the French doctrine, furnished Priestley with an argument which satisfied him that this cardinal fact can be accounted for without excluding phlogiston. “Since therefore the supposition is exceedingly convenient, if not absolutely necessary, to the explanation of many other facts in chemistry, it is at least advisable not to abandon it.”