“might assist in sustaining subterraneous fires.... The solution of the phenomena of subterraneous fires would certainly be much easier on the supposition of their supplying their own _pabulum_, by means of dephlogisticated air contained in substances exposed to their heat. I therefore desired Mr Landriani, who being in Italy had a good opportunity of making inquiries on the subject, to inform me whether any of those substances, and particularly _manganese_ be found in their volcanoes; and his answer makes it rather probable that those fires are, in part, sustained by this means.” The ease with which nitre parts with its oxygen on heating furnished Priestley with the true explanation of its so-called “detonation,” “concerning which,” he says, “the most improbable conjectures have been advanced by the most eminent philosophers and chemists.” After a reference to the hypothesis of Macquer, who assumes that what he calls “a _nitrous sulphur_” is produced, Priestley points out that “the doctrine of dephlogisticated air supplies the easiest solution imaginable of this very difficult phenomenon. Let any person but attend to the phenomena of the detonation of charcoal in nitre, and that of dipping a piece of hot charcoal into a jar of dephlogisticated air, and I think it will be impossible for him not to conclude that the appearances are the very same and must have the same cause.” Of all the quantitative exercises performed by Priestley, by far the most numerous depended upon his application of nitric oxide to measure the “goodness” of air. “When,” he says, “I first discovered the property of nitrous air as a test of the wholesomeness of common air, I flattered myself that it might be of considerable practical use, and particularly that the air of distant places and countries might be brought and examined together with great ease and satisfaction; but I own that hitherto I have rather been disappointed in my expectations from it.... I gave several of my friends the trouble to send me air from distant places, especially from manufacturing towns, and the worst they could find to be actually breathed by the manufacturers, such as is known to be exceedingly offensive to those who visit them; but when I examined those specimens of air in Wiltshire, the difference between them and the very best air in this county, which is esteemed to be very good, as also the difference between them and specimens of the best air in the counties in which these manufacturing towns are situated, was very trifling.... I have frequently taken the open air in the most exposed places in this country at _different times of the year_, and in different states of the _weather_, etc., but never found the difference so great as the inaccuracy arising from the method of making the trial might easily amount to or exceed.”