“I even think that I may flatter myself so much, if it be any flattery, as to say that there is not, in the whole compass of philosophical writing, a history of experiments so truly _ingenuous_ as mine, and especially the section on the discovery of dephlogisticated air, which I will venture to exhibit as a model of the kind. I am not conscious to myself of having concealed the least hint that was suggested to me by any person whatever, any kind of assistance that has been given me, or any views or hypotheses by which the experiments were directed, whether they were verified by the result or not.” There is much else in the preface that might be quoted as illustrative of the character and mental attributes of its author. Priestley, the natural philosopher, never forgot that he was a minister of religion, and that to him theology was the greatest and most important of all the sciences, and he cannot forbear even, in what he intended to be a scientific disquisition on purely natural phenomena, from inculcating his belief in the divine origin of Christianity and his opinion concerning the doctrine of purgatory and the worship of the dead. The first chapter is concerned with the discovery of what its author called _Vitriolic Acid Air_, but which we now know as sulphur dioxide. Priestley imagined that as the liquid marine acid—that is hydrochloric acid—readily yielded an “air” on heating it might be that vitriolic acid, or oil of vitriol, would also afford a characteristic “air” when treated in a similar manner. Acting upon a suggestion of Mr Lane he heated oil of vitriol with olive oil, when he readily obtained a new species of air, which he collected over mercury as he “had been used to do it with the marine acid air; and the whole process was as pleasing and as elegant.” Priestley at once surmised that the olive oil worked by transferring its phlogiston to the vitriolic acid, and he naturally concluded that any substance rich in phlogiston would bring about the same result. He next tried charcoal.