relative density, and he knew perfectly well that they tended to escape from the bottles in which they were contained if these were uncovered and freely exposed to the air. But, so far as we can learn, he never seems to have pondered on these facts, or noted their connection with the phenomena he observed in the course of his many experiments with Wedgwood’s retorts, and of the interchange of the water vapour he introduced into them with the gases of the fire which heated them. And yet, had he perceived even a glimmer of the truth he had sufficient means at his disposal, and sufficient knowledge from his own work and that of his contemporaries, to make the great step which it was reserved to Graham to accomplish half a century later. Whilst the chief importance of the _Experiments and Observations on Different Kinds of Air_ is that it is Priestley’s _magnum opus_, to his biographer it has the additional interest of affording an insight into the personal character and intellectual attributes of its author. Few writers on scientific subjects have ever taken their readers so completely into their confidence as Priestley. Whatever he knows or thinks he tells: doubts, perplexities, blunders are set down with the most refreshing candour; one forgives the prolixity and occasional tediousness, even the little touches of self-satisfaction, in view of the transparent honesty of purpose, the single-minded pursuit of truth for its own sake, wholly apart from preconception or bias of dogma which shine on every page. As key-notes to character, even the dedications and prefaces to the several volumes have their peculiar value and charm, as evidence of the workings of an ingenuous mind. The publication of the six volumes comprising the original work—the edition of greatest value to Priestley’s biographer—extended from 1775 to 1786. Although the space at our disposal precludes any attempt at a full account of the contents, it is necessary to set these out in such detail as may serve to afford a just idea of their value, and with such comment as may be necessary to elucidate their significance. In the preface to the first volume, which made its appearance in 1775, with a dedication to Lord Shelburne, Priestley thinks it necessary to explain why he has decided, contrary to his original intention, but with the approbation of the President and of his friends in the Royal Society, not to send them any more papers on the subject of “Air” at present but to make immediate publication of all he has done with respect to it. In view, he says, of the rapid progress that has been made and may be expected to be made in this branch of knowledge, “unnecessary delays in the publication of experiments relating to it are peculiarly unjustifiable.”