Priestley’s connection with the Society influenced the progress of chemistry in this country both directly and indirectly. As already stated, he himself was greatly stimulated to accumulate chemical facts by his association with men like Boulton, Watt, Wedgwood, Keir, Darwin, who loved knowledge for its own sake, but who were at the same time quite alive to the material benefits which they and their fellow-men might derive from the pursuit of scientific inquiry. The measure of their interest may be gauged by the extent of their support, and by the readiness with which they furnished Priestley with the means to carry on his investigations. Priestley not only freely communicated to them the results of his labours, but he incidentally fixed their attention on a class of phenomena which, more than any other, were calculated to afford an insight into the real nature of chemical change, and to lead to a rational explanation of chemical phenomena. Priestley was not consciously a casuist, but there can be no question that the interpretation which his active and ingenious mind occasionally led him to place upon his work not only served to blind himself, but was the means of obscuring the truth for a time from others. We have only to read the correspondence, already more than once alluded to, to find ample proof that such was the case. In a letter to Wedgwood, of March 30, 1781, Boulton writes:— “We have long talked of phlogiston without knowing what we talked about; but now that Dr Priestley hath brought the matter to light we can pour that element out of one vessel into another; can tell how much of it by accurate measurement is necessary to reduce a calx to a metal, which is easily done, and without putting that calx into contact with any visible thing. In short, this goddess of levity can be measured and weighed like other matter. For the rest, I refer you to the doctor himself.” In the following year (March 21, 1782) we find Priestley also writing to Wedgwood:— “Before my late experiments, phlogiston was indeed almost given up by the Lunar Society, but now it seems to be re-established.” How difficult it was to convince Priestley may be seen from the following extract from a letter to his friend Franklin, who was then in Paris, written at about the same time:— “Birmingham, _June 24, 1782_.