nevertheless entirely lacking in the higher qualities of the imagination or in that power of divination which is the characteristic of men of the type of Newton. The contrast between Priestley—the social, political and theological reformer, always in advance of his times, receptive, fearless and insistent; and Priestley the man of science—timorous and halting when he might well be bold, conservative and orthodox when almost every other active worker was heterodox and progressive—is most striking. And yet, such is the irony of circumstance, Priestley’s name mainly lives as that of a chemical philosopher. When men have desired to do him honour, and have sought to perpetuate his memory by statues in public places, he is generally represented as making a chemical experiment. In reality, great as Priestley’s merit is as an experimentarian philosopher, his greater claim on our regard and esteem rests upon his struggles and his sufferings in the cause of civil, political and religious liberty. The years which Priestley spent at Calne constitute the most fruitful period of his scientific career. Practically all that he did in the way of solid achievement and of addition to the armoury of science was effected during that time. Although, after leaving Lord Shelburne, he continued to pursue scientific inquiry with his wonted zeal and industry, doubtless adding thereby to his fame among his contemporaries, posterity has set the true measure of appreciation to his later efforts. He doubtless made many hundreds of experiments in connection with more or less well-defined trains of inquiry; nevertheless, it cannot be maintained that during his subsequent period he added many first-rate facts to our knowledge, or indeed discovered any facts at all comparable in importance with those he ascertained during his life in Wiltshire. On the contrary, what he did observe—as for example the seeming conversion of water into air—too frequently led him astray and was the cause of error to himself and others. Thus Watt’s claim to be considered as an independent, if not the first and true, discoverer of the real chemical nature of water is based upon Priestley’s experimental blunders. Watt was undoubtedly accurate in his surmise, but the surmise was right in spite of, and not by reason of, Priestley’s experimental evidence. Priestley recorded his experiments with such fulness that it is now easy to perceive where he went wrong. He was constantly on the verge of a discovery, sometimes indeed of a discovery of cardinal importance, but as constantly it eluded his grasp. The experiments on the seeming conversion of water into air might have led him, when he got over his chagrin on the detection of the real cause of his error, to the recognition of the underlying truth in it, namely, the principle of the diffusion of gases. He was, of course, familiar with the fact that the various gases he discovered, or which were known to him, differed in