From the philosophers an interest in natural science spread through the mass of the people. About the year 1870 the lecture-rooms of the great teachers of chemistry, astronomy, electricity, and even anatomy were crowded with ladies and gentlemen of fashion in the French capital. A similar state of matters was noticeable in this country. Dr. Black's lecture theatre was filled by an audience which comprised many young men of good position. To know something of chemistry became an essential part of the training of all who desired to be liberally educated. The secrets of Nature were now rapidly explored; astonishing advances were made, and as a matter of course much opposition was raised. In this active, inquiring atmosphere the young science of chemistry grew towards maturity. Priestley, ever seeking for new facts, announcing discovery after discovery, attacking popular belief in most matters, yet satisfied to interpret his scientific discoveries in terms of the hypothesis with which he was most familiar, was the pioneer of the advancing science. He may be compared to the advance-guard sent forward by the explorers of a new country with orders to clear a way for the main body: his work was not to level the rough parts of the way, or to fill in the miry places with well-laid metal, but rather rapidly to make a road as far into the heart of the country as possible. And we have seen how well he did the work. In his discovery of various kinds of airs, notably of oxygen, he laid the basis of the great generalizations of Lavoisier, and, what was perhaps of even more importance, he introduced a new method into chemistry. He showed the existence of a new and unexplored region. Before his time, Hooke and Mayow had proved the existence of more than one kind of air, but the chemistry of gases arose with the discoveries of Priestley. Although Black's chief research, on fixed air and on latent heat, was completed fifteen or twenty years before Priestley's discovery of oxygen, yet the kind of work done by Black, and its influence on chemical science, mark him as coming after Priestley in order of development. We have seen that the work of Black was characterized by thoroughness and suggestiveness. The largeness of scope, the breadth of view, of this great philosopher are best illustrated in his discourses on heat; he there leads us with him in his survey of the domain of Nature, and although he tells us that hypotheses are a "mere waste of time," we find that it is by the strength of his imagination that he commands assent. But he never allows the imagination to degenerate into fanciful guesses; he vigorously tests the fundamental facts of his theory, and then he uses the imagination in developing the necessary consequences of these facts. To Black we owe not only the first rigorously accurate chemical investigation, but also the establishment of just ideas concerning the nature of heat.