presented themselves when a frost or thaw happened to prevail. The hills are not at once cleared of snow during the sunshine of the brightest winter day, nor were the ponds suddenly covered with ice during a single frosty night Much heat is absorbed and fixed in the water during the melting of the snow; and on the other hand, while the water is changed into ice, much heat is extricated. During a thaw, the thermometer sinks when it is removed from the air, and placed in the melting snow; and during severe frost, it rises when plunged into freezing water. In the first case, the snow receives heat; and in the last, the water allows the heat to escape again. These were fair and unquestionable inferences, and now they appear obvious and easy. But although many ingenious and acute philosophers had been engaged in the same investigations, and had employed the same facts in their disquisitions, those obvious inferences were entirely overlooked. It was reserved for Dr Black to remove the veil which hid this mystery of nature, and by this important discovery, to establish an era in the progress of chemical science, one of the brightest, perhaps, which has yet occurred in its history."
Dr Black explained his theory of latent heat—such was the name he himself gave to it—to the members of a literary society, April 23, 1762, and afterwards laid before his students a detailed view of the extensive and beneficial effects of this habitude in the grand economy of nature. From observing the analogy between the cessation of expansion by the thermometer, during the liquefaction of the ice, and during the conversion of water into steam, Dr Black, having explained the one, thought that the phenomena of boiling and evaporation would admit of a similar explanation. He was so convinced of the truth of this theory, that he taught it in his lectures in 1761, before he had made a single experiment on the subject At this period, his prelections on the subject of evaporation were of great advantage to Mr James Watt, afterwards so distinguished for his application of steam power. His discovery, indeed, may be said to have laid the foundation of that great practical use of steam, which has conferred so immense a blessing upon the present age.
In 1766, on Dr Cullen being removed from the chair of chemistry at Edinburgh, to that of medicine, Dr Black, as formerly, supplied the vacant place. In this new scene, he saw that his talents would become more conspicuous, and of more extensive utility. He was therefore encouraged to devote himself, with still more enthusiastic zeal, to his duties as a chemical teacher. In this he was so far successful, that chemistry at length became a fashionable study in the Scottish capital, and a necessary part of the education of every gentleman. After this period, however, he retired from the field of chemical research, which now began to be occupied by a great number of distinguished philosophers. The cause of this was the delicate state of his health, aided, perhaps, a little by that indolence, or rather perhaps absence of ambitious motive, which has been already alluded to. It is to be regretted that, for the same reason, he can scarcely be said to have published any thing to the world, by which his discoveries might be permanently secured to the honour of his own name. From the period of his accession to the chemical chair at Edinburgh, he was, for thirty years, a most distinguished member of the professional society, which then adorned the capital, and has since given such an Augustan eclat to the latter age of the eighteenth century. Whatever obstruction his health proved in the way of publishing, it never marred the active discharge of his duties. His courses became every year plainer and more familiar, and were attended by a larger number of pupils. The simplicity and elegance of his experiments were always much admired. His manner and appearance were peculiarly pleasing. His voice in lecturing was low and fine, and his articulation so distinct that it was perfectly
