It was during this period, however, that he inves tigated and brought to maturity another discovery of the utmost importance, we mean his theory of latent heat; a discovery which constitutes the foundation of the whole doctrine of heat as at present taught by chemists, and which has been attended with more be. neficial effects to the world than any other discovery made during the 18th century ; since it occasioned the improvements in the steam engine by Mr Watt, an instrument which has operated a complete change in our manufactures. The decisive experiment was made in 1761, and Dr Black drew up an account of this theory, and read it to a literary society in Glas gow on the 23d April 1762. No account of this theory was ever published by its author ; but ever after it made a most important part of his chemical lectures. It became in consequence soon generally known in every part of Europe.
About this time likewise he made a set of experi ments, to determine whether the expansions of the thermometer corresponded correctly with the increase of heat. He satisfied himself that they did, so, and that the thermometer measured the increase of tem perature correctly. The result of his experiments was read to the literary society of Glasgow on the 28th March 1760.
In the year 1766, Dr Cullen, chemical professor in Edinburgh, was appointed professor of medicine, and Dr Black, with the unanimous approbation of the city and university, was pitched upon as his successor. In this new scene, his talents were moreconspicuous;and more extensively useful. The celebrity of the medical school of Edinburgh brought him pupils from all quarters, while the increasing importance of chemistry, both in the eye of the philosopher and manufacturer, made the number of chemical students increase every year.
Dr Black, deeply impressed with the importance of the station which he filled, devoted himself complete ly to the improt,..nent of his lectures on the elements of chemistry. His great object was to make them intelligible to all his students, however defective their previous education had been. He never ventured to indulge in hypothesis or conjecture, neither did he introduce any refined speculations, or touch upon those topics that would have required previous read ing and study. He confined himself entirely to mat ters of fact, and illustrated bis lectures by plain and beautiful experiments, the best adapted for the sub ject under discussion, and just sufficient for his pur pose. There was no parade of apparatus, nor bril liant display of showy but useless experiments ; every thing appeared in its proper place, and exactly suited the object in view, the conveying to his pupils an ex act knowledge of the subjects under discussion. - His manner was remarkably pleasing ; his voice was low, but fine and distinct ; his elocution was slow, but graceful ; and his style possessed a simplicity and elegance which has rarely been surpassed. He be came a favourite professor, and was undoubtedly one of the greatest supporters of the celebrity of the Edinburgh medical school.
•He filled the chemical chair in Edinburgh for nearly thirty-three years ; and in the whole of that period, during which the science had advanced with unexampled rapidity, and had undergone a complete revolution, his reputation as a lecturer had been con tinually increasing ; and though he added but little to the stock of chemical knowledge by his own dis coveries, lie made his lectures keep pace with the Progress of the science, and even embraced and taught the antiphlog,istic doctrines.
Owing partly, perhaps, to indolence, but chiefly to the delicate state of his health, he was obliged to re main a spectator of the brilliant discoveries in pneu matic chemistry, while he abstained from attempting to traverse the brilliant career which he himself had thrown open. About the year 1793, his health be gan to decline. He was unable to continue the gen tle exercise which had so long prevented the approach of any serious disease. The fatigue t.; performing the experiments in his class lie found too much for him, and he was obliged to get an assistant to take that labour off his hands. In 1796, he found the la bour of lecturing beyond his strength, and got a suc cessor appointed to relieve him of a part of the drud. gery. Next year he hardly attempted to lecture at all, or at least delivered only a part of the course ; and, unless our recollection fail us, 1797 was the last year that he read lectures in the university of Edin burgh. As he advanced in years, his constitution, which had always been weak, became more delicate and frail, so that every cold he caught occasioned some degree of spitting of blood. Yet he seemed to have this unfortunate disposition of body always un der command, so that he never allowed it to proceed far, or to occasion any distressing illness ; and lie thus spun his thread to the last fibre ; and even this does not seem to have broken, hut merely to have ended. " He guarded against illness," says his rela tion Dr Ferguson, " by restricting himself to a mode rate, or I should rather call it an abstemious diet ; and he met hi. increasing infirmities with a proportional increase of attention and care, regulating his food and exercise by the measure of his strength. It is won derful with what skill and success he thus made the most of a feeble constitution, by thus preventing the access of disease from abroad. He enjoyed a health which was feeble, indeed, but scarcely interrupted, and a mind ever undisturbed, in the calm and cheerful use of all his faculties. A life so prolonged had the advantage of present case, and the prospect, when the just period should arrive, of a calm dissolution." His only apprehension was, that of a long continued sick bed ; and this, perhaps, less from any selfish feeling, tlin from the humane consideration of the trouble and distress occasioned to attending friends ; and ne ver was this modest and generous wish more complete ly gratified.