Black's life closed, as was fitting, in a quiet and honoured old age. He had many friends, but lived pretty much alone—he was never married.
On the 26th of November 1799, "being at table with his usual fare, some bread, a few prunes and a measured quantity of milk diluted with water, and having the cup in his hand when the last stroke of his pulse was to be given, he had set it down on his knees, which were joined together, and kept it steady with his hand, in the manner of a person perfectly at ease; and in this attitude he expired, without spilling a drop, and without a writhe in his countenance, as if an experiment had been required to show to his friends the facility with which he departed."
Black was characterized by "moderation and sobriety of thought;" he had a great sense of the fitness of things—of what is called by the older writers "propriety." But he was by no means a dull companion; he enjoyed general society, and was able to bear a part in any kind of conversation. A thorough student of Nature, he none the less did not wish to devote his whole time to laboratory work or to the labours of study; indeed he seems to have preferred the society of well-cultivated men and women to that of specialists in his own or other branches of natural science. But with his true scientific peers he doubtless appeared at his best. Among his more intimate friends were the famous political economist Adam Smith, and the no less celebrated philosopher David Hume. Dr. Hutton, one of the earliest workers in geology, was a particular friend of Black; his friendship with James Watt began when Watt was a student in his class, and continued during his life.
With such men as his friends, and engaged in the study of Nature—that boundless subject which one can never know to the full, but which one can always know a little more year by year—Black's life could not but be happy. His example and his teaching animated his students; he was what a university professor ought to be, a student among students, but yet a teacher among pupils. His work gained for him a place in the first rank of men of science; his clearness of mind, his moderation, his gentleness, his readiness to accept the views of others provided these views were well established on a basis of experimentally determined facts, fitted him to be the centre of a circle of scientific students who looked on him as at once their teacher and their friend.
As a lecturer Black was eminently successful. He endeavoured to make all his lectures plain and intelligible; he enlivened them by many experiments designed simply to illustrate the special point which he had in view. He abhorred ostentatious display and trickiness in a teacher.
Black was strongly opposed to the use of hypotheses in science. Dr. Robison (the editor of his lectures) tells that when a student in Edinburgh he met Black, who became interested in him from hearing him speak somewhat enthusiastically in favour of one of the lecturers in the university. Black impressed on him the necessity of steady experimental work in natural science, gave him a copy of Newton's "Optics" as a model after which scientific work ought to be conducted, and advised him "to reject, even without examination, any hypothetical explanation, as a mere waste of time and ingenuity." But, when we examine Black's own work, we see that by "hypothetical explanations" he meant vague guesses. He himself made free use of scientific (i.e. of exact) hypotheses; indeed the history of science tells us that without hypotheses advance is impossible. Black taught by his own researches that science is not an array of facts, but that the object of the student of Nature is to explain facts. But the method generally in vogue before the time of Black was to gather together a few facts, or what seemed to be facts, and on these to raise a vast superstructure of "vain imaginings." Naturalists had scarcely yet learned that Nature is very complex, and that guessing and reasoning on guesses, with here and there an observation added, was not the method by which progress was to be made in learning the lessons written in this complex book of Nature.
In place of this loose and slipshod method Black insisted that the student must endeavour to form a clear mental image of every phenomenon which he studied. Such an image could be obtained only by beginning with detailed observation and experiment. From a number of definite mental images the student must put together a picture of the whole natural phenomenon under examination; perceiving that something was wanted here, or that the picture was overcrowded there, he must again go to Nature and gain fresh facts, or sometimes prove that what had been accepted as facts had no real existence, and so at length he would arrive at a true representation of the whole process.
So anxious was Black to define clearly what he knew and professed to teach, that he preferred to call his lectures "On the Effects of Heat and Mixtures," rather than to announce them as "A Systematic Course on Chemistry."
His introductory lecture on "Heat in General" is very admirable; the following quotation will serve to show the clearness of his style and the methodical but yet eminently suggestive manner of his teaching:—
"Of Heat in General.