to the professor and his pupil, and was always mentioned by the latter with gratitude and respect Dr Black, after remaining nearly six years at the college of Glasgow, left it to terminate his studies in Edinburgh; and Dr Cullen continued to correspond with him during the time of his studies. Many of these letters have been preserved, and relate principally to the chemical investigations in which they were mutually engaged ; but Dr Thomson observes, that, "During this intercourse, Dr Cullen seems to have been careful to avoid entering on any field of inquiry, in which he anticipated that his pupil might reap distinction." A letter of Dr Black's occurs, wherein, alluding to this ungenerous procedure, he thus addresses Dr Cullen: "I received your packet of chemistry, which rejoiced me extremely. A new experiment gives me new life; but I wonder at the reserve and ceremony you use with respect to me. Did I learn chemistry from you only to be a bar to your enquiries? The subject is not so limited as to be easily exhausted, and your experiments will only advance me so much farther on." Helvetius, and many other philosophers have maintained, that all mankind must be more or less actuated by the dictates of self-interest; and difficult as it may be to analyse the motives by which human conduct is often regulated, yet it cannot be concealed that the narrow-minded policy which Dr Cullen in this instance betrayed, was significant of a selfishness altogether unworthy of the general tenor of his character.
During the period that Dr Cullen lectured on chemistry in Glasgow, his attention was particularly directed to the general doctrines of heat, on which various observations are found among his manuscripts, that have been preserved. The only essay which he published on this subject appears in the second volume of the Edinburgh Philosophical and Literary Transactions. He also, in the end of the year 1753, transmitted to the Philosophical Society of Edinburgh, a paper, entitled, "Some Reflections on the Study of Chemistry, and an essay towards ascertaining the different species of salts ; being part of a letter addressed to Dr John Clerk." This letter afforded a specimen of an elementary work on chemistry, which he at that time meditated; but which, from other multifarious occupations, he did not execute. The reputation he was now daily acquiring as a lecturer on chemistry, obtained for him the acquaintance of many persons of distinction, who were celebrated for their talents and love of science. Among these was Lord Kaimes, then Mr Home, who, being devoted to scientific pursuits, naturally found pleasure in the correspondence and society of a man, whose mind was so congenial to his own. Lord Kaimes was especially delighted to find that Dr Cullen had devoted so much attention to his favourite pursuit, agriculture; and continually urged him to publish a work on this important science. That Dr Cullen had at this period made some progress in the composition of a work on agriculture, we learn from Dr Thomson, who informs us of the existence of a manuscript, part of which is in Dr Cullen's own hand-writing, entitled, "Reflections on the principles of Agriculture." Among his papers there is also an essay "On the Construction and Operation of the Plough;" composed apparently about the same period, and read before some public society, most probably the philosophical society in the college of Glasgow. The object of this essay was to explain the mechanical principles on which ploughs have been constructed, to find out what is the importance and effect of each part, and to examine what variation each, or all of them, require according to the difference of soil in which they are employed. In the year 1752, Dr Cullen's opportunities of cultivating agriculture were increased by his undertaking to manage and to improve the fawn of Parkhead, situated about eight miles from Glasgow, which he had purchased for his brother, Robert Cullen, Esq. who was at the time employed in a mercantile situation in the West Indies. But much as the
