In 1788, he went to Virginia, as tutor to two young college friends, Messrs Randolph; and after spending more than a year in America, returned to Edinburgh. In January 1790, he proceeded to London, carrying with him some recommendatory letters from Dr Smith; he has been heard to mention, that one of the most pressing injunctions with which he was honoured by that illustrious philosopher, was to be sure, if the person to whom he was to present himself was an author, to read his book before approaching him, so as to be able to speak of it, if there should be a fit opportunity. His first intention was to deliver lectures on natural philosophy; but being disappointed in his views, he found it expedient to commence writing for periodical works, as the readiest means of obtaining subsistence. For obtaining employment of this kind, he was mainly indebted to his friend Dr William Thomson, who engaged him upon the notes of a new edition of the Bible, which he was then publishing in numbers. About three months after his arrival in London, he made an agreement with Mr Murray, the bookseller, to translate Buffon’s Natural History of Birds, which was published in 1793, in nine octavo volumes. The sum he received for it laid the foundation of that pecuniary independence which, unlike many other men of genius, his prudent habits fortunately enabled him early to attain. The preface to this work, which was published anonymously, is characterised by all the peculiarities of his later style; but it also bespeaks a mind of great native rigour and lofty conceptions, strongly touched with admiration for the sublime and the grand in nature and science. During the progress of the translation, he fulfilled an engagement with the Messrs Wedgewood of Etruria in Staffordshire, to superintend their studies; he left them in 1792. In 1794, Mr Leslie spent a short time in Holland; and, in 1796, he made the tour of Germany and Switzerland with Mr Thomas Wedgewood, whose early death he ever lamented as a loss to science and his country. About this period, he stood candidate for a chair at St Andrews, and subsequently, for that of natural philosophy in Glasgow, but without success. The fortunate candidate on the latter occasion was Dr James Brown of St Andrews, with whom Mr Leslie to the end of his life maintained a constant intimacy. In 1799, he travelled through Norway and Sweden, in company with Mr Robert Gordon, whose friendship he had acquired at St Andrews college.

At what period Mr Leslie first struck into that brilliant field of inquiry where he became so conspicuous for his masterly experiments and striking discoveries regarding radiant heat, and the connexion between light and heat, we are unable to say. But his Differential Thermometer—one of the most beautiful and delicate instruments that inductive genius ever contrived as a help to experimental inquiry, and which rewarded its author by its happy ministry to the success of some of his finest experiments—must have been invented before the year 1800, as it was described in Nicholson’s Philosophical Journal some time during that year. The results of those fine inquiries, in which he was so much aided by this exquisite instrument, were published to the world in 1804, in his celebrated "Essay on the Nature and Propagation of Heat." [Previous to this period, Mr Leslie, when not otherwise or elsewhere engaged, used to live with his brothers at Largo; and there were the experiments for his essay on heat carried on, and the book written.] The experimental devices and remarkable discoveries which distinguish this publication, far more than atone for its great defects of method, its very questionable theories, and its transgressions against that simplicity of style which its aspiring author rather spurned than was unable to exemplify; but which must be allowed to be a quality peculiarly indispensable to the communication of scientific knowledge. The work was honoured, in the following year, by the unanimous adjudication to its author, by the council of the Royal Society, of the Rumford Medals, appropriated to reward discoveries in that province, whose nature and limits he had so much illustrated and extended.

Mr Leslie thus distinguished himself by his acquirements, when, early in 1805, in consequence of the translation of professor Playfair from the chair of mathematics to that of natural philosophy in the university of Edinburgh, the former became vacant, and the subject of this memoir appeared as a candidate for the situation. It might have been expected that, where the qualifications of the individual were so decidedly above all rivalry, there could have been no hesitation in his native country to confer upon him the honour which he sought. Such there might not have been, if what is called the moderate party in the Scottish church, had not been inspired by a jealousy of his liberal principles in politics, accompanied by a desire of advancing one of their own number, to oppose his election. The person brought forward as the rival candidate was Dr Thomas Macknight, one of the ministers of the city, and son of the venerable commentator on the Epistles,--a gentleman highly qualified, no doubt, not only for this, but for almost any other chair in the university; but who, nevertheless, could not be matched against an individual so distinguished for the benefits he had conferred on science as Mr Leslie; and who was moreover liable to the disqualifying consideration that he was already engaged in an office which, to be well done, requires the whole man, while Mr Leslie stood in the light of a most useful member of society in a great measure unprovided for.

The electors in this case were the magistrates and town-council of Edinburgh, and to them Mr Leslie was recommended not only by fame, but by the warmest testimonials from Sir Joseph Banks, Mr Dempster of Dunnichen, Dr Hutton of Woolwich, Baron Masseres, and Dr Maskelyne. In the supposition that these men were disposed to discharge their trust with fidelity, they could have no hesitation in preferring Mr Leslie; and it is to be related to their credit, that they had no such hesitation. On learning the bent of their resolution, the ministers of Edinburgh held various private meetings, as if to indicate the more pointedly that they had a peculiar interest of their own in the matter; and it was resolved to oppose Mr Leslie’s election on the grounds of what they deemed an infidel note in his essay on heat; employing for this purpose a clause in the fundamental charter of the college, directing the magistrates to take the advice of the Edinburgh clergy in the election of professors.

The note alluded to was one in reference to the unphilosophical theories which once attempted to explain the phenomena of gravitation by means of invisible aethers. Mr Leslie, in treating this point, found it convenient to refer to Mr Hume’s theory of cause and effect, in which, as is well known, he makes use of certain generally received doctrines to invalidate the argument for the existence of the Deity. In making the reference, it did not seem to Mr Leslie to be necessary that he should condemn the ultimate use made of these doctrines by Mr Hume, since he was only engaged in a physical examination. His note, therefore, stands as follows: "Mr Hume is the first, so far as I know, who has treated of causation in a truly philosophic manner. His Essay on Necessary Connexion seems a model of clear and accurate reasoning. But it was only wanted to dispel the cloud of mystery which had so long darkened that important subject. The unsophisticated sentiments of mankind are in perfect unison with the deductions of logic, and imply at bottom nothing more in the relation of cause and effect, than a constant invariable sequence." From these words, however, it was evident, in the opinion of his clerical opponents, "that Mr Leslie, having, with Mr Hume, denied all such necessary connexion between cause and effect, as implies an operating principle in the cause, has, of course, laid a foundation for rejecting all argument that is derived from the works of God, to prove either his being or attributes."

When Mr Leslie was informed of the grounds on which the Edinburgh ministers rested their opposition, he addressed a letter to the Rev. Dr Hunter, professor of divinity, and one of the few clergymen of the city who were not opposed to him, laying before him some explanations of the note, to which he begged him to call the attention of his brethren. These explanations were chiefly what are stated above, and are thus followed up: "I have the fullest conviction that my ideas on the question to which the note refers, would appear to coincide, in every essential respect, with those of the most enlightened adversaries of Mr Hume’s philosophy. But, limited as I am to a few moments of time, I can only disavow (which I do with the greatest sincerity and solemnity,) every inference which the ingenuity of my opponents may be pleased to draw from the partial view I have taken of the general doctrine, to the prejudice of those evidences on which the truths of religion are founded. If I live to publish another edition of my work, I pledge myself to show in an additional paragraph, how grossly and injuriously I have been treated on this occasion. * * * It is painful to be called on, after the habits of intimacy in which I have lived with the most exemplary characters in both parts of the island, to repel a direct charge of atheism; but whatever may be the effect of such calumnies on the minds of strangers, it affords me much consolation to think, that they will be heard with contempt and indignation by those who know the real state of my sentiments, and particularly by such as are acquainted with the strictness of those religious principles in which I had the happiness to be educated from my earliest years."

This letter was laid before the ministers at a meeting held by them on the 12th of March (1805); but being, to use their own phrase, by no means satisfied with it, they appointed a committee, consisting of Dr Grieve, Mr David Black, Mr David Dickson, and Dr Inglis, to proceed to the town-council and protest against the election of Mr Leslie. As the council was to be that day engaged in the election, the committee went accordingly to their chamber, and presented a protest which had been prepared, in which, besides stating the grand objection of the note and their inferences from it as to Mr Leslie’s religious principles, they stated that, "in the event of his being elected, notwithstanding this representation, they reserved to themselves full power of questioning the validity (if such election, and of employing whatever means may, to them, be found competent for preventing Mr Leslie’s induction into the office of professor; with full power, in the event of his induction, to prosecute for his ejection from said office in any competent court, civil or ecclesiastical." Immediately after this paper was given in and its bearers had left the hall, the council elected Mr Leslie.

At the meeting of the presbytery of Edinburgh on the 27th, the committee of the city clergy gave in a representation stating these transactions, along with a copy of their protest, and requested the reverend court to take such steps in the matter as they might judge proper. It was here determined by vote to carry the affair before the synod; a step formally necessary for bringing it under the decision of the highest national church court, the general assembly.

At the meeting of this court, on the 22nd of May, the case of Mr Leslie came before it in the shape of a complaint by the Rev. Sir Henry Moncrieff and other members of the synod, against the reference of the case to the general assembly. It was thus apparent that the leaders of the more zealously pious party of the church had taken the part of Mr Leslie against their accustomed opponents. The interest which the public could not have failed to entertain respecting the question, even if confined to its native merits, was excited to an uncommon degree by this complication of the phenomenon. The case, nevertheless, furnished only an unusually striking example of what must always be the result of a party system in any deliberative body. It happened to be convenient for the "moderate" party on this particular occasion, to show an anxious desire for the purity of faith and doctrine; and for this purpose they raked up a negative title in the Edinburgh clergy to be consulted in the exercise of the town-council patronage, which had not been acted upon for twenty-six years, during which time several of the very men now prosecuting had been elected to chairs in the university without regard to it. It was equally convenient for the evangelical party, though adverse to all their usual principles, to regard the suspected infidelity of Mr Leslie with a lenient and apologetic spirit, in order that they might be in their usual position regarding their opponents, and because they hoped to gain a triumph for themselves in the non-success of a prosecution, which they could easily see rested upon no valid grounds, and could hardly, in the face of public opinion, be carried to its utmost extent, even though a majority of servile votes could have been obtained for the purpose.

In the course of the long debate which followed the introduction of the case, some very strong testimonies were brought forward in favour of Mr Leslie’s moral and religious character. A letter from the minister of Largo testified that, during the two past years, while Mr Leslie resided in the parish, he had paid a becoming respect to religion, and that, if great abilities highly improved, an unstained moral character, and a tender discharge of every filial duty, recommend to confidence and esteem, these belonged to him. Another letter, from the clergyman of the neighbouring parish, after remarking that insinuations of the kind disseminated respecting Mr Leslie, yielded but a contemptible support to any cause, stated the following particulars: "I have lived in habits of intimacy with Mr Leslie for some time past; I have had an opportunity of knowing his religious and political sentiments; I have been furnished, in short, with satisfactory evidence of his attachment to our ecclesiastical and civil establishment. His father officiated long as a worthy elder of our church: his son was once a student of divinity; and though he has not prosecuted his theological studies, having been much engaged in other literary pursuits, I never heard that he had ceased to respect the doctrines or discipline of the church of Scotland. On the contrary, the leading doctrines of Christianity he regards with reverence."

‘There was also read a letter from Mr Leslie to a friend, of date, February 22nd, 1805, in which he thus expressed himself: "It was my lot to receive a most virtuous and religious education, in the bosom of a family eminently distinguished by its exemplary lives; and the impressions of my early years, no distance of time, or change of circumstances can ever efface. If my mind is more enlarged by culture, I have likewise learned to see more deeply the importance of those truths which bind men together in society, and which, visiting their inmost recesses, appal the guilty and hold forth comfort to the wretched. I have ever been a sincere lover of peace, of decency, and good order. My time has been almost wholly spent in abstract researches, and the study of the sublime operations of nature. The questions, so much agitated of late, served with me only to amuse a few leisure moments; and even at that eventful period, when the minds of men, and particularly of young men, were so violently inflamed, I escaped in a great measure the contagion. I sighed, indeed, for the improvement of our species; but the slightest appearance of tumult, or popular violence, was most abhorrent to my temper. I never had the remotest connexion with any party or political association whatever. In the spirit of mildness, I endeavoured to think and act for myself. My sentiments of loyalty had been confirmed by what I had seen during a short stay in America, where I witnessed the disgusting and pernicious influence assumed by an ignorant, licentious, and dissolute rabble. * * It is our native island that presents the truly cheering picture of equal laws mildly administered, and holds up a body of religious institutions at once rational, decent, and impressive. I venerate the great principles of our Christian faith, and am solicitous to mark, by my external behaviour, that respect which I cherish. Raising my affections above this little spot of earth, the restless scene of intrigue, and strife, and malice, I look forward with joy and expectation to that better country beyond the grave."

Among the most powerful speakers on the side of Mr Leslie was Sir Henry Moncrieff, who observed that the question expressly and simply referred to a civil right of the Edinburgh ministers. This right, he showed, had never been before exercised in the election of a professor of mathematics, and in all probability would be confined by a court of law to the professorships existing at the institution of the university, of which that of mathematics was not one. The right, however, if right it was, had in reality been exercised: the clergy had gone to the council and given their advice, and, though it had not been followed, still it had been received. Sir Henry also commented in strong terms upon the fact, that the whole of this prosecution, threatening so much to Mr Leslie, had been conducted in such a way as to allow him no possibility of appearing in his own defence. "It is a circumstance," further continued this nervous orator, "which I cannot help mentioning, that the ministers of Edinburgh, in their zeal to find any sort of heresy in Mr Leslie’s note, have unfortunately announced a doctrine in opposition to that which they would fix on him, which is capable of an interpretation more hostile to religion than any thing that they have imputed to his book. In asserting such a necessary connexion between cause and effect as implies an operating principle in the cause, they express a doctrine of which I can scarcely mention the pernicious tendency. If the necessity is applied to the first cause, it is not far from blasphemy. If it is restricted (as I suppose it was meant to be) to the second cause, it is substantially the doctrine of materialism, and leads directly to atheism. (Here Mr Ritchie interrupted the speaker, to remind him that he had qualified the expression, and restricted his meaning to a conditional or contingent necessity.) True, sir, he did so. He did the very thing which he will not allow Mr Leslie to do. He gives an explanation for himself and his friends, when he perceives the consequences of the original expressions they had employed. He qualifies the necessity they asserted by the term ‘conditional,’ by which he means to restrict it, and he expects that we are to take his explanation without a murmur; although, when Mr Leslie would confine the assertion in his note to ‘objects of physical examination,’ he obstinately fixes him down to his original expressions, and rejects the limitation as utterly inadmissible. Unfortunately, sir, the doctrine of the ministers of Edinburgh, with regard to such a necessary connexion between cause and effect as implies an operating principle in the cause, stands in its original state in the protest which they gave to the town-council, it is recorded in the council books; and there it must remain in all future times, without any explanation whatever, be its tendency or its heresy ever so mischievous.

"The use," he continued with exquisite sarcasm, "which may be made of incautious expressions, may be as forcibly illustrated from the protest of the ministers of Edinburgh, as from the note of Mr Leslie. But there is this material distinction between the two cases: Mr Leslie, at least, understood the precise meaning of his assertions, as far as they related to the subject of which he was writing; but my reverend brethren enunciated their dogma in perfect innocence and simplicity, completely unconscious of its true import and tendency!"

Near midnight, on the second day of the debate, it was determined by 96 against 84 to dismiss this vexatious case without further notice. On the vote being announced, a shout of applause—an unwonted sound in the general assembly—burst from the crowd assembled in the galleries.

Mr Leslie entered without further opposition upon the duties of his chair, and upon a course of experimental discovery by which he was to confer lustre upon the university. Through the assistance of one of his ingenious contrivances—his hygrometer—he arrived in 1810 at the discovery of that singularly beautiful process of artificial congelation, which enabled him to convert water and mercury into ice. "We happened," says a brother professor, "to witness the consummation of the discovery—at least, of the performance of one of the first successful repetitions of the process by which it was effected; and we shall never forget the joy and elation which beamed on the face of the discoverer, as, with his characteristic good nature, he patiently explained the steps by which he had been led to it."

In 1809 Mr Leslie published his Elements of Geometry, which immediately became a class-book, and has since gone through four editions. He also published, in 1813, an "Account of Experiments and Instruments depending on the relation of Air to Heat and Moisture." In 1817 he produced his "Philosophy of Arithmetic, exhibiting a Progressive view of the Theory and Progress of Calculation," a small octavo; and, in 1821, his "Geometrical Analysis, and Geometry of Curve Lines, being volume second of a Course of Mathematics, and designed as an Introduction to the study of Natural Philosophy." In 1822 he published "Elements of Natural Philosophy," for the use of his class—reprinted in 1829—and of which only one volume appeared. "Rudiments of Geometry," a small octavo, published, 1828, and designed for popular use, was his last separate work. Besides these separate works, he wrote many admirable articles in the Edinburgh Review, three profound treatises in Nicholson’s Philosophical Journal, a few in the Transactions of the Royal Society of Edinburgh, and several very valuable articles on different branches of physics in the Supplement to the Encyclopaedia Britannica. In 1819, on the death of professor Playfair, whose promotion had formerly made room for him in the chair of mathematics, he was elevated to the professorship of natural philosophy, by which his powers were of course brought into a far wider field of display and of usefulness, than they had been for the preceding fourteen years. Among the preliminary treatises of the seventh edition of the Encyclopaedia Britannica, which began to be published in 1830, he wrote a "Discourse on the History of Mathematical and Physical Science, during the eighteenth century," which may be described as one of the most agreeable and masterly of all his compositions.

The income enjoyed by Mr Leslie was for many years so much above his necessities, that he was able, by careful management, to realise a fortune not far short of ten thousand pounds. Part of this he expended, in his latter years, upon the purchase and decoration of a mansion called Coates near his native village, where he spent all the intervals allowed by his college duties. Early in the year 1832, at the recommendation of the lord chancellor (Brougham), he was invested with a knighthood of the Guelphic order, at the same time that Messrs Herschel, C. Bell, Ivory, Brewster, South, and Harris Nicolas, received a similar honour. Sir John Leslie was not destined long to enjoy the well-merited honour. In the end of October, while superintending some of the improvements about his much-loved place, he incautiously exposed himself to wet, the consequence of which was a severe cold. Among the various foibles which protruded themselves through the better powers and habitudes of his mind, was a contempt for medicine, and an unwillingness to think that he could be seriously ill. He accordingly neglected his ailment, and was speedily seized with erysipelas in one of his legs; a disorder at that time raging in Scotland with all the symptoms and effects of a malignant epidemic. On Wednesday, October 31st, he again exposed himself in his grounds, and from that day, the malady advanced very rapidly. On the evening of Saturday, November 3d, he breathed his last.

The scientific and personal character of Sir John Leslie has been sketched with so bold and free a pencil by Mr Macvey Napier, his brother in both academic and literary labours, that we make no apology for presenting it to the reader, in lieu of any thing of our own:

"It would be impossible, we think, for any intelligent and well-constituted mind, to review the labours of this distinguished man, without a strong feeling of admiration for his inventive genius and vigorous powers, and of respect for that extensive knowledge which his active curiosity, his various reading, and his happy memory had enabled him to attain. Some few of his contemporaries in the same walks of science, may have excelled him in profundity of understanding, in philosophical caution, and in logical accuracy; but we doubt if any surpassed him, whilst he must be allowed to have surpassed many, in that creative faculty—one of the highest and rarest of nature’s gifts—which leads, and is necessary to discovery, though not all-sufficient of itself for the formation of safe conclusions; or in that subtilty and reach of discernment which seizes the finest and least obvious relations among the objects of science—which elicits the hidden secrets of nature, and ministers to new combinations of her powers. There were some flaws, it must be allowed, in the mind of this memorable person. He strangely undervalued some branches of philosophical inquiry of high importance in the circle of human knowledge. His credulity in matters of ordinary life was, to say the least of it, as conspicuous as his tendency to scepticism in science. It has been profoundly remarked by Mr Dugald Stewart, that ‘though the mathematician may be prevented, in his own pursuits, from going far astray, by the absurdities to which his errors lead him, he is seldom apt to be revolted by absurd conclusions in other matters. Thus, even in physics,’ he adds, ‘mathematicians have been led to acquiesce in conclusions which appear ludicrous to men of different habits.’ Something of the same kind was observable in the mind of this distinguished mathematician, for such also he was. He was apt, too, to run into some startling hypothesis, from an unwarrantable application of mathematical principles to subjects altogether foreign to them; as when he finds an analogy between circulating decimals, and the lengthened cycles of the seasons. In all his writings, with the exception, perhaps, of his last considerable performance, the discourse prefixed to the seventh edition of the Encyclopaedia Britannica, even in the sober field of pure mathematics, there is a constant straining after ‘thoughts that breathe and words that burn,’ and a love of abstract, and figurative, and novel modes of expression, which has exposed them to just criticism, by impartial judges, and to some puny fault-finding, by others, more willing to carp at defects than to point out the merits which redeem them. But when even severe criticism has said its worst, it must be allowed, that genius has struck its captivating impress, deep and wide over all his works. His more airy speculations may be thrown aside or condemned; but his exquisite instruments, and his original and beautiful experimental combinations, will ever attest the fruitfulness of his mind, and continue to act as helps to farther discovery. We have already alluded to the extent and excursiveness of his reading. It is rare, indeed, to find a man of so much invention, and who himself valued the inventive above all the other powers, possessing so vast a store of learned and curious information. His reading extended to every nook and corner, however obscure, that books have touched upon. He was a lover, too, and that in no ordinary degree, of what is commonly called anecdote. Though he did not shine in mixed society, and was latterly unfitted by a considerable degree of deafness for enjoying it, his conversation, when seated with one or two, was highly entertaining. It had no wit, little repartee, and no fine turns of any kind, but it had a strongly original and racy cast, and was replete with striking remarks and curious information.

"He had faults, no doubt, as all ‘of woman born’ have: he had prejudices, of which it would have been better to be rid; he was not over charitable in his views of human virtue; and he was not quite so ready, on all occasions, to do that justice to kindred merit as was to be expected in so ardent a worshipper of genius. But his faults were far more than compensated by his many good qualities—by his constant equanimity, his cheerfulness, his simplicity of character, almost infantine, his straightforwardness, his perfect freedom from affectation, and, above all, his unconquerable good nature. [The person of Sir John Leslie was, in later life, far from gainly. It was short and corpulent with a florid face, and somewhat unsightly projection of the front teeth, and tottered considerably in walking. He was, moreover, very slovenly in his mode of dressing—a peculiarity the more curious, as it was accompanied by no inconsiderable share of self-respect and an anxiety to be thought young and engaging. The mixture of great intellectual powers with the humbler weaknesses of human nature, can seldom have been more strikingly exemplified than in his case; though it is evident that, as his weaknesses were very much those to which unmarried men in advanced life are supposed to be most peculiarly liable, they might have probably been obviated in a great measure, if he had happened to spend his life in the more fortunate condition of matrimony.] He was, indeed, one of the most placable of human beings; and if, as has been thought, he generally had a steady eye, in his worldly course, to his own interest, it cannot be denied that he was, notwithstanding, a warm and good friend, and a relation on whose affectionate assistance a firm reliance could ever be placed.

"There is one other matter which, in justice to the illustrious dead, we cannot pass over in silence; we mean the permanent service rendered to the class of Natural Philosophy by the late Sir John Leslie in the collection of by far the finest and most complete set of apparatus in the kingdom. Augustus boasted that he found Rome built of brick, and left it a city of palaces and temples constructed of marble. Without any exaggeration, something analogous may be predicated of Sir John Leslie in regard to the apparatus of this class. He found it a collection of antiquated and obsolete rubbish; he left it the most complete and perfect of its kind in this kingdom; and if it had pleased God to spare him a few years longer, it would, beyond all doubt, have been rendered the first in Europe or the world. The renovation which he effected was, indeed, most radically complete. The whole of the old trash was thrown aside, and its place supplied by new instruments, constructed on the most improved principles by the most celebrated artists, both in this country and on the continent; while its absolute amount was increased tenfold, and adapted, in the happiest manner, to the present advanced state of science. His perseverance and enthusiasm in this respect were indeed boundless; and as his predecessors were not experimentalists, in the same sense in which he was, and had made little or no effort to accommodate the apparatus to the progress of science, or even to repair the wear and tear of time, he had the whole to create, in the same way as if the class had only been founded when he was first promoted to the chair. By his own continued and admirably-directed efforts, aided by the liberality of the patrons, who generously made him several grants in furtherance of the object which he had so much at heart; and also by very considerable pecuniary sacrifices upon his own part, for which he has never as yet got the credit that is so justly due to him; he at length succeeded in furnishing the apparatus-room in the manner in which it may now be seen by any one who chooses to visit it, and thus conferred upon the university a benefit for which it ought to be for ever grateful to his memory. This may sound strange in the ears of those who have been accustomed to hear it said, as it has often been, most falsely, that Sir John Leslie was a bad experimenter. The truth is, that of all his great and varied gifts, none was more remarkable than the delicacy and success with which he performed the most difficult experiments, excepting perhaps his intuitive sagacity in instantly detecting the cause of an accidental failure; and it is a known fact, that, after he had discovered and communicated to the world his celebrated process of artificial congelation, particularly as applied to the freezing of mercury, some of the first men of science in London failed of performing it, till the discoverer himself, happening to be on the spot personally, showed them wherein consisted the fault of their manipulation, and at once performed the experiment which had previously baffled all their efforts. It is equally well known to those who were acquainted with him, that the most elegant in form as well as the most delicate in operation of the beautiful instruments invented by himself, were constructed by his own hand, and that this, to him most agreeable employment, constituted the recreation of his leisure hours. The apparatus-room, indeed, contains many specimens of his workmanship in this line, and they are of such a description as would not do any discredit to the most practised and skilful artist. To his immediate successor his acquisitions and his labours will, therefore, be of incalculable importance; but the merit which really belongs to him can only be duly estimated by those who know what he found, when he became professor of natural philosophy, and can compare it with the treasures which he has left behind him."