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Dr. Balfour Stewart was born in Edinburgh on November 1, 1828, and died in Ireland on December 18, 1887, having just entered his sixtieth year. He was educated for a mercantile profession, and in fact spent some time in Leith, and afterwards in Australia, as a man of business. But the bent of his mind towards physical science was so strong that he resumed his studies in Edinburgh University, and soon became assistant to Professor J. D. Forbes, of whose class he had been a distinguished member. This association with one of the ablest experimenters of the day seems to have had much influence on his career; for Forbes's researches (other than his Glacier work) were mainly in the department of Heat, Meteorology, and Terrestrial Magnetism, and it was to these subjects that Stewart devoted the greater part of his life. In the classes of Professor Kelland, Stewart had a brilliant career; and gave evidence that he might have become a mathematician, had he not confined himself almost exclusively to experimental science.

In 1858, while he was still with Forbes, Stewart completed the first set of his investigations on Radiant Heat, and arrived at a remarkable extension of Prevost's “Law of Exchanges." His paper (which was published in the 'Transactions of the Royal Society of Edinburgh') contained the greatest step which had been taken in the subject since the early days of Melloni and Forbes. The fact that radiation is not a mere surface phenomenon, but takes place like absorption throughout the interior of bodies, was seen to be an immediate consequence of the new mode in which Stewart viewed the subject. Stewart's reasoning is, throughout, of an extremely simple character, and is based entirely upon the assumption (taken as an experimentally ascertained fact) that in an enclosure, impervious to heat and containing no source of heat, not only will the contents acquire the same temperature, but the radiation at all points and in all directions will ultimately become the same, in character and in intensity alike. It follows that the radiation is, throughout, that of a black body at the temperature of the enclosure. From this, by the simplest reasoning, it follows that the radiating and absorbing powers of any substance must be exactly proportional to one another (equal, in fact, if measured in proper units), not merely for the radiation as a whole, but for every definitely specified constituent of it. In Stewart's paper (as in those of the majority of young authors) there was a great deal of redundant matter, intended to show that his new views were compatible with all that had been previously known, and in consequence his work has been somewhat lightly spoken of, even by some competent judges. These allow that he succeeded in showing that equality of radiation and absorption is consistent with all that was known; but they refuse to acknowledge that he had proved it to be necessarily true. To such we would recommend a perusal of Stewart's article in the 'Philosophical Magazine' (vol. 25, 1863, p. 354), where they will find his own views about the meaning of his own paper. The only well-founded objection which has been raised to Stewart's proof applies equally to all proofs which have since been given, viz., in none of them is provision made for the peculiar phenomena of fluorescence and phosphorescence.

The subject of radiation, and connected properties of the lumini-ferous medium, occupied Stewart's mind at intervals to the very end of his life, and led to a number of observations and experiments, most of which have been laid before the Royal Society. Such are the “Observations with a Rigid Spectroscope," and those on the "Heating of a Disk by rapid Rotation in vacuo" in which the present writer took part. Other allied speculations are on the connexion between “Solar Spots and Planetary Configurations," and on “Thermal Equilibrium in an Enclosure containing Matter in Visible Motion."

From 1859 to 1870 Stewart occupied, with distinguished success, the post of Director of the Kew Observatory. Thence he was transferred to Manchester as Professor of Physics in the Owens College, in which capacity he remained till his death. His main subject for many years was Terrestrial Magnetism; and on it he wrote an excellent article for the recent edition of the 'Encyclopædia Britannica.' A very complete summary of his work on this subject has been given by Schuster in the 'Manchester Memoirs' (4th series, vol. 1, 1888). In the same article will be found a complete list of Stewart's papers.

Among the separate works published by Stewart, his 'Treatise on Heat,' which has already reached its fifth edition, must be specially mentioned. It is an excellent introduction to the subject, though written much more from the experimental than from the theoretical point of view. In the discussion of radiation, however, which is given considerable length, a great deal of theoretical matter of a highly original character is introduced.

On another work, in which Stewart took a great part, 'The Unseen Universe,' the writer cannot speak at length. It has passed through many editions, and has experienced every variety of reception — from hearty welcome and approval in some quarters to the extremes of fierce denunciation, or of lofty scorn, in others. Whatever its merits or demerits it has undoubtedly been successful in one of its main objects, viz., in showing how baseless is the common statement that “Science is incompatible with Religion." It calls attention to the simple fact, ignored by too many professed instructors of the public, that human science has its limits; and that there are realities with which it is altogether incompetent to deal.

Personally, Stewart was one of the most loveable of men, modest and unassuming, but full of the most weird and grotesque ideas. His conversation could not fail to set one a-thinking, and in that respect he was singularly like Clerk-Maxwell. In 1870 he met with a frightful railway accident, from the effects of which he never fully recovered. He passed in a few months from the vigorous activity of the prime of life to grey-headed old age. But his characteristic patience was unruffled and his intellect unimpaired.

He became a Fellow of the Royal Society in 1862, and in 1868 he received the Rumford Medal.

His life was an active and highly useful one; and his work, whether it took the form of original investigation, of accurate and laborious observation, or of practical teaching, was always heartily and conscientiously carried out. When a statement such as this can be truthfully made, it needs no amplification.