ix 



something which it would be difficult or impossible for him to find 

 in the published books. He was in consequence a popular and 

 successful teacher, and nearly all the younger German physicists 

 are his pupils. 



In Kirchhoff science has lost a man who combined, to an excep- 

 tional degree, mathematical talent with observational skill and 

 experimental knowledge. 



A. S. 



Dr. Balfour Stewart was born in Edinburgh on November 1st, 

 1828, and died in Ireland on December 18th, 1887, having just 

 entered his sixtieth year. He was educated for a mercantile profes- 

 sion, 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 Edin- 

 burgh 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 imme- 

 diate 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 



c 2 



