X 



a whole, but for every definitely specified constituent of it. In 

 Stewart's paper (as in those of the majority of yonng 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 conseqnence 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 recom- 

 mend 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 con- 

 nexion 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 ' Encyclopaedia 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 at considerable length, a great deal of theoretical matter of a 

 highly original character is introduced. 



Of another work, in which Stewart took a great part, ' The 

 Unseen Universe," the writer cannot speak at length. It has 



