122 THE LIFE OF JAMES D. FORBES. [CHAP. 



the fatigue of such a career as your tastes and talents 

 would enable you to pursue. 



' 1 hope that under any circumstances you will not 

 lose sight of your physical pursuits in purely mathe- 

 matical ones, which are of a comparatively narrow 

 character. It is in the field of contingent truth that 



O 



the triumphs most congenial to the human mind in a 

 healthful condition are to be gained. The disposition at 

 Cambridge strongly aims in this direction, and I am 

 convinced that you will reap as much credit and more 

 advantage by studying mathematical physics as pure 

 mathematics. I cannot conceive a better exercise than 

 Airy's tract on Light, which contains some hard mathe- 

 matics, but the acquisition of the clear physical views it 

 presents is much harder. I am glad you should feel 

 any interest in so unpopular a subject as polarized heat. 

 I have now greatly extended my experiments, made 

 the effects much more obvious, and made some new 

 singular discoveries. 



* The Royal Society of Edinburgh have done me the 

 honour to award me their Keith Medal. 



* My summer was chiefly in the Pyrenees and Auvergne. 

 In the former I studied hot springs, in the latter volcanos. 

 I do not wonder that you were appalled by the difficulties 

 of the measure of absorption of the atmosphere, which 

 in fact involves the same difficulties with the theory of 

 refraction, which has been a celebrated problem amongst 

 mathematicians. . . .' 



When his third professorial session was over his cor- 

 respondence was resumed. 



To M. QUETELET, Observatory -, Brussels. 



'Mayllth, 1836. 



' . . . I have been so much occupied with my experi- 

 ments upon polarized heat, and with several papers 

 which I have communicated to our Royal Societies, as 

 well as with my annual lectures, that I have had but 

 little time for correspondence or for reducing old obser- 



