vi 



the double refraction of heat unaccompanied by light ; and, though some- 

 what indirect, it will hardly be regarded by competent judges as otherwise 

 than conclusive. Iceland spar and other doubly-refracting substances 

 absorb invisible heat too rapidly to be used for affecting directly the sepa- 

 ration of the rays, which requires a very considerable thickness of the 

 crystal. I also succeeded in repeating Fresnel's experiment of producing 

 circular polarization by two internal reflections. The substance used was, 

 of course, rock-salt.' 5 



For these researches the Royal Society awarded to their author the 

 Rumford Medal, in 1838. Taken in conjunction with the experiments of 

 Melloni on the absorption, &c. of radiant heat, they afforded the conclusive 

 proof of the identity of thermal and luminous radiations, — a fact of the very 

 greatest consequence to the further progress of one of the most fascinating 

 branches of physical science. 



In 1842 Professor Forbes communicated to the Royal Society a paper 

 " On the Transparency of the Atmosphere, and the Law of Extinction of 

 the Solar Rays in passing through it," which was adopted as the Bakerian 

 Lecture for that year, and for which in 1 843 he received a Royal Medal. 



Another prominent work of Forbes is that long series of observations on 

 the Nature and Motion of Glaciers, which he pursued with intense applica- 

 tion, and, as there is too good reason to believe, to the serious injury of his 

 health. He had spent several vacations on the Continent, and had wan- 

 dered over the mountains of Switzerland and Savoy, studying the Geology 

 and Physical Geography of those regions ; but he specially devoted the 

 summers of 1842-44 and 1846 to the exploration of the glaciers of the 

 Alps. Into this pursuit he threw an enthusiasm and concentration of 

 energy which few men are capable of. In 1843 appeared his well-known 

 work, f Travels in the Alps,' in which he blends interesting descriptions of 

 scenery with scientific observations and reasonings ; and the fruits of his 

 earlier labours on the glaciers are there given. Shortly before this he 

 commenced the interesting series of sixteen " Letters on Glaciers " published 

 in the 'Edinburgh New Philosophical Journal ' from 1842 to 1851, which 

 contain the results of his continued study ; and he also set forth and 

 discussed his views on the constitution and motion of glaciers, in an elabo- 

 rate Memoir published in the Philosophical Transactions for 1846, en- 

 titled "Illustrations of the Viscous Theo^ of Glacier Motion." 



Not contented to limit his observations to Switzerland and Savoy, he in 

 1851 made an excursion to Norway to study the glacier phenomena of 

 that country,' and gave an account of his work in ' The Glaciers of Norway 

 visited in 1851,' which was published in 1853. 



Professor Forbes looked on the progressive motion of a glacier as com- 

 parable to that of a plastic mass, moulding and adapting itself to the varia- 

 tions in the width and depth of its channel and the inclination of its bed, and 

 moving faster in the middle than at the sides, as would happen with such a 

 yielding substance. A view substantially similar had been previously pro- 

 mulgated by Bordier in 1773, the contemporary of De Saussure, and more 



