xiv.] FORBES' SCIENTIFIC WORK. 461 



'for effecting directly the separation of the rays, which 

 'requires a very considerable thickness of the crystal. 

 1 1 also succeeded in repeating Fresnel's experiment of 

 'producing circular polarization by two internal reflec- 

 ' tions. The substance used was, of course, rock-salt/ 

 Taken in conjunction with the experiments of Melloni 

 and others on the absorption, &c., of radiant heat, this 

 splendid series of researches formed 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. 



The discovery of the polarization of heat will certainly 

 form an epoch in the history of Natural Philosophy. 



.Most of Forbes' experiments are now easy to repeat, 

 even on a large scale, as class illustrations, so much 

 have galvanometers been improved since dynamical 

 notions of construction have been introduced by Sir W. 

 Thomson. The astatic pair of stout, long, needles com- 

 mon, till very lately, to all the better instruments, has 

 within a few years been rapidly disappearing, and the 

 repetition of Forbes' experiments need now give no one 

 any trouble ; but the discoveries he made are not the 

 less meritorious on that account any more than Fara- 

 day's grand discovery of magneto-electricity by a feeble 

 motion of a delicate galvanometer is rendered of less 

 account by the tremendous currents developed of late 

 in the machines of Wilde and others. Rather let us be 

 ashamed that, with the more perfect appliances at our 

 < nmniand, we have not added to Forbes results others 

 of equal or greater value, certainly now within our 

 reach, if we knew where to look for them, and destined, 

 perhaps, to be the class CXJM -i -iments of the next gene- 

 >n. 



Of great importance also are Forbes' researches <>n 

 the Conduction of llit by iron bars, and his quantita- 

 tive measurements of the comlm ti\ ity of iron at various 

 temperatures. !!< had < all. <1 attention, as early as 1833, 

 to tne fact that the ord.T <>f im-tals jis regards thermal 



