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



Polarization of Heat, the excessively modest account of 

 which in the Dissertation above ^mentioned forms one 

 grand defect visible to all who are acquainted with the 

 immense value of Forbes' discoveries, and with the ex- 

 traordinary difficulties under which, although he had the 

 very best instruments of the time, his experiments were 

 necessarily conducted. Yet, even from his own modest 

 statement, the reader may see that something of import- 

 ance was discovered. He says: * I have just referred to 

 ' my own early experiments on the subject (which were 

 'likewise inconclusive), in order to explain that it was 

 ' natural, on hearing of the application of the thermo- 

 ' multiplier to measure radiant heat, that I should wish 

 ' to repeat them with the new instrument. This I did 

 ' in 1834. I first succeeded in proving the polarization 

 ' of heat by tourmaline, which Melloni had announced 

 ' did not take place ; next, by transmission through a 

 ' bundle of very thin mica plates, inclined to the trans- 

 ' mitted ray ; and afterwards by reflection from the mul- 

 ' tiplied surfaces of a pile of thin mica plates placed at 

 ' the polarizing angle. I next succeeded in showing that 

 'polarized heat is subject to the same modifications 

 ' which doubly-refracting crystallized bodies impress upon 

 'light, by suffering a beam of heat, even when quite 

 ' obscure, after being polarized by transmission, to pass 

 ' through a depolarizing plate of mica, the heat traversing 

 ' a second mica bundle before it was received on the pile. 

 ' As the plate of mica used for depolarization was made 

 c to rotate, in its own plane, the amount of heat shown 

 ' by the galvanometer was found to fluctuate just as the 

 ' amount of light received by the eye under similar cir- 

 ' cumstances would have done. This experiment, which, 

 ' with the others just mentioned, was soon repeated and 

 ' confirmed by other observers, still remains the only 

 ' one proving the double refraction of heat unaccompanied 

 ' by light ; and though somewhat 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 



