V 



He thus entered on a Professorate of seven-and-twenty years, carried on 

 with an energy and success rarely equalled, never surpassed. He began 

 to teach the Natural Philosophy Class in Session 1833-34, when he was 

 only four-and-twenty, yet from the very first he rivetted the attention of 

 one of the largest and most distinguished classes of students that Edinburgh 

 University ever contained. One great feature of his teaching, as I have 

 always understood, was that while it stimulated the powers and won the 

 admiration of his most scientific listeners, he yet made himself attractive 

 to all students who were intelligent, though less highly gifted. 



In these lectures the whole range of Natural Philosophy was gone 

 through, and the laborious and well-sustained study required for the due 

 performance of his professorial work had doubtless prepared him for the 

 task, which he successfully accomplished, of writing the well-known " Dis- 

 sertation on the Progress of Mathematical and Physical Science, principally 

 from 1775 to 1850," which was published in the last edition of the 1 Ency- 

 clopaedia Britannica.' Meanwhile, however, Professor Forbes was indefa- 

 tigable in original investigation, for which he had so early shown an apti- 

 tude ; and the separate titles of his contributions to Transactions of 

 Societies and Scientific Journals, as given in the Royal Society's ' Catalogue 

 of Scientific Papers,' amounted in 1863 to one hundred and eighteen. 

 These embrace various subjects belonging to Physics, Meteorology, Geo- 

 logy, and Physical Geography. The most important of his earlier experi- 

 mental investigations was that in which he succeeded in demonstrating the 

 polarization of heat. Melloni had been pursuing an inquiry in the same 

 direction, but had failed to obtain the result he was in quest of. Professor 

 Forbes was more fortunate. The steps of his investigation are thus stated 

 by himself in the dissertation above referred to. 



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 

 transmitted ray, and afterwards by reflexion from the multiplied 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 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 circumstances would have done. 

 This experiment, which, with the others just mentioned, was soon re- 

 peated and confirmed by other observers, still remains the only one proving 



