L54 IIISTOEY OF THEIttlOTICS. 



light, we cannot retain the doctrine that heat radiates by the emana- 

 tion of material particles, without supposing those particles of caloric 

 to have poles ; an hypothesis which probably no one would embrace 

 for, besides that the ill fortune which attended that hypothesis in th< 

 case of light must deter speculators from it, the intimate connexion of 

 heat and light would hardly allow us to suppose polarization in the 

 two cases to be produced by two different kinds of machinery. 



But, without here tracing further the influence which the polariza- 

 tion of heat must exercise upon the formation of our theories of heat, 

 we must briefly notice this important discovery, as a law of pheno- 

 mena. 



The analogies and connexions between light and heat are so strong, 

 that when the polarization of light had been discovered, men were 

 naturally led to endeavor to ascertain whether heat possessed any 

 corresponding property. But partly from th e difficulty of obtaining any 

 considerable effect of heat separated from light, and partly from the 

 want of a thermometrical apparatus sufficiently delicate, these attempts 

 led, for some time, to no decisive result. M. Berard took up the sub- 

 ject in 1813. He used Malus's apparatus, and conceived that he found 

 heat to be polarized by reflection at the surface of glass, in the same 

 manner as light, and with the same circumstances. 25 But when Pro- 

 fessor Powell, of Oxford, a few years later (1830), repeated these ex- 

 periments with a similar apparatus, he found 26 that though the heat 

 which is conveyed along with light is, of course, polarizable, " simple 

 radiant heat," as he terms it, did not offer the smallest difference in 

 the two rectangular azimuths of the second glass, and thus showed no 

 trace of polarization. 



Thus, with the old thermometers, the point remained doubtful. 

 But soon after this time, MM. Melloni and Nobili invented an appara- 

 tus, depending on certain galvanic laws, of which we shall have to 

 speak hereafter, which they called a tkermomultiplier / and which 

 was much more sensitive to changes of temperature than any previ- 

 ously-known instrument. Yet even with this instrument, M. Melloni 

 failed ; and did not, at first, detect any perceptible polarization of heat 

 by the tourmaline ;" nor did M. Nobili, 28 in repeating M. Berard's ex- 

 periment. But in this experiment the attempt was made to polarize 

 neat by reflection from glass, as light is polarized : and the quantity 



- b Ann. Chim. March, 1813. " c Edin. Journ. of Science, 1830, vol. ii. p. 303. 

 87 Ann. de Chimic, vol. Iv. 2S flib/iotheque Univcrselle 



