414 



RADIATION OF HEAT. 



called polarization. In fact, a mirror, placed in a certain inclined position, 

 above or below one of these two rays, is capable of reflecting them in the or- 

 dinary way ; but if placed in the same oblique position, on either side of them, 

 it becomes utterly incapable of reflecting them. The other ray possesses a 

 similar quality, but the position of the non-reflecting side is reversed. Now, 

 the two rays into which a non-luminous calorific ray, transmitted through such 

 a crystal, is resolved, are found to possess precisely the same property 

 they are polarized. 



A ray of light falling on a reflecting surface at a certain angle, the magni- 

 tude of which will depend on the nature of the surface, is found, when reflect- 

 ed in the ordinary way, to be polarized or put into the physical state just now 

 mentioned, to result from the double refraction of a crystal. It is capable of 

 being reflected by an oblique mirror placed above or below it, but it is incapa- 

 ble of being reflected by the same mirror, similarly placed, on either side. A 

 non-luminous calorific ray, whether proceeding from the prism, or from a hot 

 body reflected, is found to undergo the same effect, and to be also polarized. 



In the experimental investigation of the phenomena attending thp radiation 

 of heat, it is necessary to distinguish the effect of radiated heat from the cas- 

 ual variation of the temperature of the air in the apartment in which the exper- 

 iment may be conducted. The use of the thermometer would, in this case, be 

 attended with material inconvenience, inasmuch as it would be extremely diffi- 

 cult to distinguish the effect of the heat radiated, from the casual change of 

 temperature of the medium in which the thermometer is placed. A second 

 thermometer, it is true, might be used in such experiments, the variations of 

 which would show the change of temperature of the medium ; but this second 

 thermometer could never be placed exactly in the same position as the ther- 

 mometer affected by the radiant heat : and it would not follow that the changes 

 of temperature of two different parts of the same chamber would, necessarily, 

 be exactly alike. An instrument, therefore, which is not affected by any 

 change of temperature in the medium in which it is placed would be capable 

 of giving much more accurate indications for such a purpose. Such an in- 

 strument was invented and applied by Sir John Leslie, in his experiments on 

 radiant heat, the results of which have, so justly, placed that distinguished 

 philosopher in the first rank of modern discoverers in physics. 



The differential thermometer of Leslie consists of a small glass tube, fig. 5, 

 at each extremity of which is placed two thin hollow bulbs, F E, of glass, and 

 the tube is bent into the rectangular form, E A B F, and supported on a stand 

 S, the bulbs being presented upward. This tube contains a small quantity of 

 sulphuric acid, tinged red with carmine, to render it easily visible, filling the 

 greater part of the legs and horizontal branch. To one of the legs, F B, a 



