386 KNOBLAUCH OX RADIANT HEAT. 



SO that each of them was exposed to the rays of heat for as short 

 n time as possible. The following observations will show that 

 the object was attained by this means : — 



1. When a surface has been heated (for the purpose of exa- 

 mining the heat reflected from it by three diathermanous bodies) 

 by exposure to the rays of heat for four minutes, it exerts no 

 perceptible action upon the thermoscope ; for the needle returns 

 almost immediately to its original position as soon as the source 

 of heat is removed, the position of the surface itself being unal- 

 tered as regards the pile. 



The deflection produced by the direct radiation, and which 

 was controlled before each new insertion of a diathermanous 

 substance, is therefore not perceptibly increased even in this 

 space of time, which should occur if the heat of the reflecting 

 body itself wei'e added in a constantly increasing quantity to the 

 reflected heat. 



Within the 1*5 to 2 hours which comprised a series of ob- 

 servations, and during which time a cube was exposed to the 

 rays of heat at the most four times, the temperature of the water 

 was not raised nioie than 0°'5 R. in any cube by the radiation. 

 However, as the heat acquired by the surfaces inclined towards 

 the thermal pile merely produces a deflection of about 1° in the 

 multiplier, the errors arising from this cause in the observations 

 made after the insertion of the diathermanous bodies cannot ex- 

 ceed half degrees, within which the differences of the numbers 

 subsequently given may be considered as accurate. 



2. On repeating the experiment sevex'al times, after the inser- 

 tion of the diathermanous substances, the same deviation of the 

 needle is constantly observed ; hence the quantity of heat which 

 passes through it is always the same. If the deflection which 

 ensues on direct radiation were produced on the repetition of 

 the experiment, partly by the heat of the reflecting surfaces 

 themselves, the needle, on the insei'tion of the diathermanous 

 media, should exhibit less deviation than before, because the 

 heat radiated by these bodies is comparatively less perfectly 

 transmitted by these substances than the reflected heat of an 

 Argand lamp (which was used in these experiments). 



Thus in five experiments a recess of 6°*25-6°"5 of the needle 

 was found when red glass was used, 4°*5 with blue glass, and 

 3°'5-3°"75 with alum, when the heat of the lamp was reflected 

 by black velvet, and when the deflection produced before the 



