KNOBLAUCH OX RADIANT HEAT. 



203 



by certain rays of heat in a greater degree than by others^ 

 whether this occurs at a lower or higher temperature*. 



That free radiant heat is really the agent concerned in the 

 cases we have detailed, follows with certainty from the following 

 observations : — 



1. When, after the insertion of one of the above bodies be- 

 tween the source of heat and the thermal pile, the needle of the 

 multiplier has arrived at a certain deflection, if the source of 

 heat be removed whilst the inserted substance retains a fixed 

 position as regards the pile, the needle at the same time returns 

 to the same point, whatever may have been the extent of the 

 amount of deflection. Hence this does not arise from the in- 

 serted body itself becoming heated. 



2. If the thermal pile be removed from the field of rectilinear 

 radiation of the source of heat, whilst it preserves a constant 

 distance from the inserted substance, which remains exposed to 

 the heating rays, the needle immediately returns to the same 

 point which it attains on the removal of the source of heat ; a 

 further proof that the deviation observed cannot be ascribed to 

 the heat absorbed by the former substance. 



3. In almost all the bodies experimented upon, the indication 



of the thermoscope is diminished when it is coated on both 



sides with lamp-black ; and when thus its power of absorption 



and radiation is increased at the expense of the transmission. 



* I obtained the following results when radiant heat from a Leslie's cube at 

 212° F., red-hot platinum, the flame of alcohol and an Argand lamp was trans- 

 mitted through prepared rock salt : — 



Thus the heat of the red-hot jilatinuin and the flame of alcohol, the tempera- 

 tures of which are undoubtedly different, radiate through the prepared rock-salt 

 in the same manner, whilst the heat from the source at 212° F., conformably with 

 Melloni's discovery, pai'ses through it better than that of the .Argand lamp. 



