350 SCIENTIFIC AMUSEMENTS. 



9. Radiant Action : Invisible Light. 

 CHAPTEK XXIV. 



KADIANT HEAT. 



One of the simplest possible observations is that when one stands 

 in the sunshine in summer time so as to be illuminated by means 

 of visible light a sensation of warmth is developed, which is con- 

 siderably diminished by moving into the shade of some solid large 

 object impervious to visible light, and consequently casting a 

 " shadow ; " in other words, a radiant action accompanying visible 

 light, but producing a different sensation, is also present in sun- 

 light. If an ordinary artificial fire burning in a grate be examined, 

 it will be noticed that on .interposing a pane of glass between 

 the fire and the face, the sensation of warmth due to the radiation 

 of heat will be diminished, although the fire is practically just as 

 visible as before, showing that glass is less transparent for that 

 form of radiant energy termed heat than for visible light. If, 

 instead of a pane of glass, a deal board or metal teatray be used, 

 still less heat will be felt, whilst all visible light will be cut off, both 

 the wood and metal being quite opaque to either form of radiation. 



If, whilst direct radiation is thus screened off, a mirror be so 

 held by another person that the fire can be distinctly seen by 

 reflection at not too great a distance, a sensation of warmth on the 

 face will be noticeable, which will disappear on altering the position 

 of the mirror, so that the blaze is no longer visible; therefore 

 radiant heat is reeded in the same way as visible light. A 

 delicate thermometer will indicate analogous results even more 

 perfectly than the face. 



Yet again, if a large convex lens (Expt. 365) be so held that a 

 bright image of the sun is formed on a sheet of white paper, the 

 paper will become strongly heated where the image is produced, 

 and will be fired, the lens acting as a " burning glass " and con- 

 verging the heat rays to a "focus" situated in much the same 

 position as that of the visible rays. 



In like manner all experiments possible with visible light may 

 be performed with radiant heat with exactly analogous results 

 when the methods of detection of heat employed are sufficiently 

 delicate to enable such observations to be made. In short, that 



