THE MECHANICAL ACTION OF LIGHT. 271 



does not very much interfere with the movement. I place yellow 

 glass in front, and the movement is scarcely diminished at all. Very 

 deep-colored glass, you see, diminishes it a little more. Blue and 

 green glass make it go a little slower, but still do not diminish the 

 speed one-half. I now place a screen of water in front : the instru- 

 ment moves with diminished velocity, rotating with about one-fourth 

 its original speed. 



Taking the action produced by a candle-flame as .... 100 



Yellow glass reduces it to 89 



Red " " " 11 



Blue " " " 56 



Green " " " 56 



Water " " 26 



Alum " " 15 



I now move the candle a little distance off, so as to make the in- 

 strument move slower, and bring a flask of boiling water close to it. 

 See what happens. The luminous index no longer moves steadily, 

 but in jerks. Each disk appears to come up to the boiling water with 

 difficulty, and to hurry past it. More and more sluggishly do they 

 move past, until now one has failed to get by, and the luminous beam, 

 after oscillating to and fro a few times, comes to rest. I now gradu- 

 ally bring the candle near. The index shows no movement. Nearer 

 still. There is now a commencement of motion, as if the radiometer 

 were tryiug to push past the resistance offered by the hot water; but 

 it is not until I have brought the candle to within a few inches of the 

 glass globe that rotation is recommenced. On these pith radiometers 

 the action of dark heat is to repel the black and white surfaces almost 

 equally, and this repulsion is so energetic as to overcome the rotation 

 caused by the candle, and to stop the instrument. 



With a radiometer constructed of a good conductor of heat, such 

 as metal, the action of dark heat is different. Here is one made of 

 silvered copper, polished on one side and lampblacked on the other. 

 I have set it moving with a candle slightly the normal way. Here is 

 a glass shade heated so that it feels decidedly warm to the hand. I 

 cover the radiometer with it, and the rotation first stops, and then 

 recommences the reverse way. On removing the hot shade the 

 reverse movement ceases and normal rotation recommences. 



If, however, I place a hot glass shade over a pith radiometer, the 

 arms at once revolve the normal way, as if I had exposed the instru- 

 ment to light. The diametrically opposite behavior of a pith and a 

 metal instrument when exposed to the dark heat radiated from a hot 

 glass shade is very striking. The explanation of the action is not 

 easy, but it depends on the fact that the metal is one of the best con- 

 ductors of heat, while pith is one of the worst. 



One more experiment with this metallic radiometer. I heat it 

 strongly with a spirit-lamp, and the arms spin round rapidly. Now 



