Effects of Heat and Light on Inorganic Bodies. 415 



body absorbing it, or it may do work in separating the molecules of the 

 body, the condition being, that the rate of vibration of the light waves 

 fits the pitch of the interior spaces of the body. 



There is a little instrument, which was invented by 

 Wm. Crookes, that shows how radiant energy can be 

 converted into the motion of a mill. It is called a 

 Radiometer. It consists of an air-tight glass globe, 

 inside of which is a wheel consisting of four arms ra- 

 diating from an upright shaft standing on a needle 

 point, which allows it to revolve without friction. To 

 each arm is attached a little plate of aluminum, which 

 is black on one side and bright on the other. The air 

 is almost completely exhausted from the globe. The 

 light ( with heat ) of even a candle will cause the mill 

 to turn, a hot sunbeam making it revolve with surpris- 

 ing rapidity. It goes in the direction indicated by the 

 arrow in the cut, the bright side of the plates in ad- 

 vance. There is no way to make a perfect vacuum in 

 Radiometer *^ e glbe ; and there is sure to be a quantity of resid- 



ual air after the most complete exhaustion practicable. The motion is 

 due to the action on the plates of this residual air which is set in motion 

 by the external heat. When the radiometer is exhausted to a certain de- 

 gree, the mill rotates most freely, but the exhaustion of air can be car- 

 ried to so great ai/ extent that the motion begins to be retarded, which 

 proves that the action is due to the effect of light or heat on the resid- 

 ual gas remaining in the glass after exhaustion. When air in several 

 radiometers is replaced by oxygen, hydrogen, carbonic acid, &c. , and 

 the globes are all exhausted to the same degree, the mills rotate at dif- 

 ferent rates, supposed by C arpenter to be due to the different rates of 

 molecular vibration of the several gases. It is possible to compel the 

 arms to remain. still while the globe rotates. This has been done by 

 fixing a magnet on the arms of the mill, then floating the radiometer in 

 an upright position in water. Light then caused the mill to rotate. 

 Then a strong magnet was brought near and the arms stopped, but the 

 globe began to slowly rotate in the opposite direction. When the mag- 

 net was taken away, the mill began at once to go in the original direc- 

 tion, and the glass globe came to rest. The most powerful action of 

 the radiometer is produced by the red rays those strongest in heating 

 qualities. 



