1876.] 



Mr. W. Crookes on the Radiometer. 



309 



the bulbs is a small four-armed fly with roasted mica disks blacked on 

 one side. The fly can be balanced on either cup. In the smaller bulb 

 there is about a quarter of an inch between the vanes and the glass, 

 whilst in the larger cup there is a space of half an inch. The mean of 

 several experiments shows that in the small bulb the fly rotates about 50 

 per cent, faster than in the large bulb, when exposed to the same source 

 of light. 



One of the arms of another radiometer was furnished with roasted 

 mica disks blacked on alternate sides. The other arm was furnished with 

 clear mica disks. The two arms were pivoted independently of each 

 other, and one of them was furnished with a minute fragment of iron, 

 so that by means of a magnet I could bring the arms in contact, the 

 black surface of the mica then having a clear plate of mica in front of 

 it. On bringing a lighted candle near the instrument, and allowing it 

 to shine through the clear plate on the blackened mica, the clear plate 

 is at once driven away till the arm sets at right angles to the other. 



Two currents of force, acting in opposite directions, can exist in the 

 same bulb. I have prepared a double radiometer in which two flies are 

 pivoted one over the other, and having their blackened sides turned in 

 opposite directions. On bringing a lighted candle near, the flies rapidly 

 rotate in opposite directions. 



Experiment shows that the force can be reflected from a plane surface in 

 such a manner as to change its direction. If an ordinary radiometer is 

 exposed to light the black surface is repelled, owiug to the excess of 

 pressure acting between it and the glass. If, however, a plate of mica 

 were to arrest this force and reflect it back again, the motion should be 

 reversed. Experiment shows that this is the case. A two-disk radio- 

 meter was made, having flat opaque mica disks blacked on one side. In 

 front of the black surface of the mica, about a millimetre off, is fixed a 

 large disk of thin clear mica. On bringing a candle near, the molecular 

 pressure streaming froui the black surface is caught by the clear plate 

 and thrown back again, causing pressure behind instead of in front ; and 

 the result is rapid rotation in the negative direction, the black side now 

 moving towards the light. 



To still further test this view of the action, I made another radiometer, 

 similar to the above, but having a clear mica disk on each side of the 

 ordinary mica vane. This prevents the reflection of the pressure back- 

 wards, and causes it to expend itself in a vertical plane, the result being 

 an almost total loss of sensitiveness. 



The above actions can be explained on the " evaporation and conden- 

 sation " theory, as well as by that of molecular movement ; and I there- 

 fore devised the following test to decide between these two theories. 

 A radiometer has its four disks cut out of very clear and thin plates 

 of mica, and these are mounted in a somewhat large bulb. At the 



