Mr. W. Crookes on the Radiometer. 471 



the mode of action of the residual gas. The explanation, as given 

 by Mr. Johnstone Stoney, appears to me the most probable ; 

 and having stood almost every experimental test to which I have 

 submitted it, I may assume for the present that it expresses the 

 truth. According to this the repulsion is due to the internal 

 movements of the molecules of the residual gas. When the mean 

 length of path between successive collisions of the molecules is 

 small compared with the dimensions of the vessel, the molecules 

 rebounding from the heated surface, and therefore moving with 

 an extra velocity, help to keep back the more slowly moving mole- 

 cules which are advancing towards the heated surface ; it thus 

 happens that though the individual kicks against the heated surface 

 are increased in strength in consequence of the heating, yet the 

 number of molecules struck is diminished in the same propor- 

 tion, so that there is equilibrium on the two sides of the disk, even 

 though the temperatures of the faces are unequal. But when the 

 exhaustion is carried to so high a point that the molecules are 

 sufficiently few and the mean length of path between their successive 

 collisions is comparable with the dimensions of the vessel, the swiftly 

 moving, rebounding molecules spend their force, in part or in 

 whole, on the sides of the vessel; and the onward crowding, 

 more slowly moving molecules are not kept back as before, so that 

 the number which strike the warmer face approaches to, and in the 

 limit equals, the number which strike the back, cooler face ; and as 

 the individual impacts are stronger on the warmer than on the 

 cooler face, pressure is produced, causing the warmer face to 

 retreat. 



I have tried many experiments with the view of putting this 

 theory to a decisive test. The repulsive force being due to a reac- 

 tion between the fly and the glass case of a radiometer, it follows 

 that, other things being equal, the fly should revolve faster in a small 

 bulb than in a large one. This cannot well be tested with two 

 different radiometers, as the weight of the fly and the amount of 

 friction would not be the same in each ; but I have constructed a 

 double radiometer which shows this fact in a very satisfactory 

 manner. It consists of two bulbs, one large and the other small, 

 blown together so as to have a wide passage between them. In 

 the centre of each bulb is a cup, held in its place by a glass rod ; 

 and in 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 in- 

 dependently of each other ; and one of them was furnished with 

 a minute fragment of iron, so that by means of a magnet I could 



