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Mr. W. Crookes on the Radiometer. [Nov. 16^ 



lished, leave no reasonable doabfc that the presence of residual gas* is 

 the cause of the movement of the radiometer. But few theories are 

 sufficiently strong not to require reinforcement ; and in the present case 

 very much remains to be ascertained as regards 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 experi- 

 mental 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 re- 

 bounding from the heated surface, and therefore moving with an extra 

 velocity, help to keep back the more slowly moving molecules 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 proportion, so that there is equilibrium on the 

 two sides of the disk, even though the temperatures of the faces are un- 

 equal. 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 onw^ard 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 reaction 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 



* It is a question whether the residual gas in the apparatus, when so highly 

 attenuated as to have lost the greater part of its viscosity, and to be capable of 

 acquiring molecular movement palpable enough to overcome the inertia of a plate of 

 metal, should not be considered to have got beyond the gaseous state, and to have 

 assumed a fourth state of matter, in which its properties are as far removed from those 

 of a gas as this is from a liquid. 



