176 Mr. W. Crookes on the Otheoscope. [Apr. 26, 



III. " On Repulsion resulting from Radiation. — Preliminary Note 

 on the Otheoscope." By William Crookes, F.R.S. &c. Re- 

 ceived April 23, 1877. 



I communicated to the Royal Society in November last an account of 

 some radiometers which I had made with the object of putting to 

 experimental proof the "molecular pressure" theory of the repulsion 

 resulting from radiation. Continuing these researches, I have constructed 

 other instruments, in which a movable fly is caused to rotate by the 

 molecular pressure generated on fixed parts of the apparatus. 



In the radiometer, the surface which produces the molecular distur- 

 bance is mounted on a fly, and is driven backwards by the excess of 

 pressure between it and the sides of the containing vessel. Regarding 

 the radiometer as a heat-engine, it is seen to be imperfect in many re- 

 spects. The black or driving surface, corresponding to the heater of the 

 engine, being also part of the moving fly, is restricted as to weight, 

 material, and area of surface. It must be of the lightest possible con- 

 struction, or friction will greatly interfere with its movement ; it must 

 not expose much surface, or it will be too heavy ; and it must be a very 

 bad conductor of heat, so as to retain the excess of pressure on one side. 

 Again, the part corresponding to the cooler of the engine (the side of 

 the glass bulb) admits of but little modification. It must almost neces- 

 sarily be of glass, by no means the best material for the purpose ; it is 

 obliged to be of one particular shape ; and it cannot be brought very near 

 the driving surface. 



A perfect instrument would be one in which the heater was stationary ; 

 it might then be of the most suitable material, of sufficient area of surface, 

 and of the most efficient shape, irrespective of weight. The cooler should 

 be the part which moves ; it should be as close as possible to the heater, 

 and of the best size, shape, and weight for utilizing the force impinging 

 on it. By having the driving surface of large size, and making it of a 

 good conductor of heat, such as silver, gold, or copper, a very faint amount 

 of incident radiation suffices to produce motion. The black surface acts as 

 if a molecular* wind were blowing from it, principally in a direction 

 normal to the surface. This wind blows away whatever easily movable 

 body happens to be in front of it, irrespective of colour, shape, or material ; 

 and in its capability of deflection from one surface to another, its arrest 

 by solid bodies, and its tangential action, it behaves in most respects like 

 an actual wind. 



Whilst the radiometer admits of but few modifications, such an instru- 



* Molecular, not molar. There is no wind in the sense of an actual transference of 

 air from one place to another. This molecular movement may be compared to the 

 movement of the gases when water is decomposd by an electric current. In the water 

 connecting the two poles there is no apparent movement, although eight times as much 

 matter is passing one way as the other. 



