180 Mr. G. J. Stoney on Crookes's Radiometer. 



of light of mean refrangibility, and about one sixteenth of 

 the diameter of the disks which float in human blood. Let us 

 suppose, then, that a layer of this thickness is heated, irre- 

 spective of convection, when ordinary air is inside the ap- 

 paratus. 



7. On the foregoing assumptions, we can compute what the 

 state of things will be when the chamber is exhausted. When 

 the pressure is made to vary, it appears from the doctrine of 

 probabilities that the value of \ (using X to designate the length 

 of the average excursion of a molecule, i. e. the distance a 

 molecule on the average travels in the intervals between two 

 of its encounters with other molecules) will vary inversely as 

 Si, 8 being the density. Now the thickness of the layer of 

 graduated temperature depends on X, and will vary in the 

 same ratio as it. We have supposed the density in our 

 vacuum-chamber to be -~^ of an atmosphere ; it will follow 

 that the thickness of the heated layer in this attenuated 

 medium would be 10,000 times what it is in ordinary air, 

 and would therefore become half a sixth-metre X 10,000^, which 

 is more than a decimetre. It therefore reaches quite to the 

 walls of our little vacuum-chamber ; and this very materially 

 alters the state of affairs. 



8. In fact we have on one side glass at a temperature of, 

 suppose, 15°, on the other a disk at a temperature of 15°*1, 

 and between them a space which is only a part of what 

 would be required to establish a complete gradient of tem- 

 perature in the intervening air. This is equivalent to saying 

 that some of the additional momentum communicated to 

 molecules of air by the heated disk, instead of expending 

 itself in interaerial collisions and thus increasing the general 

 temperature and pressure of the air, makes its way across the 

 intervening stratum to the opposite walls of glass, where it 

 occasions an increased pressure against them, of which the re- 

 sultant is directed perpendicularly from the disk. The mo- 

 mentum of the accelerated molecules which reach the glass 

 falls after the contact of the molecules with the glass to the 

 feebler type corresponding to its lower temperature ; and it is 

 chiefly momentum of this feebler type whish makes its way 

 to regions behind the disk. An excess of force equal and 

 opposite to that on the glass acts against the front of the disk, 

 and is sufficient to account for the phenomena which Mr. 

 Crookes has investigated *. For its amount may be approx- 

 imated to as follows. 



* This little heat-engine may be otherwise described as follows : — 

 There is active motion of the molecules of air in all parts of the chamber ; 



