Mr. Gr. J. Stoney on Crookes's Radiometer. 179 



the temperature varies gradually, having on one side the tem- 

 perature of the surrounding air, and on the other the tempera- 

 ture of the disk. 



5. If the chamber enclosing the apparatus contained air 

 at atmospheric pressure and temperature, this layer would be 

 thin. It would consist of air which has been expanded by the 

 warmth of the disk, while the air in the rest of the chamber 

 would by this expansion be in a trifling degree compressed. 

 In other words, the molecules whose activity has been in- 

 creased by contact with the heated disk would, in their en- 

 counters with other molecules, keep back some of them, and 

 in this way reduce the number of molecules striking the heated 

 disk, while this process would slightly crowd molecules into 

 the rest of the chamber, and thus increase the number coming 

 into collision with unheated surfaces. In this way the pres- 

 sure everywhere is in a small degree raised, but everything is 

 adjusted so that there is no excess of pressure anywhere ; and 

 this adjustment takes place in an exceedingly short period of 

 time, so short that no sensible motion of the disk can establish 

 itself while it is being effected. In fact the number of molecules 

 per cubic millimetre in atmospheric air is known to be about a 

 unit-eighteen*; the molecules are dashing about with velocities 

 of which the average is about 500 metres per second; each 

 meet i with about 1000 millions of encounters with others in 

 every second ; and the adjustment accordingly takes place with, 

 what is promptitude as compared with visible motions (see 

 paper on the Internal Motions of Gases, in the Philosophical 

 Magazine of August 1868). 



6. It is necessary for our purpose to form some estimate 

 of the thickness of the layer of warmed air. In the absence 

 of direct experiments f , I will assume that this layer of gra- 

 duated temperature would in ordinary air be one fourth- 

 metre thick (about as thick as a sheet of paper) if the disk 

 were 20° Centigrade hotter than the air. This seems a very 

 moderate estimate, judging from the copiousness of the con- 

 vection currents which would quickly establish themselves if 

 there were such a difference of temperature. And from this 

 assumption it follows that if the temperature of the disk had 

 been raised 0*1 of a degree before the chamber was exhausted, 

 which I have assumed to be about the elevation of tempera- 

 ture that actually takes place in the radiometer, the thickness 

 of the warmed layer of air would have been half a sixth-metre. 

 This is a very small thickness. It is about the wave-length 



* That is, a million of millions of millions. 



t It will appear from the sequel that the Radiometer may be made the 

 means of determining by experiment the thickness of this layer. 



