140 Mr. William Croohes [April 4, 



Because of tins preponderatiDg motion, and also because they will 

 thereby less effectually keep back bees crowding in from the sides, 

 there will now be a greater proportionate pressure both on the hot 

 face of the diaphragm and on that part of the box which is in front 

 of it. Hence the pressure on the hot side will now exceed that on the 

 cool side of the diaphragm, which will consequently have a backward 

 movement communicated to it. 



I may diminish the size of the bees as much as I like, and by 

 correspondingly increasing their number the mean free path will 

 remain the same. Instead of bees let me call them molecules, and 

 instead of having a few hundreds or thousands in the box let me have 

 millions and billions and trillions ; and if we also diminish the mean 

 free path to a considerable extent, we get a rough outline of the 

 kinetic theory of gases. (I may just mention that the mean free path 

 of the molecules in air, at the ordinary pressure, is the ten-thousandth 

 of a millimetre.) 



Three years ago I had the honour of bringing before you the 

 results of some researches on the Eadiometer. Let me now take up 

 the subject where I then left off. I have here two radiometers which 

 have been rotating before you under the influence of a strong light 

 shining upon them. 



The explanation of the movement of the radiometer is this, — the 

 light, or the total bundle of rays included in the term " light," falling 

 upon the blackened side of the vanes, becomes absorbed, and thereby 

 raises the temperature of the black side : this causes extra excitement 

 of the air molecules which come in contact with it, and pressure is 

 produced, causing the fly of the radiometer to turn round. 



I have long believed that a well-known appearance observed in 

 vacuum tubes is closely related to the phenomena of the mean free 

 path of the molecules. When the negative pole is examined while 

 the discharge from an induction-coil is passing through an exhausted 

 tube, a dark space is seen to surround it. This dark space is found 

 to increase and diminish as the vacuum is varied, in the same way 

 that the ideal layer of molecular pressure in the radiometer increases 

 and diminishes. As the one is perceived by the mind's eye to get 

 greater, so the other is seen by the bodily eye to increase in size. If 

 the vacuum is insufiicient to permit the radiometer to turn, the 

 passage of electricity shows that the "dark space" has shrunk to 

 small dimensions. It is a natural inference that the dark space is 

 the mean free path of the molecules of the residual gas. 



The radiometer which has just been turning under the influence 

 of the lime-light is not of the ordinary kind. Fig. 1 will explain its 

 construction. 



It is similar to an ordinary radiometer with aluminium disks for 

 vanes, each disk coated on one side with a film of mica. The fly is 

 supported by a hard steel instead of glass cup, and the needle point 

 on which it works is connected by means of a wire with a platinum 

 terminal sealed into the glass. At the toj) of the radiometer bulb 



