THE RADIOMETER. i 7 



different surfaces exposed to one and the same medium? and jour 

 question is a perfectly legitimate one ; for it is just here that the new 

 phenomena seem to belie all our previous experience. If, however, you 

 followed me in my very partial exposition of the mechanical theory of 

 gases, you will easily see that on this theory it is a more difficult ques- 

 tion to explain why such a difference of pressure does not manifest 

 itself in every gas medium and under all conditions between any two 

 surfaces having different temperatures. 



"We saw that gas-pressure is a double effect, caused both by the 

 impact of molecules and by the recoil of the surface attending their 

 rebound. We also saw that when molecules strike a heated surface 

 they rebound with increased velocity, and hence produce an increased 

 pressure against the surface, the greater the higher the temperature. 

 According to this theory, then, we should expect to find the same at- 

 mosphere pressing unequally on equal surfaces if at different tempera- 

 tures ; and the difference in the pressure on the lampblack and mica 

 surfaces of the vanes, which the motion of the radiometer-wheel neces- 

 sarily implies, is therefore simply the normal effect of the mechanical 

 condition of every gas medium. The real difficulty is, to explain why 

 we must exhaust the air so perfectly before the effect manifests itself. 



The new theory is equal to the emergency. As has been already 

 pointed out, in the ordinary state of the air the amplitude of the mo- 

 lecular motion is exceedingly small, not over a few ten-millionths of an 

 inch a very small fraction, therefore, of the height of the inequalities 

 on the lampblack surfaces of the vanes of a radiometer. Under such 

 circumstances, evidently the molecules would not leave the heated sur- 

 face, but simply bound back and forth between the vanes and the sur- 

 rounding mass of dense air, which, being almost absolutely a non- 

 conductor of heat, must act essentially like an elastic solid wall con- 

 fining the vanes on either side. For the time being, and until replaced 

 by convection-currents, the oscillating molecules are as much a part of 

 the vanes as our atmosphere is a part of the earth ; and on this sys- 

 tem, as a whole, the homogeneous dense air which surrounds it must 

 press equally from all directions. In proportion, however, as the air is 

 exhausted, the molecules find more room and the amplitude of the mo- 

 lecular motion is increased, and when a very high degree of exhaustion 

 is reached the air-particles no longer bound back and forth on the 

 vanes without change of condition, but they either bound off entirely 

 like a ball from a cannon, or else, having transferred a portion of their 

 momentum, return with diminished velocity, and in either case the force 

 of the reaction is felt. 1 



1 The reader will, of course, distinguish between the differential action on the opposite 

 faces of the vanes of the radiometer and the reaction between the vanes and the glass 

 which are the heater and the cooler of the little engine. Nor will it be necessary to re- 

 mind any student that a popular view of such a complex subject must be necessarily par- 

 tial. In the present case we not only meet with the usual difficulties in this respect, but, 

 VOL. xiii. 2 



