Properties of Matter in the Gaseous State, 109 



In addition to this, Prof. Reynolds has not noticed the 

 action of the tangential stresses in the case of the radiometer. 

 That they play an important part in the phenomena is evident. 

 For example, in the case of a small plane plate, hot on one 

 side and cold on the other, represented in fig. 12, p. 836 of 

 Prof. Reynolds's paper, if it were surrounded by a properly 

 shaped and heated envelope, as far as Prof. Reynolds shows, 

 the pressures all over the envelope might be the same; and so 

 the envelope would have no tendency to move, owing to them, 

 while the pressures on the plate would tend to move it. Such 

 a state of affairs is manifestly impossible; so that it is evident 

 that the tangential forces must be of importance. Maxwell, 

 indeed, goes so far as to assert that if the whole stress were 

 taken into account, and if there were no discontinuity near 

 the surface of solids, then no motion would take place. This 

 seems very likely, but is entirely opposed to the drift of this 

 part of Prof. Reynolds's explanation. On p. 840 Prof. Rey- 

 nolds italicizes the assertion, The action of the radiometer does 

 not depend on the distance betiveen the hot and cold plates. Yet, 

 only four pages before, he asserts that these pressures do de- 

 pend on the quantity of heat passing; and this even he will 

 acknowledge depends on the distance between the hot and cold 

 plates. He goes on to say that no action will take place, how- 

 ever close the plates are placed, if they be not of limited ex- 

 tent. If he means by no action no currents in the gas, he is 

 certainly right; but if he means that there will be no inequali- 

 ties of pressure, I think I have shown that this is contrary to 

 the results of all his own equations, and was due to his over- 

 looking the possibility of any stress besides the kind of one he 

 happened to be considering. 



Though Prof. Reynolds does not seem to agree with Max- 

 well that there would be no tendency to motion unless the 

 discontinuity at the surface of solids produced currents of gas, 

 yet his discussion of the motion of the radiometer seems almost 

 to imply this. In conclusion, then, I think that Prof. Rey- 

 nolds has done invaluable service to science by calling atten- 

 tion to the vital importance of taking account of the disconti- 

 nuity at the surface of contact of the solid and the gas, and to 

 the interesting phenomena that occur in consequence. Though 

 I very much prefer Maxwell's method of calculation, as being 

 capable of much greater extension and as savouring much less 

 of assumption, yet Prof. Reynolds's has the great advantage 

 that it is continually reminding one of the necessity for con- 

 sidering the discontinuity, which even Maxwell overlooked 

 until reminded of its importance by Prof. Reynolds's able 

 investigation. 



40 Trinity College, Dublin, 

 January 3, 1881. 



