114 Profs. W. B. Ayrton and J. Periy on 



the dimensions of the ball of the radiometer*. Conceivably 

 these assumptions may naturally arise from the idea that in 

 a "vacuum," so termed^ the molecules of gas are few and 

 widely scattered. This, we have shown, is by no means the fact. 

 It may therefore be worth while investigating from the above 

 result of Professor Maxwell what the mean path of the mole- 

 cules of gas is at the degree of rarefaction at which the rota- 

 tion of the radiometer was observed to be at a maximum. This 

 maximum is stated in Mr. Crookes's paper to have occurred 

 in the case of hydrogen at a rarefaction of 50 millionths. 

 The mean path being in direct proportion to the rarefaction, 

 its value in this case will be ^-^^htttt x ^^^^n^^^ = A ^^ ^^ ^"^^ 

 about. This is but a small fraction of the diameter of the bulb. 

 It is remarked that the special gauge used could not indicate 

 the pressure of the mercury vapour of the pump, and that there- 

 fore the actual values for rarefaction attained might have been 

 somewhat under the estimated values. This would go to make 

 the mean length of path, Avhen the rotation was at its maxi- 

 mum, a still smaller fraction of the diameter of the bulb than 

 above ; and it is stated that at higher rarefactions the rota- 

 tion notably began to fall off. The length of path of the mo- 

 lecules of other gases is also less — about half that of hydrogen. 

 My object here is not in any way to deal with the cause of 

 the rotation of the radiometer, but rather to adduce from the 

 best obtainable data certain facts which may tend to throw a 

 light upon the true physical condition of the rarefied gas en- 

 closed in the instrument ; and it is certain that, whatever the 

 true explanation may be, it cannot be otherwise than forwarded 

 by such a course. 

 London, June 1877. 



XYI. Ice as an Electrolyte. By W. E. Ayrton and John 

 Perry^ Professors in the Imperial College of Engineering, 

 Tolcio, Japan^. 



[Plate 11.] 



"T?OR the purpose of measuring the resistance of ice at 

 -*- various temperatures, its power to act as an electrolyte, 

 and its specific inductive capacity, the following piece of ap- 

 paratus was constructed. A B C D (Plate II. fig. 1) is a cop- 

 per box 17*4 centims. in diameter, rigidly fixed by means of 



* " Wir yei'diinnen das Gas so wait, dass der Weg, den ein Moleciil 

 zwisclien zwei Ziisammeustosseu mit anderen Moleciilen dmcliechnittlich 

 zuriicklegt, selir gTOSs ist im Verlialtniss zii den Dimensionen der Kngel 

 des Eadiometers." (Pogg. Ann. July 1876, part 8, p. 579.) 



t Communicated by the Physical Society . 



