458 



Prof. G. G. Stokes. 



[Feb. 17, 



special appliances for exalting the faint action to be detected, the 

 above-named phenomena can be prodnced at still higher pressnres. 



It mnst be remembered that we know nothing of the absolute length 

 of the free path or the absolute velocity of a molecnle ; these may vary 

 almost from zero to infinity. We mnst limit ourselves to the mean 

 free path and the mean velocity, and all that these experiments show 

 is that a few molecules can travel more than a hundred times the 

 mean free path, and with perhaps a corresponding increase over the 

 mean velocity, before they are stopped by collisions. With weak elec- 

 trical power the special phosphorogenic action of these few molecules 

 is too faint to be noticed ; but by intensifying the discharge the action 

 of the molecules can be so increased as to render their presence visible. 

 It is also probable that the absolute velocity of the molecules is in- 

 creased so as to make the mean velocity with which they leave the 

 negative pole greater than that of ordinary gaseous molecules. This 

 being the case, they will not easily be stopped or deflected by collisions, 

 bnt will drive through obstacles, and so travel to a greater distance. 



If this view is correct, it does not follow that gas and ultra gas can 

 coexist in the same vessel. All that can be legitimately inferred is, 

 that the two states insensibly merge one into the other, so that at an 

 intermediate point we can by appropriate means exalt either the phe- 

 nomena due to gas or to ultra gas. The same thing occurs between the 

 states of solid and liquid, and liquid and gas. Tresca's experiments 

 on the flow of solids prove that lead and even iron, at the common 

 temperature, possess properties which strictly appertain to liquids,, 

 whilst Andrews has shown that liquid and gas may be made to merge 

 gradually one into the other, so that at an intermediate point the sub- 

 stance partakes of the properties of both states. 



"Note on the Reduction of Mr. Crookes's Experiments on 

 the Decrement of the Arc of Vibration of a Mica Plate Os- 

 cillating within a Bulb containing more or less rarefied 

 Gas/' By Professor G. G. Stokes, Sec. R.S. Received 

 February 17, 1881. 



(Abstract.) 



The determination of the motion of the gas within the bulb, winch 

 wonld theoretically lead to a determination of the coefficient of visco- 

 sity of the gas, forms a mathematical problem of hopeless difficulty. 

 Nevertheless we are able, by attending to the condition of similarity 

 of the motion in different cases, to compare the viscosities of the diffe- 

 rent gases for as many groups of corresponding pressures as we please. 

 Setting aside certain minute corrections, which would have vanished 



