126 Mr. W. L. Upson : Observations 



apparent. It is o£ no avail to say that the disturbances from 

 without are very small, because a very small impulse may 

 change the direction, and because the disturbances continue 

 acting- for an immense time. In the H theorem, when H is 

 near its minimum, say H = H + t i', where x is very small, 



—j— is a small quantity of the second order, compared with 



x ; so that in the cycle, if there be a cycle, H will remain 

 near its minimum for very long periods of time. The same 



is true of K and — =— in the diffusion theory. 



dt J 



The effect of disturbances coming from without is, as I 

 pointed out in ' Nature ' for November 1894, always to main- 

 tain or to renewpro tanto the state — "molecular ungeordnet, 5 ' 

 whatever that may mean — which is assumed to exist at every 



instant, as the necessarv basis of proof that — r- , or — — , is 



r at 7 dt 



negative. They must therefore be sufficient to destroy the 

 possible cyclic motion. In that sense the diffusion of gases 

 will, in any experiments that can practically be made, be 

 found to be irreversible. 



VII. Observations on the Electric Arc. 

 By Walter L. Upson, E.E., M.S., Princeton*. 



[Plates VI. & VII.] 



A LTHOUGrH numerous investigators have studied the 

 J^JL. phenomena of the electric arc in air, between carbon 

 terminals, and also between metal terminals, yet the use of 

 the electric arc in the production of electric oscillations raises 

 some new questions with regard to it. 



While I was working in the Pender Electrical Laboratory 

 of University College, London, during the present session, 

 Prof. J. A. Fleming, F.R.S., suggested therefore to me 

 that it would be interesting to make a further examination 

 of the arc between metal and carbon terminals, in air and 

 hydrogen. 



I am indebted to him for the facilities for carrying out the 

 work, and also for many suggestions during its progress. 

 The following apparatus was first constructed with the kind 

 assistance of Prof. W. C. Clinton, B.Sc. 



* Communicated by the Physical Society : read June 14, 1907. 



