1880.] Mr. Spottiswoode on Elcdricitij in Traimtu. 427 



WEEKLY EVENING MEETING, 



Friday, May 21, 1880. 



The Duke of Northumberland, D.C.L. LL.D. President, 

 in the Chair. 



William Spottiswoode, Esq. D.C.L. M.A. LL.D. Pres.K.S. MM.L &c. 



Electricity in Transitu. 



The subject which I have proposed for this evening's discourse does 

 not offer the wide perspective of modern investigation opened out by 

 that of Professor Huxley, nor can it claim the manifold and varied 

 sympathies evoked by the lectures which have followed on the suc- 

 ceeding Fridays. It belongs rather to the region of minute philo- 

 sophy, and will on that account perhaps require more than usual 

 patience and attention. Following the lines of a research on which 

 Mr. Moulton and myself have been for some time engaged, I hope to 

 extend by one or two steps our knowledge of the internal mechanism 

 of that complicated, and still somewhat mysterious subject, the Elec- 

 tric Discharge. And in so doing I must leave aside, or at least only 

 incidentally touch upon, many collateral points of interest which have 

 presented themselves in our inquiry ; because my main object will 

 be ultimately to bring ourselves face to face with those important 

 elements which we have called the small time-quantities of the phe- 

 nomenon ; that is, the times during which the different parts of the 

 discharge are effected. These quantities are, however, so transient 

 in duration, so evanescent in magnitude, that they elude all direct 

 observation even with our most delicate instruments ; and therefore, 

 abandoning all attempts at absolute measurement, we have endea- 

 voured, as it were, to lay wait for them as they pass, and, catching up 

 any waif or stray indication that they may leave behind in transitu^ to 

 form such relative estimate of the quantities in question as may prove 

 possible. 



It is well known that when an electrical discharge is effected in 

 air or other gas at atmospheric pressure, it passes in an irregular 

 bright line or spark. If the discharge be made in a closed tube, and 

 the tube be gradually exhausted, the discharge becomes thicker as the 

 exhaustion proceeds, until it completely fills the tube with light. 

 During the process of exhaustion, the discharge, when effected in a 

 suitable manner, exhibits the phenomena of stratification in its various 

 phases ; while at the same time a very marked dissymmetry between 



