r 



29 



may still be an intermittent phenomenon ; but all that 

 is known of the period of such intermittence is, 

 that it must recur at exceedingly short intervals. 

 And in connexion with this subject, it may be 

 added that, whatever be' the ultimate explanation of 

 the strange stratification which the voltaic discharge 

 undergoes in rarefied gases, it is clear that the alternate 

 disposition of light and darkness must be dependent on 

 some periodic distribution in space or sequence in time 

 which can at present be dealt with only in a very general 

 way. In the exhausted column we have a vehicle for 

 electricity not constant like an ordinary conductor, but 

 itself modified by the passage of the discharge, and 

 perhaps subject to laws differing materially from those 

 which it obeys at atmospheric pressure. It may also be 

 that some of the features accompanying stratification form 

 a magnified image of phenomena belonging to disruptive 

 discharges in general ; and that consequently so far from 

 expecting among the known facts of the latter any clue 

 to an explanation of the former, we must hope ultimately 

 to find in the former an elucidation of what is at present 

 obscure in the latter. A prudent philosopher usually 

 avoids hazarding any forecast of the practical application 

 of a purely scientific research. But it would seem that 

 the configuration of these stri^ might some day prove 

 a very delicate means of estimating low pressures, and 

 perhaps also for effecting some electrical measurements. 



Now, it is a curious fact that almost the only small 

 quantities of which we have as yet any actual measure- 

 ments are the wave lengths of light ; and that all others, 

 excepting so far as they can be deduced from these, 

 await future determination. In the meantime, when 

 unable to approach these small quantities individually, 

 the method to which we are obliged to have recourse 



