436 Mr. Spottiswoode on Electricity in Transitu. [May 21, 



luminosity, or stria, and of a blank space — not necessarily identical, 

 although correlative quantities ; they suggest that the brilliancy of 

 light, with so little attendant heat, may be due not only to the 

 slight density of the medium, but also to its brevity of duration; 

 they suggest that, for action of such rapidity as that of individual 

 discharges, the mobility of the medium may count as nothing, and 

 that for these infinitesimal periods of time gas may itself be as rigid 

 and as brittle as glass. Time is an element in all mechanical action ; 

 and the converse of such brittleness is not unknown in experiments 

 where substances, for all practical purposes hard and unconformable, 

 have under the long-continued action of gravity, or of even moderate 

 pressure, proved viscous and self-adapting in form. 



" Quid magis est durum saxo, quid moUius unda ? 

 Dura tamen moUi saxa cavautur aqua." 



[W. S.] 



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