ELECTRIC LIGHTING. 291 



retains the sensation of light for at least this interval after 

 the light has disappeared. It is equally certain, from Wheat- 

 stone's experiments, that the lightning flash does not actually 

 endure for the ioo,oooth part of a second. Adopting this 

 last number, though it falls far short of the truth the actual 

 duration being probably less than i,ooo,oooth of a second 

 we see that so far as the eye is concerned, an amount of 

 light which was really emitted during the ioo,oooth part of a 

 second is by the eye judged to have been emitted during 

 an interval 4,000 times as long. It is certain, then, that the 

 eye's estimate of the intensity of the illumination resulting 

 from a lightning flash is far short of the truth. This is 

 equally true even in those cases where lightning is said to 

 be for awhile continuous. If the flashes for a time succeed 

 each other at less intervals than a twenty-fifth of a second, 

 the illumination will appear continuous. But it is not 

 really so. To be so, the flashes should succeed each other 

 at the rate of at least 100,000, and probably of more than 

 1,000,000 per second. 



While the lightning flash shows the brilliancy which the 

 electric illumination can attain, it shows also the intense 

 heat resulting from the electric discharge. This might, 

 indeed, be inferred simply from the brilliancy of the light, 

 since we know that this brilliancy can only be due to the 

 intense heat to which the particles along the track of the 

 electric flash have been raised. But it is shown in a more 

 convincing manner to ordinary apprehension by the effects 

 which the lightning flash produces where in the common 

 way of speaking it strikes. The least fusible substances 

 are melted. Effects are produced also which, though at 

 first not seemingly attributable to intense heat, yet in reality 

 can be no otherwise explained. Thus, when the trunk of a 

 tree is torn into fragments by the lightning stroke, though 

 the tree is scorched and blackened, a small amount of heat 

 would account for that particular effect, while the destruc- 

 tion of the tree seems attributable to mechanical causes. It 



is, indeed, from effects such as these that the idea of the 



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