ELECTRIC LIGHTING. 295 



or mixed fluid of the body, attracting the negative fluid and 

 repelling the positive. When the tension of the opposite 

 electricities is great enough to overcome the resistance of 

 the air, they re-combine, the spark resulting from the heat 

 generated in the process of their combination.' This 

 explanation is all very well ; but it assumes much that is 

 in reality by no means certain, or even likely. All we know 

 is, that whereas before the spark is seen the electrical con- 

 ditions of the conductor and the object presented to it were 

 different, they are no longer different after the flashing forth of 

 the spark. It is as though a certain line (straight, crooked, 

 or branched) in the air had formed a channel of communica- 

 tion by which electricity had passed, either from the con- 

 ductor to the object or from the object to the conductor, 

 or possibly in both directions, two different kinds of elec- 

 tricity existing (before the flash) in the conductor and the 

 object, as the old-fashioned explanation assumes. 1 Again, 

 we know that the passage of electricity along the air-track, 

 supposing there really is such a passage, but in any case the 

 observed change in the relative electrical conditions of the 

 conductor and the object, is accompanied by the generation 



1 It is supposed by many, that when the spark is long enough we 

 can note the direction in which it travels ; and observations of the 

 motion of lightning from the earth to the cloud have been collected, 

 as showing that the usually observed direction of the flash is sometimes 

 reversed. In reality, no one has ever seen a lightning flash travel 

 either one way or the other. If the attention is fixed on the storm 

 cloud, as usual when a lightning storm is watched, every flash appears 

 to pass from the cloud to the earth. If, on the contrary, at the 

 moment when the attention is fixed on some terrestrial object the 

 lightning flashes near that particular object, the flash will seem to pass 

 from the object to the cloud. In either case the motion is apparent 

 only. If there is motion at all, the passage of the electric spark 

 occupies less than the ioo,oooth part of a second, and of course it is 

 utterly impossible that any eye could tell at which end of its track the 

 flash first appeared. In every case the flash seems to travel from the 

 end to which attention was more nearly directed. The apparent mo- 

 tion corresponds to the chance direction of the eye. 



