ELECTRICITY. 75 



If, again, we examine the electricity of the atmosphere, 

 when, as is usually the case, it is positive with respect to that 

 of the earth, we find that each successive stratum is positive 

 to those below it and negative to those above it ;. and the 

 converse is the case when the electricity of the atmosphere is 

 negative with respect to that of the earth. 



If other electrical phenomena be selected, other changes in 

 the matter affected will be found to have taken place. The 

 electric spark, the brush, and similar phenomena, the old 

 theories regarded as actual emanations of the matter or 

 fluid, Electricity ; I venture to regard them as produced by an 

 emission of the material itself from whence they issue, and 

 a molecular action of the gas, or intermedium, through or 

 across which they are transmitted. 



The colour of the electric spark, or of the voltaic arc (i.e. 

 the flame which plays between the terminal points of a power- 

 ful voltaic battery), is dependent upon the substance of the 

 metal, subject to certain modifications of the intermedium : 

 thus, the electric spark or arc from zinc is blue ; from silver, 

 green ; from iron, red and scintillating ; precisely the colours 

 afforded by these metals in their ordinary combustion. A 

 portion of the metal is also found to be actually transmitted 

 with every electric or voltaic discharge : in the latter case, 

 indeed, where the quantity of matter acted upon is greater 

 than in the former, the metallic particles emitted by the 

 electrodes or terminals can be readily collected, tested, or even 

 weighed. Moreover, the transverse lines in the spectrum of 

 the electric discharge differ for different metals used as ter- 

 minals. It would thus appear that the electrical discharge 

 arises, at least in part, from an actual repulsion and severance 

 of the electrified matter itself, which flies off at the points of 

 least resistance. 



A careful examination of the phenomena attending the 

 electric spark or the voltaic arc, which latter is the electric 

 disruptive discharge acting on greater portions of matter, 

 tends to modify considerably our previous idea of the nature 

 of the electric force as a producer of ignition and combustion. 

 The voltaic arc is perhaps, strictly speaking, neither ignition 

 nor combustion. It is not simply ignition ; because the 



