PHYSICS, EIGHTEENTH CENT. ELECTRICITY. 329 



agreed to so reverse them. There is the same confusion in the mean- 

 ing of these terms on account of Franklin's excess-and-defect-of-single- 

 rluid theory. Be it observed that Franklin does not admit a positive 

 electricity and& negative electricity, although he speaks of bodies being 

 positively or negatively electrified. While these Franklinian phrases may 

 be used when we are expressing the facts in terms of the two-fluU 

 theory,- we cannot, consistently with Franklin's theory, speak of the 

 passage of negative electricity, etc. 



One other observation with regard to electrical theories. If there 

 is anything known at all in the present day concerning the nature of 

 electricity, it is the certainty that electricity is NOT a fluid. This is a 

 mere fiction which is consciously adopted, at least by scientific men, in 

 order to assist the conception or description of the phenomena. The 

 reason why electricity should be pictured as a fluid, while in the case 

 of gravitation no such notion is required, will appear hereafter. The 

 popular fallacies on the subject arise from the general tendency to 

 assume the expression of a fact as its explanation, and to confound a 

 phrase with a principle. As in the subsequent description of electrical 

 discoveries, the expressions " electric fluid" "flow of electricity" etc., 

 etc., may often occur, the reader will do well to constantly bear in 

 mind that these expressions are nothing more than expedients to avoid 

 prolixity. The cause or real nature of electricity is absolutely un- 

 known. We are completely ignorant of what it is that acts in these 

 phenomena, but we know something of how it acts. 



The second claim to a distinguished place for Franklin in the history 

 of electricity rests on his demonstration of the identity of electricity 

 and lightning. Not that Franklin was by any means the first person 

 to whose mind the idea occurred. The resemblances between light- 

 ning and the electric spark were too obvious to escape notice, and 

 even in the seventeenth century these resemblances were explicitly 

 mentioned by more than one author. The ABBE NOLLET in France 

 had stated several reasons for believing that thunder and lightning were 

 the electricity of nature. Franklin puts forward the same view hypo- 

 thetically in a letter dated November yth, 1749, and he enumerates 

 all the points in which electricity and lightning were known to resemble 

 each other. He notices the irregular and angular course of the light- 

 ning-flash, and observes that long electrical sparks sometimes exhibit 

 the same appearance. Lightning strikes elevated and pointed objects, 

 such as trees, towers, steeples, masts of vessels, etc. ; and he declares 

 that pointed conductors are more affected by electricity than smooth 

 -surfaces. Lightning- inflames combustible substances ; electricity can 

 do the same. Lightning sometimes fuses metals ; electricity can be 

 made to do the same. The similarity of lightning and the discharge 

 of the Leyden jar is seen in such effects as killing of animals, rending 

 of certain substances, reversal of the poles of magnetic needles, etc. 

 That the effects of lightning should be so much more powerful than 



