8 POPULAR SCIENCE MONTHLY. 



We are thus confronted by the idea, long ago suggested by Weber 

 and by Helmholtz, that the agency we call electricity is atomic in 

 structure, that is to say, we can only have it in amounts which are all 

 exact multiples of a certain small unit. Electricity therefore re- 

 sembles those articles of commerce like cigars, which we can buy in 

 exact numbers, 1, 10, 50, 100, 1000, but we cannot buy half a cigar or 

 five sixths of a cigar. If then the law which holds good for electricity 

 in association with atoms during electrolysis holds good generally, a 

 very important advance has been made in establishing the fact that 

 there is a small indivisible unit of it which can be multiplied but not 

 divided, and every quantity of electricity, small or large, is an exact 

 integer multiple of this unit, the electron. 



Theories of Electricity. 



Various answers have been given at different times to the question 

 — What is electricity? It has been defined as an imponderable fluid, 

 as a force, as a mode of motion, a form of energy, an ether strain or 

 displacement or a molecular motion. 



At one time physicists have considered it as a single entity or fluid ; 

 at others it has been pronounced to be duplex in nature and positive and 

 negative fluids or electricities have been hypothecated. 



The state of electrification has been looked upon at one period as 

 due to an excess or defect of a single electricity, at others as a conse- 

 quence of the resolution of some neutral fluid into two components. 

 An electrical charge on a conductor has been regarded as something 

 given to or put upon the conductor and also as a state of strain or 

 displacement in the surrounding non-conductor. The intelligent but 

 non-scientific inquirer is often disappointed when he finds no simple, 

 and as he thinks essential, answer forthcoming to the above question, 

 and he asks why it cannot be furnished. 



We must bear in mind, however, that scientific hypotheses as to 

 the underlying causes of phenomena are subject to the law of evolution 

 and have their birth, maturity and decay. Theory necessarily suc- 

 ceeds theory, and whilst no one hypothesis can be looked upon as ex- 

 pressing the whole truth, neither is any likely to be destitute of all 

 truth if it sufficiently reconciles a large number of observed facts. 



The notion that we can reach an absolutely exact and ultimate ex- 

 planation of any group of physical effects is a fallacious idea. We 

 must ever be content with the best attainable sufficient hypothesis that 

 can at any time be framed to include the whole of the observations 

 under our notice. Hence the question, What is electricity? no more 

 admits of a complete and final answer to-day than does the question, 

 What is Life? Though this idea may seem discouraging, it does 

 not follow that the trend of scientific thought is not in the right 



