Mechanical and Galvanic Electricity. 213 



derstood in order to be admitted" Notwithstanding that in 

 support of the opinion thus quoted, the much-respected au- 

 thority of both Cavendish and Faraday is arrayed, it is one 

 which I cannot so understand as to admit. 



I am unable to form any other idea of intensity, than that 

 of the ratio of quantity to space. Thus the intensity of the 

 pressure of an elastic fluid, is as the quantity to tlie space in 

 which it is confined. The space being the same, the intensity 

 of the pressure will be directly as the quantity ; the quantity 

 being constant, the pressure will be inversely as the space. 



Agreeably to an analogous mode of reasoning, the intensity 

 of the light or the heat emanating from a radiant bod}', is al- 

 ways estimated to be inversely as the surfaces on which it may 

 be diffused or concentrated ; and hence the inference that the 

 intensity is as the square of the distance inversely, or as the 

 area of the receiving surface of a lens or mirror, to that of 

 the focus into which the rays are collected. 



It follows that if there be in any two cases, like quantities 

 of electricity evolved by mechanical, and by galvanic appa- 

 ratus, the space occupied by the fluid generated by the latter, 

 must be as much greater, as its intensity is less. 



In a memoir which I published upon this subject some 

 years ago, I endeavoured to show that the spaces occupied by 

 equivalent charges of galvanic and mechanical electricity were 

 not such as to justify the idea that the former required for its 

 existence a larger space than the latter. But on this subject, 

 it is not now necessary to recur to the facts which I then ad- 

 duced, since I find it conceded in one of the recent Memoirs 

 of Farada}', that the spaces occupied by the electricity evolved 

 by galvanic apparatus, as compared with those occupied by 

 mechanical electricity, are almost infinitely small. " A grain 

 of water or of zinc, contains as imich of the electric fuid as 

 •woidd supply eight hundred thotisand charges of a batteiy con- 

 taining a coated surface of fifteen himdred square inches" 

 " Four grains of zinc, with one of xvater, may yield as much 

 electricity as is evolved during a thunder storm." 



It follows inevitably that the electric matter evolved by 

 galvanic action is, previous to its evolution, in a state almost 

 infinitely intense, as compared with that of the same matter, 

 when evolved by a machine or meteorological changes. Yet 

 to the currents induced in this matter, in the first-mentioned 

 form, an opposite state is ascribed, as respects intensity, to 

 that in which it has previously existed. 



It may be said with respect to currents, that the space being 

 tlie same, the intensity will not only be directly as the quantity, 

 but also inversely us the time in which it passes, or, in other 



