612 THE PRINCIPLES OF SCIENCE. [CHAP. 



extremely minute particles in pure water, acquires an 

 oscillatory movement, often so marked as to resemble dan- 

 cing or skipping. I conceive that this movement is due to 

 the comparatively vast intensity of chemical action when 

 exerted upon minute particles, the effect being 5,000 or 

 10,000 greater in proportion to the mass than in fragments 

 of an inch diameter (p. 406). 



Much that was formerly obscure in the science of elec- 

 tricity arose from the extreme differences of intensity and 

 quantity in which this form of energy manifests itself. 

 Between the brilliant explosive discharge of a thunder-cloud 

 and the gentle continuous current produced by two pieces 

 of metal and some dilute acid, there is no apparent analogy 

 whatever. It was therefore a work of great importance 

 when Faraday demonstrated the identity of the forces in 

 action, showing that common Motional electricity would 

 decompose water like that from the voltaic battery. The 

 relation of the phenomena became plain when he succeeded 

 in showing that it would require 800,000 discharges of his 

 large Leyden battery to decompose one single grain of 

 water. Lightning was now seen to be electricity of ex- 

 cessively high tension, but extremely small quantity, the 

 difference being somewhat analogous to that between the 

 force of one million gallons of water falling through one 

 foot, and one gallon of water falling through one million 

 feet. Faraday estimated that one grain of water acting on 

 four grains of zinc, would yield electricity enough for a 

 great thunderstorm. 



It was long believed that electrical conductors and in- 

 sulators belonged to two opposed classes of substances. 

 Between the inconceivable rapidity with which the current 

 passes through pure copper wire, and the apparently com- 

 plete manner in which it is stopped by a thin partition of 

 gutta-percha or gum-lac, there seemed to be no resem- 

 blance. Faraday again laboured successfully to show that 

 these Were but the extreme cases of a chain of substances 

 varying in all degrees in their powers of conduction. Even 

 the best conductors, such as pure copper or silver, offer 

 resistance to the electric current The other metals have 

 considerably higher powers of resistance, and we pass 

 gradually down through oxides and sulphides. The best 

 insulators, on the other hand, allow of an atomic induction 



