180 THE PRINCIPLES OF SCIENCE. 



predict with certainty a whole class of highly interesting 

 phenomena. To him&quot; especially do we owe the important 

 advance which has been recently made to a new era in 

 the history of science, when the famous philosophical 

 system of Bacon will be to a great extent superseded, 

 and when, instead of arriving at discovery by induction 

 from experiment, we shall obtain our largest accessions of 

 new facts by reasoning deductively from fundamental 

 principles. 



The theory of electricity is a necessary part of the 

 general theory of matter, and is rapidly acquiring the 

 power of prevision. As soon as Wheatstone had proved 

 experimentally that the conduction of electricity occupied 

 time, Faraday remarked in 1838, with wonderful sagacity, 

 that if the conducting wires were connected with the 

 coatings of a large Ley den jar, the rapidity of conduction 

 would be lessened. This prediction remained unverified 

 for sixteen years, until the submarine cable was laid be 

 neath the Channel. A considerable retardation of the 

 electric spark was then detected by Siemens and Latimer 

 Clark, and Faraday at once pointed out that the wire 

 surrounded by water resembles a Leyden jar on a large 

 scale, so that each message sent through the cable verified 

 his remark of 1838. 



The joint relations of heat and electricity to the metals 

 constitute almost a new science of thermo-electricity. Sir 

 W. Thompson was enabled by theory to anticipate the 

 following curious effect, namely, that an electric current 

 passing in an iron bar from a hot to a cold part produces 

 a cooling effect, but in a copper bar the effect is exactly 

 opposite in character, that is the bar becomes heated 1 . 

 The action of crystals with regard to heat and electricity 

 was partly foreseen on the grounds of theory by Poisson. 



c Tyndall s Faraday, pp. 73, 74 ; Life of Faraday, vol. ii. pp. 82, 83. 

 d Tail s Thermodynamics, p. 7 7. 



