XXIV.] EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 543 



pressure of 1 300 atmospheres, water did not freeze uutil 

 cooled down to— i8°C. Another remarkable prediction 

 of Professor Thomson was to the effect that, if a metallic 

 spring be weakened by a rise of temperature, work done 

 auainst the spring in bending it will cause a cooling effect. 

 Although the eflect to be expected in a certain apparatus 

 was only about four-thousandths of a degree Centigrade, 

 Dr. Joule ^ succeeded in measuring it to the extent of three- 

 thousandths of a degree, such is the delicacy of modern 

 heat measurements. I cannot refrain from quotingDr. Joule's 

 reflections upon this fact. " Thus even in the above de- 

 licate case," he says, " is the formula of Professor Thomson 

 completely verified. The mathematical investigation of the 

 thermo-elastic qualities of metals has enabled my illustrious 

 friend to predict with certainty a whole class of highly in- 

 teresting phenomena. To him 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 system of Bacon will be to a great extent superseded, 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 previ.sion. As soon as Wheatstone had proved 

 experimentally that the conduction of electricity occupies 

 time, Faraday remarked in 1838, with wonderful sagacity, 

 that if the conducting wires were connected with the 

 coatings of a large Leyden jar, the rapidity of conduction 

 would be lessened. This prediction remained unverified 

 for si.xteen years, until the submarine cable was laid be- 

 neath the Channel. A considerable retardation of the 

 electric spark was then detected, and Faraday at once 

 pointed out that the wire surrounded by water resendjles 

 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 a new science of thermo-electricity by which 



' Pliiioi^nphical Trnnsndiovs, 1858, vol. cxlviii. p. 127. 



* Tyiidall'.s Faraday, i^y). 73, 74 ; Life 0/ Faraday, \o\. ii. pp. 82, 83. 



