232 GENEEAL CONDITIONS OF SCIENTIFIC KESEAKCH. 



years, until the submarine cable was laid beneath 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 Ley den jar on a large scale, so that 

 each message sent through the cable verified his remark 

 of 1838.' 'Sir W. Thomson 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.' ' The existence of the metals potassium and sodium 

 was foreseen by Lavoisier, and their elimination by Davy 

 was one of the chief experimented crucis which established 

 Lavoisier's system. The existence of many other metals 

 which eye had never seen was almost a necessary inference, 

 and theory has not been found at fault. No sooner, too, 

 had a theory of organic compounds been conceived by 

 Professor A. W. Williamson than he foretold the formation 

 of a complex substance consisting of water in which both 

 atoms of hydrogen are replaced by atoms of acetyle. This 

 substance, known as the acetic anhydride, was afterwards 

 produced by Grerhardt. In the subsequent progress of 

 organic chemistry, occurrences of this kind have been 

 multiplied almost indefinitely.' 1 



Eecently, also, Sir William Thomson has predicted 

 that in the phenomena of ' electro-torsion ' of iron wire, 

 < if the wire, rod, or tube experimented upon be stretched 

 by a heavy weight, then no doubt, the torsions, as well as 

 the elongations, under varying magnetic influences, will 

 be the reverse of those discovered.' 2 Mendlejeef has 

 also predicted the existence of several new elementary 



1 Jevons, Principles of Science, vol. ii. pp. 180, 181. 



2 Philosophical Transactions of the lloyal Society, 1874, p. 562. 



