DISCOVERY BY EXAMINING EXCEPTIONAL CASES. 499 



contain ; there are also several well-known exceptions to 

 the general statements, that bodies expand when heated, 

 and that a solid body is more freely soluble in a hot liquid 

 than in a cold one. 



In some cases a simple and apparently unimportant 

 exceptional fact completely invalidates a very general 

 theory ; for instance, the long-established doctrine that 

 expansion is a direct result of rise of temperature is com- 

 pletely disproved by the fact that iodide of silver, whilst 

 in the solid crystalline state, and being cooled from 1 6' C. 

 to 18- C., enlarges in volume from 1000 to about 1018. 1 

 Expansion by heat, therefore, is not a direct effect of the 

 heat, but of a molecular change, produced by the heat. 



According to the principle of the dissipation of energy, 

 discovered by Sir William Thomson, in the year 1852, 

 whenever one form of energy or force is converted into 

 another there is always more or less of the power con- 

 verted into the form of heat, which becomes dissipated 

 and uniformly distributed, and thus rendered unavailable 

 for further production of power. In this way all kinds of 

 energy in the universe are being gradually rendered in- 

 capable of producing mechanical effect or motion. Accord- 

 ing to this theory, no known process of nature is exactly 

 and completely reversible ; and no form of energy can, 

 after having been converted into its equivalent of another 

 form, be reconverted into the original amount of the pri- 

 mitive kind of energy. Assuming this theory to be true, 

 it is an exception to the general principle of activity of 

 nature. 



The facts that carbon, crystalline selenium, and tellu- 

 rium conduct electricity better when heated, 2 are also 

 exceptional cases to the general statement, that ele* 



1 Rodwell, Proceedings of the Royal Society, 1874, No. 157, p. 107. 



2 Dr. Siemens, Telegraphic Journal, Sept. 1, 1876, p. 238. 



K K 2 



