184 THE PRINCIPLES OF SCIENCE. 



solidifies and rises in temperature to 32 Fahr. A like 

 effect is still more beautifully displayed in the well known 

 lecture-room experiment, of the suspended crystallization 

 of a solution of sodium sulphate, in which a sudden rise 

 of temperature of 30 or even 40 Fahr. is often manifested. 



The science of electricity is full of the most varied and 

 interesting cases of inversion. As Professor Tyndall has 

 remarked, Faraday had a profound belief in the reciprocal 

 relations of the physical forces. The great starting-point 

 of his researches, the discovery of electro-magnetism, was 

 clearly an inversion. 



Oersted and Ampere had proved that with an electric 

 current and a magnet in a particular position as ante- 

 cedents, motion is the consequent. If then a magnet, a 

 wire and motion be the antecedents, an opposite electric 

 current will be the consequent. It would be an endless 

 task to trace out the results of this fertile relationship 

 when once fully understood. No small part of Faraday's 

 researches was occupied in ascertaining the direct and 

 inverse relations of magnetic and diamagnetic, amorphous 

 and crystalline substances in various circumstances. In 

 all other relations of electricity the principle of inversion 

 holds. The voltameter or the electro-plating cell is the 

 inverse of the galvanic battery. As heat applied to a 

 junction of antimony and bismuth bars produces electricity, 

 it necessarily follows that an electric current passed 

 through such a junction will produce cold. Thus it is 

 apparent that inversion of cause and effect is a most 

 fertile ground of prediction and discovery. 



The reader should carefully notice, however, that the 

 inversion of natural phenomena is exactly true only of 

 the character of the effect, not the amount. There is 

 always a waste of energy in every work, because a certain 

 part of it is dissipated in the form of conducted or radiated 

 heat, and escapes beyond our use. Theoretically speaking, 



