238 



rent evidence, will any one be still bold enough to assert that the 

 conservation of energy is a myth ? 



52. Examples without number might be adduced of the con- 

 servation of energy, in which the equivalence has not yet been, but 

 probably ere long will be, determined quantitatively; but a few 

 must suffice. Whenever resistance is offered to the passage of 

 an electric current, heat is generated in proportion to the 

 resistance in the circuit, and the heat is evolved at the expense 

 of current, that is to say, there is a transformation of electric 

 into thermic energy ; and this may exist in any degree, from the 

 least perceptible elevation of temperature in the conductor, to 

 its actual deflagration and volatilisation, as in the carbon points of 

 an electric lamp, or the deflagration of gold-leaf by the discharge 

 of a Leyden battery. And it has elsewhere been shown by the 

 writer"^ that under suitable conditions the converse transforma- 

 tion of heat into electricity takes place. If the adjacent ends 

 of a bar of antimony {a) and one of bismuth (b) be soldered 

 together, it has long been known that when a sufficiently weak 

 current of electricity is transmitted through this thermo-electric 

 element passing from a to b, heat is produced at the point of 

 junction, but if passing in the direction from b to a, cold is pro- 

 duced ; but when the element is placed in a Wheatstone's 

 bridge, the galvanometer shows a loss of current when heat is 

 gained, and a gain of current when heat is lost. This leads 

 irresistibly to the conclusion that an interchange of thermic and 

 electric energy takes place at the point of junction of the two 

 metals. It may be observed that bismuth presents this property 

 in a higher degree than any other known substance ; and it is 

 altogether a remarkable metal, excelling also in the property of 

 diamagnetism, and sharing with water the property of expanding 

 on passing from the fluid to the solid state by cooling. 



53. The dynamo-electric machine is another conspicuous 

 example. While at rest it manifests no properties either of 

 electricity or magnetism, but when kinetic energy produced by the 

 muscular force of the arm is expended in turning the winch of the 

 machine, magnetism is produced, and the electro-magnet becomes 

 active; this again induces an electric current in the revolving 

 armature, which in its turn becomes light and heat in a platinum 

 wire, through which it may be transmitted ; or if employed in 

 doing any mechanical work, it becomes kinetic energy. 



54. Lastly, the sense of vision may be quoted as a highly 

 probable example of the conservation of energy, it being not 

 inconsistent with any known fact to suppose that the action of 

 light on the retina is a true photographic process, not per- 



* L. E. and D. PhAl. Mag., vol. xxxii., p. 378. 



