rays 



28 HABIT AND INTELLIGENCE. [cHAP. 



are also capable of transformation the one into the other. 

 This relation of quantitative equivalence must indeed neces- 

 sarily exist between all forms of energy, in consequence of 

 the law of the conservation of energy, by which energy is 

 unalterable in qiiantity through all its transformations : 

 it may be transformed, but its quantity can be neither 

 increased nor diminished. 

 Radiation One of the most remarkable properties of heat is its 

 of heat, property of transferring itself from one body to another, 

 across vacant space, by radiation. But when heat assumes 

 the radiant form, as it does, for instance, when on its way 

 from the sun to the earth, it obeys laws totally different from 

 those which are properly the laws of heat — it obeys laws 

 which are, so far as experiment can inform us, perfectly 

 identical with those of light — it becomes capable of reflec- 

 Radiant tion, refraction, and polarization. Eadiant heat is thus a 

 and\he ' distinct form of energy from heat of temperature, or what 

 actinic is properly called heat ; while, on the other hand, it differs 

 from light only as two differently coloured rays of light 

 differ from each other. And the same is true of the invisible 

 chemical rays of the solar spectrum, or what have been called 

 the actinic rays ; they differ from light or from radiant heat 

 only as two differently coloured rays of light differ from 



energy ; for the quantity of heat, or of any other form of energy into which 

 the electricity of an electrised body is capable of being transformed, is not 

 simply proportional to what electricians call the quantity of the electricity 

 wherewith it is charged, but to the quantity of the electricity multiplied 

 into its tension : or, what comes to the same thing, supposing the extent 

 of electrised surface to be given, the quantity of heat into which the elec- 

 tricity is capable of being transformed is proportional, not to the quantity, 

 but to the square of the quantity. This may be compared with the law 

 that, supposing the mass of a moving body to be given, the energy due to 

 its motion is proportionate, not to the velocity, but to the square of the 

 velocity. (See p. 24.) 



The definition of electric intensity and of quantity of electricity is, 

 that if an electrised ball of metal is brought into contact with an unelec- 

 trised one of the same size, the electricity will spread over both ; and 

 when it has so spread, its quantity is said to be unchanged, but its intensity 

 reduced to one-half. In this diminution of intensity there is a trans- 

 formation of energy into heat. (See De La Rive's Electricitv% English 

 translation, vol. ii. p. 219, et seq.) 



