II.] 



TRANSFORMATIONS OF ENERGY. 29 



each other. ^ Light, radiant heat, and the actinic rays, all 

 beyond any reasonable doubt consist of vibrations in a 

 medium that fills all space. They are consequently to be 

 classed as one and the same form of energy, which I classed 



m r i. i-i 4- 1 together as 



intend in future to call radiance. ine tact tliat only radiance, 

 some of the rays of the sun's radiance are luminous 

 depends not so much on their own nature as on the 

 properties of the nervous apparatus by which we see ; 

 and it is not at all improbable that to some animals rays 

 may be luminous — that is to say, may give the sensation 

 of light — which are obscure to us.^ 



When radiance is absorbed, it is transformed back into Absorp- 



_ - ) T tiou of 



heat ; as, for mstance, when the sim s radiance warms an i-adiance. 

 object on which it falls. Eadiation is the transformation 

 of heat into radiance ; absorption is the transformation of 

 radiance back into heat. 



If radiance is a form of energy, concerning which there 

 can be no doubt whatever, every ray must have some 

 energy. The energy of the luminous and actinic rays, how- 

 ever, when separated from the obscure ones, is very small, 

 as shown by their very small heating power. The rays of 

 the moon are usually believed to have no heating power at 

 all, but Professor Piazzi Smith found that the heat of the Heating 

 moonbeams on the Peak of Teneriffe, as measured by the ^°^l''^_^ ° 

 thermo-electric multiplier, are equal in heating power to a beams. 

 candle at a distance of twenty-six feet.^ 



I have now enumerated the principal kinds of actual 

 energy, namely, Energy of Motion, Heat, Electricity, and 



1 This is not quite accurate, for all rays are heating rays : if it were not 

 so, there would be radiance incapable of transformation into heat, and we 

 know that all energy is capable of transformation into heat. But the 

 maxima of heating power, of illuminating power, and of chemical power, 

 occur in different rays . 



2 It has been suggested that the eyes of cats and other nocturnal animals 

 are more sensitive than ours to the highly refi-angible rays which abound 

 in twilight. 



3 Piazzi Smith's Teneriffe, p. 212. Piazzi Smith found that when the 

 moon was at an altitude of about 42° and the weather perfectly serene, its 

 heating power was equal to about a third of that of a candle fifteen feet off. 

 By the law of the inverse square, the heating power of a candle at fifteen 

 feet is three times what it is at twenty-six feet. 



