Prof. Minchin's Experiments in Photoelectricity. 237 



according to Violle. Converting these into foot-pounds' 

 weight per square foot per second, we have 



63*42 foot-pounds' weight per sq. foot per second (Pouillet), 

 91-35 „ „ „ „ (Violle). 



The quantity of solar energy existing at any instant in 

 one cubic foot of space at the surface of the earth is, of course, 

 obtained by dividing these numbers by the velocity of the 

 radiations in feet per second (roughly 10 9 ), and this quantity is 

 infinitesimally small. Perhaps it is the infinitesimal value of 

 the energy in a cubic foot which impresses people so strongly 

 with the insufficiency of the solar radiation for doing work ; 

 but it is obvious that with this quantity we are not concerned. 

 It is the amount which is contained at any instant in the 

 immensely long column through which the energy travels in 

 a second that is important. 



The number assumed by Clerk Maxwell (* Electricity and 

 Magnetism/ vol. ii. p. 402) is 83*4, which is very nearly 

 what Pouillet's becomes at the superior liruit of the atmo- 

 sphere. 



Thus we see that the amount of power at our disposal is 

 small — yet, perhaps, not hopelessly small. 



But is it necessary to accept this deduction from Pouillet's 

 experiments at all ? I think not, because it is not by any 

 means certain that a blackened surface, such as that in 

 Pouillet's pyrheliometer, catches up and transforms into heat 

 every form of energy in the solar beam. There may be 

 forms of energy which take no notice of blackened surfaces 

 and which refuse to be converted into heat by means of 

 them. Perhaps the proper receptive surfaces for them remain 

 to be discovered ; and it is this consideration chiefly which 

 gives hope to the experimenter in Photoelectricity. The 

 dissipation of every form of energy in the Universe into the 

 final form of heat of uniform temperature is a dogma and 

 nothing else — and, moreover, a dogma which leads to the 

 most dismal results. Can its advocates trace the process in 

 the case of energy incident on the retina, telephotographically 

 transmitted to the brain, and converted into — or, at any rate, 

 productive of — thought ; and can they show that after this 

 process nothing but heat remains ? 



It is not to the purpose to say that, u so far as we know," 

 all forms of energy must run into heat finally, and be diffused 

 throughout the Universe, because we may even still imagine 

 that there are many very astonishing physical facts beyond 

 what we know. Indeed, even in the unspeakably dismal 

 event of the final conversion of the whole " visible " Universe 



