392 



NATURE 



[December i8, 19 19 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] • 



Power from the Sun. 



In the very interesting Trueman Wood lecture 

 delivered at the Royal Society of Arts on December lo, 

 Sir Oliver Lodge discussed the utilisation of solar 

 radiation, and recommended, as the best method ot 

 effecting this purpose, the promotion of agriculture 

 of every kind. According to Sir Oliver Lodge, the 

 green leaves of trees and vegetables generally are able 

 to absorb and utilise solar energy without much regard 

 for any hampering limit to efficiency such as the 

 second law of thermodynamics, but in saying this he 

 appears to be unaware of the researches of Dr. Horace 

 Brown, who has shown that the actual amount of 

 energy stored is less than 2 per cent, of that which 

 reaches the vegetation. 



Now- the total amount of solar energy intercepted 

 by the earth is prodigious, being in the aggregate 

 some 200 billion h.p., or, on an average, about- 

 4,000,000 h.p. per square mile of that portion 

 of the earth's surface that is exposed not too obliquely 

 to the sun's rays. Absorption by the clouds and by 

 the atmosphere, though important, is not so great 

 as might be expected, with the result that even in this 

 latitude and in this climate the energy constantly 

 received throughout the hours of daylight exceeds 

 1000 h.p. per'acre. 



If, then, some method could only be devised for 

 efficiently converting this energy into a form in which 

 it could be readily applied for motive power and other 

 purposes, the gain and the convenience would be 

 enormous ; for, to take a single instance, sufficient 

 energy to run all the machinery in a factory through- 

 out the viforking day could be collected from an area 

 in many cases not greater than that subtended by the 

 factory's roof. 



Now, of course, for reasons which Sir Oliver Lodge 

 fully discussed, it is hopeless to expect to be able to 

 effect anything of this nature with the heat engine, 

 for with this we should scarcely reach the 2 per cent, 

 efficiency nearly attained by vegetation. But is there 

 any, need to allow the radiation to turn itself into heat 

 at all? Solar radiation, as is well known, consists 

 of electromagnetic waves in the ather — waves exactly 

 similar in kind to those employed in wireless tele- 

 graphy. The only difference is that, whereas the 

 length of the waves used in wireless telegraphy is a 

 matter of hundreds or thousands of metres, the wave- 

 length in the case of solar radiation is only a very 

 minute fraction of a millimetre. 



Even with wireless waves the resulting frequency is 

 tt)0 great to allow of the electric currents thcv induce 

 being directly utilised. The telephones and other 

 instruments employed offer too much impedance to 

 allow such currents to pass, while, apart from tfiis, 

 no mechanical device could move with sufficient 

 rapidity to respond to such frequencies. In wireless 

 telegraphy, however, a method has been devised for 

 converting these rapidly alternating or oscillating electric 

 currents into currents which, though pulsating, are uni- 

 directional. This is accomnlished bv the applicat'on of 

 thermionic or crystal rectifiers or non-return valves, 

 which only allow the currents in one direction to pass 

 and suppress altogether the currents in the opposite 

 direction. In this way the comparativelv useless high- 

 frequency oscillatory currents are converted into 

 NO. 2616, VOL. 104] 



rapidly pulsating unidirectional currents which behave 

 like continuous currents, and will operate telephone^ 

 and other electromagnetic devices. Moreover, though 

 in wireless telegraphy it is customary to use the cur- 

 rents in a single direction only, and to suppress the 

 inverse currents altogi^her, there is no difficulty about 

 utilising both currents by turning them into separate 

 circuits with valves set opposite ways. Under such 

 conditions, seeing that the separate valves let through 

 their respective currents with- but little loss, the 1 

 efficiency of the conversion from the radiant energy ' 

 absorbed to that utilisable in the form of electric 

 current i.s quite high, probably not less than 50 per 

 cent., and perhaps considerably more. 



Is it too rash to suggest the possibility of sonn 

 analogous method being applicable to convert intc) 

 utilisable electric currents the electromagnetic waves 

 of which the radiant energy from the sun consists ? 

 The method is quite successful with wireless waves 

 having frequencies of millions per second, but can it 

 be applied to the sun's waves, the frequency per 

 second of which is of the order of billions? No 

 doubt the problem is a difficult one, but we ' live in 

 an age of marvels, and what would have been said of 

 modern wireless methods only a few years ago ? 



One thing seems certain. The energy in the sun's 

 radiation is there, and there, too, in most abundant 

 quantity. To make use of it, moreover, requires no 

 Maxwellian "demon" such as is necessary to rencer 

 available the general stock of heat energy at uniform 

 temperature. Nor, again, does what is suggested run 

 counter to anv thermodynamical law such as would 

 preclude full advantage being taken of the great 

 efficiencv that is rendered possible by the enormous 

 temperature of the sun. 



.\nvwav, the problem of the application of solar 

 radiation to the production of power otherwise than 

 b\' moans of heat engines seems worthy of attention, 

 and is a problem that would appear much more likely 

 to meet with a speedv solution than the difficult 

 and obscure question of the liberation and utilisation 

 of the internal energy of the atom. 



A. A. C.AMPRF.LI. SwiNTON. 



66 Victoria .Street, London, S.W.i, 

 December 15. 



Heat of Reaction and Gravitational Field. 



A .SIMPLE relation between the variation of mass in 

 a physical change of state or chemical reaction and 

 the rate of variation with gravitational potential of 

 the corresponding change of total interna! energy can 

 be deduced as follows: — Let m, and m, denote the 

 masses of the initial and final states of the chemical 

 svstem and O the heat evolved, say at constant tem- 

 perature and pressure, and at the gravitational poten- 

 tial Z. Considering the following isothermal cycle : 



((7) State I to state 2 at Z, 



(h) State 2 at Z to state 2 at Z-f SZ, 



(c) State 2 to state i at Z-I-8Z, 



(d) State I at Z + 8Z to state i at Z, 



and equating the total change of energy to zero, we 

 get the equation 



W.r'"'-'"- 



For all ordinarv reactions, experiment has shown 

 that >ii,-nu_. if not zero, must be very small. It 

 follows, however, from the theory of relativitv that 

 if the reaction be exothermic )ii,>;ii,, whilst if it be 



endothermic m,<m,. Hence in the former case , ;;; 



is positive, whilst in the latter case it is negative. If 

 we can apply the energy theorv to the highly exenergir 



