162 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



rent withdrawal from such a device, a phase of the program which 

 has just commenced. 



As to whether photogalvanic cells of this or similar types have 

 practical importance as solar energy converters it is too early to 

 hazard an opinion. Certainly their study has the merit of presenting 

 problems in photochemistry which, while complex, are not so complex 

 as to defy analytical treatment. In that respect they satisfy the 

 condition which the scientist has learned to impose on himself, namely, 

 not to ask questions of Nature which are so difficult that he cannot 

 yet begin to understand her answer. 



In summary, I have tried to point out that the best-known method 

 of utilizing solar energy by artificial means is the relatively simple 

 one of first converting the energy to heat; that, today, engineering 

 data are inadequate properly to determine the value of such heat, 

 whether for conventional use as heat or for conversion to power; 

 that, if heat is converted to power, we are limited in possible efficiency 

 by the second law of thermodynamics ; that consequently it is necessary 

 to turn to the fields of photochemistry and photoelectricity where 

 theoretical limitations on expected output are less severe; that in 

 turning to these fields it is found that the problems which arise are 

 of so complicated a nature as to point plainly to the need for a long- 

 range program of research into fundamental phenomena, research 

 divorced almost completely for the time being from any considera- 

 tions of a practical nature. To summarize this summary, with respect 

 to the future of solar energy utilization, your guess is as good as 

 mine. 



REFERENCES 



Abbot, C. G. 



1929. The siin and the welfare of m'an. Smithsonian Sci. Ser., vol. 2. 



New York. 

 1939. Utilizing heat from the sun. Smithsonian Misc. Coll., vol. 98, No. 5. 



ACKEEMANN, A. S. E. 



1915. The utilisation of solar energy. Journ. Roy. Soc. Arts, vol. 63, No. 

 3257, pp. 538-562, April. 

 Beooks, F. a. 



1936. Solar energy and its use for heating water in California. Univ. Cali- 

 fornia, Coll. Agr. Bull. 602, November. 

 Epstein, Leo F., Kaeush, F., and Rabinowitch, E. A. 



1941. A spectrophotometric study of thionine. Journ. Opt. Soc. Amer., 



vol. 31, No. 1, pp. 77-84. 

 HoTTEL, H. C. and Woebtz, B. B. 



1942. The performance of flat-plate solar heat collectors. Trans. Amer. 



Soc. Mech. Eng. February. 

 Rabinowitch, E. 



1940a. The photogalvanic effect. I. The photochemical properties of the 



thionine-iron system. Journ. Chem. Phys., vol. 8, No. 7, pp. 551-559. 



1940b. The photogalvanic effect. II. The photogalvanic properties of the 



thionine-iron system. Journ. Chem. Phys., vol. 8, No. 7, pp. 560-566. 



