152 HOWARD E. PULLING 



I. SOLAR RADIATION 



As botanists we are but little concerned with the origin and 

 maintenance of the sun's radiation as a subject for investigation. 

 Unless, however, the fundamentals of the subject are grasped 

 our reasoning and measurements are quite likely to lead to 

 wholly erroneous botanical generalizations; a misfortune that 

 is realized in practically all our texts in plant physiology. 



Radiation is produced by vibrations of small particles, atoms 

 or electrons, called radiators for brevity. An increase in tem- 

 perature (absorption of energy) increases the rates of vibrations 

 of these radiators, increasing their output of energy and the 

 frequency with which they vibrate. 1 Thus the intensity of 

 radiation is increased and the wave length shortened. Several 

 laws, of which Wien's, Stefan's and Planck's are the most 

 general, connect the temperatures of certain types of radiating 

 bodies with measurable features of their spectra. From these 

 laws 2 the temperature of a Kirchoff radiator (so-called " black 

 body, " i.e., a uniformly heated enclosure radiating by tempera- 

 ture only) that would approximately reproduce the sun's spec- 

 trum has been calculated. Such a calculation leads to a tem- 

 perature of about 6000° absolute, which is often erroneously 

 stated to be approximately equivalent to the sun's actual 

 temperature. 



Since we have little accurate knowledge concerning the 

 original character of the radiation in the sun's interior (this 

 original spectrum is, of course, considerably altered in passing 

 through the cooler outer layers of the sun) 3 and since chemical 



1 See: Preston, T., The theory of heat. 2nd ed. New York, 1894. 

 Idem. The theory of light. New York, 1895. 



Wood, R. W., Physical Optics. New York, 1905. 



Drude, P., Lehrbuch der Optik. 2te Aufl. Leipzig, 1906. Or any other good 

 text dealing with radiation. 



2 See, for example, Preston's Heat (cited in note 1). A good account is 

 given by Lewis, W. C. McC, A system of physical chemistry. Vol. 2, pp. 359- 

 542. New York, 1916. 



3 A brief discussion is given by Abbot, C. G., Ann. Astrophys. Obs. Smith- 

 sonian Inst. 3: 12; 139; 194-202. 1913; Idem. The sun's energy spectrum and 

 temperature. Astrophys. J. 34: 197-208. 1911; Idem. The sun. New York, 

 1911. 



