THE ORIGIN OF ORGANIC MATTER 27 



ably received the most thorough study. This is the case not only on the 

 part of the physicist in the laboratory but as well by the astronomer and 

 meteorologist for solar radiation. The measurement of the total supply of 

 solar radiation as well as the wave lengths and wave frequencies which 

 constitute this radiation have been the subject of careful and exhaustive 

 investigation. The pioneer work in this field was done by the American 

 physicist, Langley, and has been greatly extended by the investigations 

 of Abbot and his collaborators. In most of these investigations * the 

 intensity of solar radiation has been measured by means of the heat pro- 

 duced when the radiation is absorbed on a black surface at right angles 

 to the rays, and has been converted into calories per square centimeter per 

 minute. The values thus obtained depend, among other factors, upon ele- 

 vation above sea-level, i.e. the thickness of atmosphere which the rays must 

 penetrate, and the angle distance which the sun is from the zenith. Thus 

 Abbot states ("The Sun," p. 284) "The maximum intensity of solar radia- 

 tion as measured near sea level at Washington when the sun is not more 

 than 45° from the zenith usually ranges from 1.15 to 1.45 calories per 

 square centimeter per minute on cloudless days, depending on the clearness 

 and dryness of the air. At IMount Wilson in California, over one mile 

 above sea level, the values observed range from 1.45 to 1.62 calories, and 

 on Mount Whitney in California, nearly three miles in altitude, the ob- 

 served values reach 1.75 calories." 



Observations have been made over extended periods of time at many 

 different points on the earth with a number of different instruments. The 

 total annual insolation (for clear sky) varies with the latitude, is greatest 

 at the equator and diminishes toward the poles. It should be emphasized 

 that the values of total insolation have only an indirect bearing on possible 

 photosynthetic activity, because the latter activity depends upon a number 

 of factors besides insolation which greatly complicates the situation. 



In Figure 1 is shown the maximum radiation on normal and horizontal 

 surfaces during the course of the year. The lower maxima at normal 

 incidence during the summer are due to the dust and moisture in the 

 atmosphere during this season.^ 



Solar radiation is greatly affected both in intensity and composition 

 by its passage through the earth's atmosphere. This is due to the fact 

 that the lower layers of the air contain dust particles and water vapor 

 which absorb and refract the light rays. Because of the fact that the 

 quantity and quality of the dust and the distribution of water vapor are 

 changing from day to day it is not possible to predict accurately the in- 



* Annals of the Astrophvsical Observatory of the Smithsonian Institution, 2 

 (1908); 3 (1913); 4 (1922). Abbot. C. G., The Sun, Appletons, New York, 

 1911 Bigelow, F. H., Treatise on the Sun's Radiation and Other Solar Phe- 

 nomena, New York, 1918. Kimball. IT. II., Bull. Mt. Weather Observatory, 1, 

 parts 2 and 4; 2, part 2; 3. part 2 (1910). Pulling, H. E., Sunlight and Its Meas- 

 urement. The Plant World, 22, 151-171; 187-209 (1919). Dorno, C, Studie 

 uber Licht und Luft des Hochgebirges. Braunschweig, 1911. 



° Kimball, Monthly Weather Rev., 43, 102 (1915). 



