EFFECT OF TROPICAL SUNLIGHT ON THE ATMOSPHERE. 273 



ultra-violet light reaching the surface of the earth in the region around 

 the equator must be much greater than it is in those nearer the pole. 



Arrhenius, in his recent papers on the effect of light radiation pres- 

 sure in moving cosmic dust, presents several ideas which may be of 

 considerable importance in explaining the distribution of sunlight on 

 the earth's surface. The cosmic dust shot out from the sun is charged 

 and, consequentl}-, as it approaches the earth's surface, it tends to collect 

 toward the magnetic poles, leaving a region around the equator com- 

 paratively free from dust. As this fine dust strongly absorbs the blue 

 and ultra-violet portions of the sun's spectrum, both in the chromosphere 

 of the sun and the atmosphere of the earth, it is evident that tropical 

 regions, comparatively free from it, will receive more of these rays than 

 regions near the poles. As the distribution of dust around the sun and 

 the rate with which it is expelled from the sun are intimately connected 

 with the phenomena of sun spots, it seems evident that relations will 

 be discovered between sun spots and the ionization of the air, and, 

 in general, the rate of reactions carried on under the influence of sun- 

 light. 



In connection with nry experiments indicating ionization of the air 

 by tropical sunlight, a very interesting question arises as to its ionization 

 on the different parts of the earth's surface. In the last few, years 

 hundreds of measurements of this ionization have been made, and, in 

 general, it has been ascribed to radioactive substances. This assumption 

 has very strong support in many experimental facts, especially as the 

 rate of decay of the emanation agrees in many cases with that of known 

 radioactive matter. 



Eutherf ord 14 states : 



There can be little doubt that the large part of the radioactivity (ionization) 

 of the atmosphere is due to the radium emanation which is constantly diffusing 

 into the atmosphere from the pores of the earth. Since radioactivity has been 

 observed in the atmosphere at all points at which observations have, so far, 

 been made, radioactive matter must be distributed in minute quantities throughout 

 the soil of the earth. The volatile emanations escape into the atmosphere by 

 diffusion, or are carried to the surface in spring water, or by the escape of 

 underground gases and cause the radioactive phenomena observed in the atmos- 

 phere. 



Rutherford " also mentions that the results point to the conclusion that a 

 large part, if not all of the ionization at the earth's surface, is due to radioactive 

 matter distributed in the atmosphere. A constant rate of production of 30 ions 

 per second per cubic centimeter of air, which has been observed in the open air 

 at the surface of the earth in various localities, would be produced by the 

 presence in each cubic centimeter of the air of the amount of emanation liberated 

 from 2.4X10" 5 grams of radium bromide in radioactive equilibrium. However, 

 it is not likely that the ionization of the upper part of the atmosphere is due 



14 Radioactivity. Cambridge. (1905), 523. 



15 hoc. cit. 



