28 PRINCIPLES OF STRATIGRAPHY 



prevalent. In dust storms the amount is, of course, extreme, but 

 ordinary air sometimes carries a surprising amount. Thus, in Eng- 

 land, during the fogs of February, 1891, the amount of dust de- 

 posited on the previously washed glass roofs of the plant house at 

 Kew was 30 grams to 20 square yards, while at Chelsea 40 grams 

 were deposited over the same area. This represents 22 pounds to 

 the acre, or six tons to the square mile. Of this amount, uncon- 

 sumed carbon (soot) formed 42.5 and 39 per cent., respectively, 

 and 41.5 and 33.5 per cent, was other mineral matter, including 

 some metallic iron. (Hann-40:77; Russell-8i.) 



OPTICAL CHARACTERS OF THE ATMOSPHERE. 



Light, The three chief effects produced by the energy of the 

 sun's rays are optical, thermal, and chemical. We may thus speak 

 of light rays, heat rays, and chemical or actinic rays, bearing in 

 mind that the difference of effect does not lie in the differences of 

 the rays, but in the nature of the surface on which they fall. 



The rays of the most importance to vegetal life are the luminous 

 or light rays, i. e., the components of radiation near the red end of 

 the visible spectrum, rather than at the blue and violet end. Experi- 

 ments with monochromatic light on Mimosa pudica have shown 

 that the plant thrives best in red light and least in blue. "The red 

 and the yellow rays are the most active in promoting the respira- 

 tion and the transpiration of the leaves and the assimilation of car- 

 bonic acid." (C. Flammarion, quoted by Hann-4o:5(5.) 



Diffusion of daylight. The sunlight on penetrating our atmo- 

 sphere is scattered or diffused by the particles of dust in the air, 

 and thus the illumination of the whole atmosphere on the sunny 

 side of the earth is produced. If it were not for the dust, it is 

 probable that the sky would be black, the stars visible by day, an^ 

 darkness prevail in all shady places, while those exposed to the sun 

 would be illumined with dazzling brilliancy. The color of the sky 

 and the sunset and sunrise tints are largely due to the presence of 

 the dust. The very brilliant sunset effects of the autumn and winter 

 of 1883 were due to the large amount of volcanic dust in all parts 

 of the upper air, derived from the explosion of the volcano Kraka- 

 toa (between Sumatra and Java) from May to August, 1883. (See 

 the illustrations in Hann (39), opposite page 50.) 



Diffused sunlight is of the greatest importance to plant life.' It 

 is the diffused light which pours over all portions of the plant 

 equally, which benefits the plant as a whole by benefiting all its parts. 



