424 Action of the Atmosplterc vpon veiohj -deepened Soil. 



that its origin is electric, and that electrical manifestations 

 appear to be quickened by the least luminous rays. 



Dr. George Wilson ascertained that in darkness dry chlorine 

 may be kept for three years in contact with colours without 

 bleaching them, but that a few weeks sufficed to produce that 

 effect in sunlight. The following are some of his results with 

 other gases : — 



Sulpliuretted Jiydrofjen, if made dry, and kept in darkness, does 

 not bleach, but recovers its l^leaching power with the assistance 

 of sunlight, and acts also readily if moist. 



Oxygen was found to be similarly affected, and also, in a fainter 

 .degree, carbonic acid. 



Nitrogen exerts a faint bleaching action under exposure to 

 sunlight, although it has no appreciable power in the dark, 

 whether moist or dry. {Brit. Assoc. Report^ 1851, p. C5.) 



Some of these gases are present in the soil in large quantity, 

 and the atmosphere is composed almost entirely of the last three ; 

 the above would show that under sunlight they act with an 

 energy unknown in its absence ; can the conclusion therefore be 

 avoided that, although we are yet in a great measure ignorant of 

 the precise influence it exerts, light doubtless affects in various 

 ways the mutual action of the atmosphere and the soil, and that 

 earth long buried, on being turned up to the surface, is exposed 

 to a number of new forces tending to make its particles enter 

 into fresh arrangements, and that among these light is not the 

 least prominent in its effects ? 



Having considered the action derived from the sunbeams upon 

 the soil, we have now to investigate the influence exerted upon 

 it by the constituents of the atmosphere ; before this can be done 

 it must be shown what these constituents are. The great mass 

 of the air is composed mainly of the two gases, oxygen and. 

 nitrogen, supposed to be not chemically combined but in a 

 state of mechanical mixture. Its composition was discovered 

 about the same time by Scheele and Lavoisier, and the researches 

 of modern chemistry show that the relative proportions of these 

 gases are as follows : — 



100 measures of atmospheric air consist of — 



Oxypien 20 or 21 volumes. 



Nitrogen SO or 70 „ 



M. Dumas, from recent careful analyses of air, has obtained 

 the following results : — ■ 



Air by AVeight. Air by Volume. 



Oxygen 23-lU 20-i)0 



Nitrogen 7G-90 79-10 



100-00 100-00 



