162 AIR AND LIFE. 



Much of it is borrowed by plants. It lias been observed in England 

 and in France that rain water collected in cities or in their immediate 

 vicinity contains more nitrogen (especially under the form of ammonia) 

 tlian that collected in the country some distance away. Towns where 

 industrial pursuits are thriving and active, where factories and furnaces 

 keep their chimneys constantly at work, produce a large quantity of 

 ammonia. London, Glasgow, and Manchester are specially noted for 

 this. Some amount of carbureted hydrogen exists in the atmosphere 

 (one ten-thousandth), and its name, marsh gas, gives a clue to its origin. 

 Sulplmreted hydrogen, also present in very small quantities, has its origin 

 in some volcanoes and in the disintegrative processes going on in dead 

 bodies or other lifeless organic materials. It is therefore often found in 

 the vicinity of graveyards and of fecal matter. It is enough to merely 

 mention the presence of a very slight proportion of boric acid, which is 

 ejected into the atmosphere by volcanoes — by some at least. 



Iodine has been detected in small quantities by Chatin, who is of the 

 opinion that its presence or absence in the air and waters bears some 

 relation to the occurrence of goiter in the human species. Very little 

 can be said in support of this view. The atmosphere undoubtedly con- 

 tains saline particles, and all observers who use the spectroscope have 

 been more or less annoyed by the fact. But these particles are present 

 under the solid form. They are positively in suspension in the air, and 

 not under the form of vapor nor of gas. No very considerable mental 

 effort is required to ascertain the origin of such particles. Dust per- 

 vades the whole atmosphere — that is, the lower strata at least — dust 

 which has been torn from the soil in all countries of the world, in the 

 deserts of Sahara, Kalahari, Gobi, or Atacama, in the lowlands, from 

 the flanks of the mountain ranges, dust that has been poured out from 

 the bowels of the earth by Cotopaxi and Kilauea, Vesuvius and Colima, 

 Erebus, and Terror, and all this dust contains a large number of saline 

 particles. The seas also contribute their share. The wind sweeps off 

 the crest of the waves, blows the foam and brine inshore, often to 

 considerable distances, with the result that the atmosphere contains a 

 proportion of the salts of the sea, which often cover with a perceptible 

 coating plants fairly distant from the shore. Farther inland the pro- 

 portion of sea salts is decreased, but while not themselves apparent 

 they exert apparent effects upon plants. 1 Another curious influence 

 is exerted by these particles in quite a different direction. It is well 

 known that aqueous solutions of salts may, under peculiar circum- 

 stances, be supersaturated ; that is, may contain a larger proportion of 

 dissolved salt than is consistent with theory. If air is allowed to come 

 in contact with the surface, such a solution often suddenly crystallizes. 

 M. Gernez, who has thoroughly investigated these phenomena, comes 

 to the conclusion that the sudden crystallization is due to the presence 



1 Cf. P. Lesago: Influence du bord de la mer sur la structure, des plantes. 



