ATMOSPIIEIMC AIU as Till-: FOOD OF PLANTS. 91 



The uiirates do not appear to be absorbed by the plant 

 to any great extent, except through the medium of tlie 

 soil, sinre they cannot exist in the state of vapor and are 

 brought down to the earth's surface by atmospheric waters. 



The full discussion of their nutritive effects must there- 

 fore be deferred until the soil comes under notice. See 

 Division II, p. 371. 



In § 10, p. 96, "Recapitulation of the Atmospheric 

 Supplies of Food to Crops," the inadequacy of the at- 

 mospheric nitrates will be noticed. 



OTHER INGREDIENTS OF THE ATMOSPHERE; viz., 3farsh Gas, 



Carbonic Oxide, Mtroiis Oxide, Hijdrochloric Acid, Sulphurous Acid, 



Sulph'jdric Acid, Organic Vapors, Suspended Solid Matters. 



There are several other gaseous bodies, some or all of which may oc- 

 cur in tlie atmosphere in very minute quantities, but whose relations to 

 vegetation, in the present state of our knowledge, appear to be of no 

 practical moment. Since, however, they have been the subjects of in- 

 vestigations or disquisition by agricultural chemists, they require to be 

 briefly noticed. 



IVInrsli «as,* C H4.— This substance is a coloiless and nearly 

 odorless gas, whicli is formed almost invarialily when organic matters 

 suffer decomposition in ab.-ence of oxygen. Wlien a lump of coal or a 

 bilU't of wood is strongly heated, poi-tions of carbon and hydrogen 

 unite to Conn this among several other substances. It is accordingly 

 one of the ingredients of the gases whose combustion forms the flame 

 of all fii-es and lamps. It is also produced in the decay of vegetal)le mat- 

 ferrf, especially when they arc immer-ed iu water, as happens in swamps 

 and stagnant ponds, and it often bubbles in large quantities from the 

 bottom of ditches, when the mud is stirred. 



Pettenkofer and Voit have lately found that marsh gas is one of the 

 f^aseous products of the respiration or nutrition of animals. 



It is combustible at high temperatures, and burns with a yellowish, 

 faintly luminous flame, to water and carbonic acid. It causes no ill ef- 

 fects when breathed by animals if it be mixed with much air, though of 

 itself it cannot support respiration. 



* Known also to chemists under the names of Light Carburetted Hydrogen, 

 Hydride of Methyl and Methauc. 



