MATERIALS WHICH PLANTS DERIVE FROM THE AIR. 29 



copper still, with the addition of a small quantity of muriatic 

 acid,* a substance which unites with ammonia, so as to pre- 

 vent the heat driving it away, and a compound of the am- 

 monia and the acid was obtained in the vessel. Liebig 

 estimated that, if a pound of rain water contained one-fourth 

 of a grain of ammonia, a field of 26,910 square feet would 

 receive annually upwards of 80 lbs. of it, or 65 lbs. of 

 nitrogen ;i so that a statute acre would each year receive, in 

 the rain which falls upon its surface, about 129 lbs. of am- 

 monia, containing 106 lbs. of nitrogen. J 



20. The other compound which the atmosphere contains 

 is a gas called Carbonic Acid, formed by the union of the 

 black inflammable substance charcoal, or, as chemists term it, 

 carbon, with oxygen. 



21. When wood or peat is burned in a close vessel or in 

 a heap covered over with sods, in such a manner that the 

 air has not free access to it, as is occasionally done by farmers 

 in preparing turf for manure, it does not consume as when 

 burned in our fires, but there is left a considerable quantity 

 of a black-coloured porous substance, lighter than the material 

 employed, and which may be exposed to the most intense 

 heat without undergoing any change, provided we exclude 

 the air. This substance is insoluble in water, and consists 

 chiefly of one of the simple elementary bodies termed carbon, 

 combined with some earthy impurities. In the process of 

 manufacturing gas for illumination, coal, which is a compound 

 of carbon with several gases, is exposed to a strong heat in 



* Muriatic acid is the liquid sold as spirits of salts. It is a solution in 

 water of a pungent suffocating gas of a sour taste, formed by the union 

 ')f hydrogen and chlorine. The solution is very sour, and, when brought 

 into contact with ammonia, immediately unites with it, forming a white 

 solid substance named chloride of ammonium, or sal-ammoniac. Tlie 

 ammonia coming off from manure-heaps may be detected by bringing 

 near the manure a narrow slip of window glass, or a feather wet 

 ^vith spirits of salts, when the white solid compound will be produced 

 on the glass. 



t By weight, 100 lbs. of ammonia consist of 82 i lbs. of nitrogen 

 and 17i lbs. of hydrogen. 



X A aseful property of charcoal, and one which renders it most inte- 

 resting to the farmer, is its remarkable power of absorbing gases and 

 of giving them out again when moistened with water : thus, of ammonia, 

 wood cliarcoal absorbs 90 times, and of oxygen D times, its own volume. 

 I he coke of the gas manufacturer usually contains from 75 to 96 per 



lit. of pure carbon- 



