MATERIALS WHICH PLANTS DERIVE FROM THE AIR. 27 



which exist in them: thus the compound of sulphuric acid 

 with potash is termed sulphate of potash. 



14. Though neither nitrogen nor hydrogen (which, I 

 have saidy unite together to form ammonia), in a sepai-ate 

 state possesses smell or taste, the compound evolves the 

 peculiar pungent odour of smelling salts, and is also distin- 

 guished from its elements by exhibiting the properties which 

 I have described as characteristic of the class of bodies 

 termed alkalies. It has a caustic alkaline taste, restores the 

 blue colour of reddened test papers, and readily enters into 

 chemical combination with acids, forming salts, several of 

 which are employed as manures. 



15. This singular pungent smelling gas has not been 

 found, like oxygen, to enter to any great extent into the 

 composition of rocks and minerals, but it is produced in large 

 quantities in various natural processes. Thus it is formed in 

 those great chemical operations which accompany volcanic 

 eruptions, and is also evolved wherever animal or vegetable 

 matters are exposed to a high temperature, or are undergoing 

 that more gradual dissolution of their parts which we term 

 decay. You may recognise its characteristic penetrating 

 odour upon burning a piece of bone or a feather, and you 

 may convince yourselves that the vapour is alkaline by bring- 

 ing near it a slip of reddened test paper. It is given off by 

 the liquid manure of the farm-yard, and you will detect its 

 smell upon opening the door of an ill- ventilated stable, and 

 in the neighbourhood of a carelessly managed manure heap. 



16. Ammonia also differs from its elements by dissolving 

 readily in water, and a strong solution of it is sold by the apo- 

 thecary. From its solubility in water, the rain which falls upon 

 the unprotected manure heap flows away laden with it, and 

 in the sewer water of our towns, enormous quantities of it are 

 swept into the sea which ^urrounds these islands. It has 

 been calculated that in the water of a single sewer in London, 

 upwards of a ton weight of ammonia is every day poured 

 into the Thames. 



17. In the distillation of coal to produce the gas used for 

 lighting our cities, a licpiid containing a very large amount 

 of ammonia is formed, which is extensively used in the arts, 

 and has also been employed with great advantage as a 

 manure, under the name of " gas liquor." {See Manures.) 



18. Ammonia is considerably lighter than common air; and 



