362 What are Animals and Plants ? 



either (1) elements, such e.g. as the gas oxygen or the metal 

 iron ; or (2) compounds of elements, such, e.g., as rust, which 

 consists of oxygen and iron united to form a third substance 

 which is neither the one nor the other. 



Very many substances can exist (as water can) in three 

 states, solid (ice), fluid (water), aeriform (vapour). 



A solid inorganic substance may be either in the form of 

 crystal (as marble) or not crystalline (as chalk), while having 

 all the time the same chemical composition. Thus both 

 marble and chalk can be resolved into (1) lime and (2) a gas, 

 commonly known as carbonic acid gas, and carbonic acid is 

 again resolvable into (1) oxygen and (2) carbon, or pure 

 charcoal. 



The aeriform envelope of this planet — that is. Air — is a 

 mixture of the two gases (1) oxygen and (2) nitrogen, with 

 some carbonic acid gas and a certain amount of ammonia 

 and the vapour of water. 



Oxygen, itself incombustible, is the great burner or aider of 

 combustion. 



Nitrogen is remarkable at once both for its own inertness 

 and for its instability ; so that it is an ingredient in all the 

 most explosive compounds, such as gunpowder, guncotton, 

 nitroglycerine and the iodide, sulphide, and chloride of 

 nitrogen. 



Of carbonic acid there are ordinarily but four cubic feet 

 in ten thousand cubic feet of air ; yet so great is the quantity 

 of it contained in the whole atmosphere that there are reck- 

 oned to be 371,475 tons of it in the column of atmosphere 

 which extends above each square mile of the earth's surface. 



Water, the earth's fluid envelope, consists of oxygen, 

 hydrogen, carbonic acid, ammonia, carbonate of lime, flint (in 

 solution), and sundry salts. It is, as it were, the mother sub- 

 stance of life, both historically and physiologically, and has 

 been a groat agent in both the production and the destruction 



