THE CHEMISTRY OF THE BODY 77 



gas will burn with a pale blue flame, and form a deposit of water on the sides 

 of the bottle. Hydrogen combines with oxygen (from the air) when it burns, 

 hence water is a compound consisting of hydrogen and oxygen. 



(c\ Fill another jar with hydrogen, and pass into it a lighted taper. The 

 taper is extinguished as soon as it enters the gas, but the gas itself burns. 

 Thus we learn that hydrogen is not a supporter of combustion. (Fig. 83.) 



If water contains oxygen, how is it that 

 water will not support combustion ? We 

 answer this by saying that the oxygen in the 

 water is not free, but is in combination with 

 hydrogen. Air supports combustion because 

 the oxygen it contains is free, the air being 

 a simple mixture of two gases, and not a 

 chemical compound. 



Fig. 82. Showing 

 how to prepare and 

 collect Hydrogen 

 gas. 



.-jjji^t . 



Fig. 83. 



Fig. 84. Showing how to 

 prepare Nitrogen from the 

 Atmosphere. 



Nitrogen is the other gas of the atmosphere, and it may be 

 separated from the oxygen with which it is mixed by the following 

 simple process : 



Place a small piece of dried phosphorus on a cork floating in a dish of water. 

 Ignite the phosphorus and then cover it quickly with a bell jar or wide- 

 mouthed bottle. As the phosphorus burns, the volume of the air becomes 'ess, 

 and the water rises to take the place of the gas consumed. The oxygen of the 

 air has combined with the phosphorus, and nitrogen only remains, this nitro- 

 gen occupying about four-fifths of the original air. After the nitrogen has 

 stood till it is quite free from the white fumes formed by the combustion of 

 the phosphorus, the bottle or jar may be turned over without losing any of the 

 gas, which may then be tested with a lighted taper, and it will be found that 

 nitrogen does not support combustion. Nitrogen is a very inactive gas, and 

 its use in the atmosphere is to modify the violent action of the oxygen, and so 

 render the air fit for the support of animal life. 



We have already learnt that water is formed by the combustion 

 of hydrogen ; and that carbonic acid gas is produced by the com- 



