ELEMENTS NECESSARY FOE. PLANT GROWTH 35 



Oxygen 



Hydrogen 



trie current may pass through, the water is first mixed 

 with a little sulphuric acid. The gases are most easily 

 collected by means of an apparatus shown in Fig. 7. The 

 volume of the hydrogen evolved is twice that of the oxygen. 



Properties. Hydrogen will burn in air with a blue 

 flame, yielding a quantity of 

 heat greater than that pro- 

 duced by the combustion of 

 the same weight of any other 

 combustible. A quarter of a 

 ton of hydrogen when burned 

 will furnish as much heat as 

 can be obtained from a ton 

 of coal. For this reason coal 

 gas, which is composed of 

 about 50 per cent of hydrogen, 

 is an economical fuel for cook- 

 ing purposes. 



Hydrogen is the lightest 

 known substance. Air is nearly 

 14| times as heavy as hy- 

 drogen gas, hence the latter 

 is used for inflating balloons and for dirigibles. Hydrogen 

 is a product of the decay of many organic bodies. When 

 burned in air, it unites with oxygen and produces steam 

 according to the equation 



H 2 +O = H 2 O. 



31. Carbon. Next to oxygen carbon enters most 

 abundantly into the composition of plants. It is a very 

 important element, although not nearly so v/idely distrib- 

 uted as oxygen. It exists in the air in the form of the 

 compound carbon dioxide gas, CCb, constituting nearly 

 four parts in ten thousand parts of air. Carbon exists 

 in the soil as the carbonates of certain metallic elements 

 such as calcium and magnesium. The diamond is a crys- 



FlG 7. Electrolysis of water. 



