INTRODUCTION 15 



The reaction commences at the ordinary temperature, and the 

 gas can be collected over wafcer. 



The characteristic properties of hydrogen are its h'ghtness 

 and the high temperature produced by its union with oxygen. 

 The former led to its employment for filling balloons, and 

 though now the more easily obtained coal-gas is generally em- 

 ployed for the purpose, it is not nearly so eflficient, and only 

 about half of it is hydrogen, the remaining half being made up 

 of heavier gases. Although the flame of hydrogen burning in 

 air or oxygen is intensely hot and can be used to melt refrac- 

 tory substances, e.^r., silica and platinum, it possesses practically 

 no illuminating powers. When mixed with air or oxygen and 

 heated to a high temperature, e.g., by contact with a flame or 

 electric spark, hydrogen, like all inflammable gases, explodes 

 violently. With hydrogen and air the temperature required 

 to start the explosion is about 650° C, and any mixture con- 

 taining from 5 to 80 per cent, of hydrogen is explosive. Free 

 hydrogen is very rarely found in any quantity in nature, 

 though certain fermentative changes to which vegetable 

 matter is liable, produce it, and it has been found in the gases 

 escaping from volcanoes. In a state of combination, however, 

 it occurs in a very large number of compounds, particularly 

 when combined with carbon, oxygen and nitrogen. 



Carbon is the element most closely associated with plant 

 and animal life. It forms a large portion of the solid matter 

 of all living beings, and the chemical processes of animal and 

 plant life are mainly those in which carbon plays an important 

 part. It exists in the combined state in many minerals, e.g.f 

 the carbonates of calcium, magnesium, iron, zinc and lead, and 

 also in a small but important constituent of the air, carbon 

 dioxide. The element occurs in three distinct forms: the 

 diamond, graphite or plumbago, and the amorphous forms as 

 charcoal, lampblack, (fee. These allotropic forms as they are called, 

 though identical in composition (being elementary carbon), 

 are possessed of very different phi'sical properties. Thus the 



