122 Introduction to the Study of Science 



constituent. The most universal and abundant is water. 

 Hydrogen forms about one ninth of the weight of water, or 

 more exactly it is one part by weight to 7.94 parts by weight 

 of oxygen. But volume for volume, hydrogen is only about 

 one sixteenth as heavy as oxygen and about one fourteenth 

 as heavy as air. The volume or bulk of hydrogen is twice that 

 of oxygen in water. 



Hydrogen dioxid is a familiar compound, of which much use 

 is made for disinfectant, aseptic, and bleaching purposes, and 

 in the restoration of old works of art. It is known commercially 

 as hydrogen peroxid. It is a very unstable compound, which 

 in decomposition readily liberates about one half of its oxygen, 

 the rest being just water. Because of its ready release of oxygen 

 it is useful in oxidizing dead and putrefying tissue, in destroy- 

 ing germs or infective substances. The solution which is com- 

 mon in the market does not usually have more than three per 

 cent of the full strength of the dioxid. 



Hydrogen combines with nitrogen to form ammonia, a com- 

 pound which is noticeable by its odor where organic matter is 

 putrefying. Ammonia, used in refrigeration and ice-manu- 

 facturing, is produced in commercial quantities from bitumi- 

 nous coal as a by-product of coal gas and coke manufacture 

 (page 163). 



Hydrogen is an essential constituent in all acids, of which 

 vinegar is a type, and in all substances which neutralize acids, of 

 which ammonia water, caustic soda, and caustic potash or lye 

 are examples. When to these compounds all fuels, except 

 pure carbon, and all foods are added, in which hydrogen is an 

 essential, the importance of the substance in the things upon 

 which life depends is made strikingly evident. 



We have now to return to the inquiry as to the sourc'e of the 

 substances which combine in the burning candle to form water. 

 Water is composed of oxygen and hydrogen, as the study of 

 the composition of water revealed. Oxygen clearly is taken 

 from the surrounding air, but hydrogen must be derived from 



