82 ORIGIN AND NATURE OF LIFE 



of water is the same in all its qualities as 

 another sample. Two atoms of hydrogen 

 unite with one of oxygen to form water, but 

 if hydrogen and the yellowish gas with bleach- 

 ing properties called chlorine happen to be 

 the two kinds of atom uniting, then it is found 

 that one atom of hydrogen and one atom of 

 chlorine unite, and the product of their union 

 is called hydrochloric acid. 



In another case, when one atom of an 

 element called nitrogen, which forms the 

 prodominant component of the atmosphere, 

 unites with three atoms of hydrogen, there is 

 formed the molecule of the gaseous body which 

 we term ammonia. Again, if one atom of 

 the solid element carbon unites with four 

 atoms of hydrogen there is formed a gas known 

 as marsh gas, or methane. As a final example, 

 there may be mentioned the liquid substance, 

 chloroform, of which one molecule is formed 

 by the union of one atom of carbon, with one 

 atom of hydrogen, and three atoms of the 

 above-mentioned gas, chlorine. These parti- 

 cular examples have been chosen in illustra- 

 tion, for a special purpose which will 

 become obvious presently. The points to be 

 observed at present are two, namely, that 

 there is always a definite number of atoms of 

 each kind of constituent element, and secondly 



