72 THE REALITIES OF MODERN SCIENCE 



Thus the molecule of water may be divided into three 

 smaller parts, of which two are similar particles of 

 hydrogen and one is a particle of oxygen. In the same 

 way common salt is divisible, each molecule giving 

 one particle of a metal known as sodium and one of 

 chlorine. The latter we know as a green poisonous gas, 

 but in this form each molecule of it consists of two of 

 the smaller particles which enter into the molecule of 

 salt. These particles into which the molecule is di- 

 visible or of which the molecule is composed are called 

 atoms. Of the atoms there are about ninety different 

 kinds. These different kinds of matter, which can- 

 not be further decomposed into other kinds, the ele- 

 mentary substances, are spoken of as "the elements. " 



This concept of an atomic composition was fore- 

 shadowed by the early Greek philosophers but was not 

 confirmed until about 1802, when it was set forth with 

 experimental evidence by John Dalton. During the 

 preceding centuries the natural desire of men to explain 

 matter had found its outlet in theories untested by 

 experiment. Matter had been classified by its proper- 

 ties and its apparent similarities rather than by its 

 experimentally determined composition. The four 

 qualities by which it was generally compared were 

 those of fire, earth, water, and air. Any chemical 

 change was regarded as due to the proportion of these 

 " elements " or " principles" which were added to the 

 original substance. But as to the concept of the " es- 

 sence" to which these principles were added, opinion 

 was divided ; some of these ancient and medieval phi- 

 losophers held that it was a material substance and 

 others that it was "ethereal." 



