36 THE REALITIES OF MODERN SCIENCE 



toward a solution. The amazing productions of modern 

 chemistry 1 have been made possible by the knowl- 

 edge that any homogeneous 2 substance is composed 

 of small particles called molecules which are all alike 

 in construction. The molecules themselves are formed 

 by the combination of still smaller particles, the atoms, 

 which were once thought to be indivisible. Of the 

 atoms there are some ninety different kinds, each a 

 chemical element. With the ways in which they com- 

 bine, the properties of their compounds, and the 

 methods by which elements may be extracted from 

 compounds, modern chemistry is concerned. 



Now Thales advanced a theory as to the composi- 

 tion of matter. Of course, his theory was all wrong 

 because he had insufficient experimental data. He 

 suggested that water was the single fundamental 

 element. The progress of science is marked by theories 

 which have been advanced by various scientists, ac- 

 cepted and held for a time by other scientists, and later 

 superseded. Let us see what a scientist means when he 

 speaks of a theory. He should mean that upon the 

 assumption of certain things as true he can then ex- 

 plain in terms of these all the known facts of the subject 

 of his theory. He does not mean that these assump- 

 tions are necessarily correct. If they help to form a 

 picture of the mechanism of known phenomena they 

 may enable him to predict what will happen under 

 somewhat different conditions. 



1 Cf. Duncan, " Some Chemical Problems of To-day." Harpers, 

 1911. 



2 The word "homogeneous" is also applied to substances which 

 are mixtures in definite proportions of two or more kinds of mole- 

 cules. 



