230 DARWINISM AND THE PROBLEMS OF LIFE 



oxygen, hydrogen, nitrogen, carbon, all the metals, 

 sulphur, phosphorus, and others. 



These elements may unite in two different ways to 

 make up the materials we find in nature — in simple 

 mixtures or in chemical combinations. 



A mixture can easily be understood. We may take a 

 piece of copper, for instance, file it down, and mix it up 

 thoroughly with ground sulphur so as to form a greyish- 

 green powder. This seems to be quite homogeneous, 

 but we find that it is not if we examine it under the 

 microscope. If we apply a higher power, we see clearly 

 that the new powder consists of grains of sulphur and 

 copper, lying side by side. 



But if we heat the mixture until it becomes incan- 

 descent, and then let it cool, we get a black substance 

 in which we cannot detect, under the most powerful 

 microscope, a single grain of sulphur or copper. A new 

 body has been formed, and this cannot at once be 

 separated into its constituents, as the mixture could be. 

 The new body has also different properties from its 

 constituents ; they have, as we say, entered into a 

 chemical combination. In the present instance the 

 compound is called sulphuret of copper. 



There are vast numbers of these chemical compounds 

 in nature. Water is one, for instance ; it consists of 

 hydrogen and oxygen. On the other hand, the 

 elements that form the chemical compounds are rarely 

 found in a free state in nature. Iron, for example, is 

 never found pure, but always combined with sulphur in 

 sulphuret of iron or other forms. 



It is possible to break up chemical compounds into 



