748 The Outline of Science 



product; it is now known to be a sort of treasure-house of dyes 

 and drugs, of perfumes and explosives. It has been called one of 

 the most useful substances in the world. 



The reasonable question at once arises, why should coal-tar 

 be such a treasury "a magic purse of Fortunatus," as Dr. Slos- 

 son says? The answer is twofold: (a) that coal-tar is a mixture 

 of organic substances which were built up in the ancient club- 

 moss and horsetail forests, the remains of which formed the 

 coal deposits, and (b) because the chemist can juggle with the 

 primary products so as to build up quite new artificial ones. When 

 the coal-tar obtained from the distillation of coal is re-distilled, it 

 yields materials like carbolic acid (phenol), so much used as a 

 disinfectant, like naphthalene, used in driving moths away from 

 furs, like benzine or benzol, and so on. The residue left after 

 about ten colourless liquids or white solids have been separated off 

 is the familiar black pitch. 



1 



The chemist's unit is a molecule, the smallest amount of an 

 element that can exist separately, and he is accustomed to picture 

 the molecule as composed of atoms linked together by hands or 

 bonds. Thus the molecule of hydrogen (H.) may be pictured as 

 H H, each atom having one hand. The atom of carbon is 



pictured as if it had four hands C , and thus the symbol of the 



H 



I 

 simplest hydrocarbon (marsh gas) is H C H, or CHi. Now 



H 



the coal-tar product called benzene (benzine is a different thing al- 

 together) has the puzzling formula CaHe, puzzling because it is 

 not easy to see how the six four-handed atoms of carbon can be 

 "satisfied" with six one-handed atoms of hydrogen. The puzzle 



