THE CENTURY'S PROGRESS IN CHEMISTRY 



came until the discovery of the law of valency. Then 

 much of the mystery was cleared away ; for it was 

 plain that since each atom in a molecule can hold to 

 itself only a fixed number of other atoms, complex 

 molecules must have their atoms linked in definite 

 chains or groups. And it is equally plain that where 

 the atoms are numerous, the exact plan of grouping 

 may sometimes be susceptible of change without doing 

 violence to the law of valency. It is in such cases that 

 isomerism is observed to occur. 



By paying constant heed to this matter of the affini- 

 ties, chemists are able to make diagrammatic pictures of 

 the plan of architecture of any molecule whose com- 

 position is known. In the simple molecule of water 

 (H 2 O), for example, the two hydrogen atoms must have 

 released one another before they could join the oxygen, 

 and the manner of linking must apparently be that rep- 

 resented in the graphic formula H O II. With mole- 

 cules composed of a large number of atoms, such graphic 

 representation of the scheme of linking is of course in- 

 creasingly difficult, yet, with the affinities for a guide, it 

 is always possible. Of course no one supposes that such 

 a formula, written in a single plane, can possibly repre- 

 sent the true architecture of the molecule : it is at best 

 suggestive or diagrammatic rather than pictorial. Never- 

 theless, it affords hints as to the structure of the mole- 

 cule such as the fathers of chemistry would not have 

 thought it possible ever to attain. 



VI 



These utterly novel studies of molecular architecture 

 may seem at first sight to take from the atom much of 



275 



