CHEMISTRY SINCE TIME OF DALTON 



is important in the architecture of a molecule. It be- 

 came certain that atoms are not thrown together hap- 

 hazard to build a molecule, any more than bricks are 

 thrown together at random to form a house. 



How delicate may be the gradations of architectural 

 design in building a molecule was well illustrated about 

 1850, when Pasteur discovered that some carbon com- 

 pounds as certain sugars can only be distinguished 

 from one another, when in solution, by the fact of their 

 twisting or polarizing a ray of light to the left or to the 

 right, respectively. But no inkling of an explanation 

 of these strange variations of molecular structure 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 numer- 

 ous, the exact plan of grouping may sometimes be sus- 

 ceptible 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,O), for example, the two hydrogen atoms must have 

 released each other before they could join the oxygen, 

 and the manner of linking must apparently be that rep- 

 resented in the graphic formula H O H. With 

 molecules composed of a large number of atoms, such 

 graphic representation of the scheme of linking is of 



