PRELIMINARY CHAPTER 

 PRINCIPLES AND DEFINITIONS 



1. Dissymmetry and Asymmetry. A survey of the litera- 

 ture on optical activity of protoplasm reveals some confu- 

 sion in the terminology. Terms such as dissymmetry and 

 asymmetry, which are so often used, are not always clearly 

 defined. Some preliminary definitions are, therefore, 

 necessary. 



Dissymmetry is a property of the individual components 

 of a system, that is, in the cases to be considered here, a 

 property of molecules, while asymmetry refers to an aggre- 

 gate of molecules. 



The term dissymmetry was used in this sense for the first 

 time by Pasteur in the classical paper that he wrote in 1848 

 on the relations between crystalline form, chemical compo- 

 sition and optical rotation and that he summarized in the 

 two well-known lectures delivered in 1860 before the Paris 

 Chemical Society on the molecular dissymmetry of natural 

 organic products. Pasteur says that when we study mate- 

 rial objects of whatever nature, as regards their form and 

 the repetition of their identical constituent parts, we soon 

 recognize that they fall into two large classes which present 

 the following characters. Those of the one class, placed 

 before a mirror, give images which are superposable on 

 the objects themselves, while the images of the others are 

 not superposable on the objects. A cube, straight stairs, a 

 branch with opposite leaves, the human body — these are of 

 the former class ; an irregular tetrahedron, winding stairs, 

 a hand — these belong to the second group. The latter are 



