THE ATOMIC VIEW OF NATURE. 449 



the valency of an element was to attach to it as many 

 lines as it possessed capacities of saturation. The capa- 

 cities of saturation or valencies thus appeared very early 

 as points of saturation, and the saturation itself as a 

 linkage. These geometrical artifices or expressions were, 

 for a long time, used merely as symbols, and to the 

 present day many eminent chemists refuse to attach to 

 them any real meaning : formulae of this kind were called 

 formulae of structure, not of constitution. One of the 47. 

 most remarkable instances of the exact use of linkages linkage, 

 to explain the difference of a series of organic compounds, 

 all closely connected with each other, is the theory of the 

 so-called aromatic compounds, derived from benzene, which 

 we owe to Kekule". It has stood the criticism of more 

 than a quarter of a century, and has led to the most 

 wonderful practical knowledge of a large number of old 

 and new compounds. 



It is not astonishing if, in the face of these remark- 

 able strides which geometrical symbols have led to, 

 an attempt has been made to form an actual con- 

 ception of the geometrical figure and grouping of the 

 atoms of which chemical molecules and compounds are 

 made up. 



Space relations are the only ones in which the differ- 

 ence of symmetry and asymmetry can be at all conceived 

 by us ; and when chemical compounds were discovered 

 which show no other difference than that one of them 

 turns the plane of polarisation of a ray of light passing 

 through it to the right, the other to the left side, the 

 time seemed ripe to seek an explanation of this in a 

 purely stereometrical difference of form or grouping. 



VOL. I. 2 F 



