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 

 formula} 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 



