THE ATOMIC VIEW OF NATUEE. 447 



outside inward a method which has rarely led to definite 

 results in scientific research. 



A department of chemical science called structural 45. 



. . I'll structural 



chemistry which has quite recently developed into ad stereo- 



^ ^ chemistry. 



stereo-chemistry has during the last fifty years of the 

 century been working by the opposite method. Even 

 those organic chemists who ridiculed the notion that 

 a chemical formula, which on the surface of the paper 

 on which it is written cannot help making use of 

 geometrical position and proximities, is in any way a 

 picture of the arrangements of atoms in real space, were 

 nevertheless forced to avail themselves of this symbolism. 

 About the middle of the century, especially through the 

 researches of Frankland, followed by those of Couper and 

 Kekule, the phenomenon of multiple proportions was ex- 

 plained by introducing the notion of saturation. An 

 element which can combine with one or more atoms of 

 the same or of different elements or definite chemical 

 compounds was looked upon as having a chemical affinity 

 which might be wholly or only partially satisfied. The 

 different compounds arising out of such combinations 

 would then represent different degrees of saturation of 

 the first element; and it was evident that elements as 

 well as compounds could be arranged according to the 

 degrees of saturation of which they were capable. A 

 compound containing elements which possessed a greater 

 capacity for saturation than the combination afforded was 

 called unsaturated. The term valency was introduced to 46. 

 denote the degrees of saturation of elements and com- ^''^''^- 

 pounds, which were therefore mono-, di-, or poly-valent, 

 according to the compounds existing in fixed simple or 



