448 SCIENTIFIC THOUGHT. 



fixed multiple proportions. In a table of the valencies 

 or saturating capacities of elements and compounds, the 

 element hydrogen forms the unit and point of reference, 

 as it does in the scale of the atomic or combining weights, 

 and very remarkable relations and analogies have been 

 established between the periodic law of Mendeleeff and 

 the valency of the different elements. Nevertheless it 

 must be remarked that the valency of an element or 

 compound does not, according to our present knowledge, 

 show such absolute fixity as the equivalents or combining 

 weights do, or as the .angles of crystallisation of chemically 

 pure substances do. 1 



The introduction of the conception of valency has had 

 an enormous influence on the development of the science 

 of chemistry, and this in a twofold direction. Its prac- 

 tical use was demonstrated by Kekule, when he placed 

 the idea of the tetravalency, or fourfold saturating capa- 

 city, of carbon in the front of his treatise of organic 

 chemistry, 2 and by so doing gave a great impetus to 

 organic research. One of the first symbols used to denote 



1 Not only are many of the ele- ; "no characteristic distinction has 

 ments, such as oxygen and phos- been found, either in physical or 

 phorus, classed differently by dif- chemical behaviour, between the 

 ferent chemists according as their ordinary compounds and the mo- 

 valency or saturating capacity is ' lecular compounds ; and therefore, 



put at a higher or lower multiple, 

 but compounds which are univer- 

 sally considered to be saturated 

 compounds, such as neutral salts 



strictly speaking, from the pheno- 

 mena exhibited, at present no other 

 conclusion can be drawn except 

 that chemical compounds do un- 



and water, form chemical combina- doubtedly exist which cannot be 



tions according to their combining ! included in the structure scheme 



numbers, which are quite defi- : which is based on the doctrine of 



nite and stable : such are the hy- ; a constant valency " (see Nernst, 



drated crystallised salts and the j ' Theoretical Chemistry,' transl. by 



double salts. These compounds 

 are called "molecular compounds." 

 Various explanations have been at- 

 tempted, but the fact remains that 



Palmer, London, 1895, p. 246). 



a A. Kekute (1829-1896), 'Lehr- 

 buch der organischen Chemie,' 1st 

 ed., Erlangen, 1859, and later. 



