ARTICLES 249 



modifications of benzyl-phenyl-allyl-methyl-ammonium-iodide 

 (C 6 H 6 CH 2 ) (C 6 H 5 ) (C 3 H 6 ) (CH 3 ) NI by combining the base with 

 Reychler's dextro-camphor sulphonic acid, 1 and fractionally 

 crystallising the salt from acetone. 



Almost immediately after it had been thus established that 

 pentavalent nitrogen can replace carbon as a centre of optical 

 activity, it was shown that sulphur and selenium can act 

 similarly. 



During the past decade what is without question the 

 greatest advance in our knowledge of optically active com- 

 pounds has been made by Dr. A. Werner, Professor of Chemistry 

 in the University of Zurich, who has resolved into active com- 

 ponents a number of complex derivatives of cobalt, chromium, 

 iron and rhodium. This work, which has received only a very 

 grudging acknowledgment in this country, is a natural develop- 

 ment of his ideas upon the constitution of these compounds 

 following upon a change of conception of that somewhat vague 

 property of the atom which we call valency. 



Valency is a term introduced in order to obtain a numerical 

 expression for the attraction exercised by one atom upon 

 another. When the atomic theory in its modern form was first 

 put forward it was inevitable that atoms should be assumed to 

 combine singly when forming their simplest compounds. 

 Determinations of molecular weight, however, based upon a 

 few simple assumptions, soon showed that this was not true, 

 but that some atoms had the power of holding more than one 

 other atom in combination, and in the case of many elements 

 that the number of other atoms with which one atom of the 

 element can combine depends on the nature of all of the com- 

 bining atoms, and on the conditions (particularly those of 

 temperature and pressure) under which combination takes 

 place. 



With the development of the idea of valency as a property 



1 This is a modification of a method which we also owe to the remarkable 

 scientific insight of Pasteur. He reasoned that, although the two active tartaric 

 acids, when combined with inactive bases, such as potash or soda, yielded salts of 

 the same solubility, it was unlikely that this would be the case when they were 

 combined with bases active in themselves, such as quinine, strychnine, or brucine, 

 and that the solubilities of such salts might differ so widely as to render them 

 separable by this means. This conclusion, which experiment confirmed, led to 

 the most generally applicable method for the separation of optically active acids 

 and bases. 



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