PREFACE 



The history of the development of chemistry and molecular physics 

 during the past few decades is largely an account of the growth of our 

 conceptions of matter in the ionic condition. Whatever the shortcomings 

 of the older ionic theory may have been, it has proved itself a powerful 

 tool for the purpose of disclosing the structure of material substances. 

 The intimate relation existing between matter and electricity, first in- 

 ferred by Helmholtz as a consequence of Faraday's laws, has been estab- 

 lished as securely as the atomic theory itself. Present day conceptions 

 as to the nature of matter are, in a large measure, the outgrowth of 

 fundamental conceptions underlying the ionic theory. It is true that 

 certain branches of molecular physics, to the development of which the 

 ionic theory has contributed, have outstripped this theory in the impor- 

 tance of the results obtained. Nevertheless, the further advance of 

 chemistry is largely dependent upon the further development of our con- 

 ceptions of matter in the ionic condition. 



A vast amount of experimental material relating to this subject has 

 accumulated during the past thirty years. It is found scattered through 

 the volumes of many journals and the transactions of scientific societies. 

 Unfortunately, this material has nowhere been collected in a form ren- 

 dering it available to those who are not primarily interested in this field. 

 The purpose of the present volume is to present the more important of 

 this material in a comprehensive and systematic manner, thus enabling 

 the reader to gain a knowledge of the contemporary state of this subject 

 without an undue expenditure of time and effort. It is hoped, too, that 

 this volume will prove useful to those investigators in allied sciences, who 

 find it difficult to ascertain the precise limitations underlying methods 

 and ideas which they often find it necessary to apply in their own 

 subjects. 



The systems treated are those in which ionic phenomena are most 

 clearly in evidence. Metallic systems are included, for, although the 

 nature of the metals is but little understood, the existence of a relation 

 between the phenomena in metallic and electrolytic systems is unmis- 

 takable. The treatment of metals is necessarily brief, since our knowl- 

 edge of them is still very uncertain. The chemical aspects of metallic 

 systems are, so far as possible, kept in the foreground. A more detailed 



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