224 



NATURE 



[November 6, 1919 



from van't Hoff's study of osmotic pressure, and 

 his extension of the gas laws to sokitions. This 

 remarkable work was followed, at a short interval, 

 by Arrhenius's hypothesis of electrolytic dissocia- 

 tion, a conception that has left its mark deep on 

 the physico-chemical research records of the past 

 thirty years. This hypothesis has been the guid- 

 mg principle in countless investigations, and 

 although it presents difliculties not yet satis- 

 factorily solved, and appears to require" modifica- 

 tion in some respects, notably in regard to the 

 yole of hydration, it holds the ground to-day as 

 the most acceptable and intelligible interpretation 

 of the properties of electrolyte solutions. The 

 history of the electrolytic dissociation theory may 

 be fairly described in Larmor's words : " In the 

 case of every successful scientific theory the time 

 must come when its first easy triumphs become 

 exhausted, and what prominently confronts the 

 investigator are its outstanding defects and diffi- 

 culties." Such is the present position in regard 

 to the ionisation theory, and during recent years 

 there has been a concentration of effort on such 

 outstanding problems as the application of the 

 mass action law to strong electrolytes, the cata- 

 lytic action of ions, and the differences existing 

 between the values of the ionisation ratio deduced 

 for one and the same electrolyte by the osmotic 

 and conductivity methods respectivelv. 



The decade in which the theories of van't Hoff 

 and Arrhenius were propounded saw also the 

 establishment of the first journal exclusively de- 

 voted to the record of physico-chemical rese'arch. 

 The first number of the Zeitschrift fur physik- 

 alische Chemie appeared in 1887, and an inspec- 

 tion of the early volumes reveals the extraordinary 

 variety and attractiveness of the problems that 

 were being attacked under the aegis of the new 

 science, and on the more definitely quantitative 

 lines for which this branch of chemistry stands. 

 It was not long before the infiuence of physical 

 chemistry began to be apparent beyond its own 

 borders in a renascence of inorganic chemistry 

 which continues to the present day. Important 

 reactions between well-known substances, re- 

 garded as completely worked out, have been ex- 

 plored afresh in the light of physico-chemical 

 principles, hnd have yielded an extraordinary 

 amount of valuable quantitative data. In this 



connection one might refer to the phase rule and 

 its practical utility in connection with the con- 

 ditions of existence of salt hydrates, the constitu- 

 tion of alloys, and various technical problems. 



Prominent among the later developments of 

 physical chemistry has been the examination of 

 matter in a condition coarser than that corre- 

 sponding with the molecular state. The study of 

 mechanical suspensions, and the investigation of 

 ! colloidal solutions with the aid of the ultramicro- 

 scope, have opened up a whole new world of 

 fascinating phenomena, and bridged the gap 

 between the visible particle and the molecule. 

 Perrin's epoch-making count of the particles at 

 different levels in a vertical column of a 

 mechanical suspension, and the evaluation of the 

 Avogadro constant which follows therefrom, have 

 notably extended the validity of the gas laws, 

 and supplied at the same time definite quantita- 

 tive proof of the molecular movements postulated 

 by the kinetic gas theory. Of extraordinary 

 interest also in this connection is the fact that 

 purely physical evidence, based on the atomic 

 character of electricity and depending on 

 measurements of the elementary electric charge, 

 gives strong support to the Avogadro conception. 

 At the present moment fresh means of attack- 

 ing the still unsolved problems of the physico- 

 chemical field are being developed. Planck's 

 quantum theory, for example, coupled with such 

 experimental work as that on the heat capacity 

 of solids at low temperatures, and on the origin 

 and relationship of spectral lines, appears likely 

 to have a notable ' influence on the future of 

 physical chemistry. The thorough investigation 

 of colloids along physico-chemical lines, which 

 is actively proceeding to-day, promises to throw 

 light on many problems which are of interest not 

 only from the purely scientific point of view, but 

 also to the industrial chemist. The sister sciences, 

 too, are vitally concerned in the exploitation of 

 this field, and, indeed, the physical chemist of 

 to-day may point with legitimate pride to the 

 fact that the principles of his science are welcomed 

 by the metallurgist, the physiologist, the geo- 

 logist, and others, as valuable aids in the elucida- 

 tion of their respective problems. This ever- 

 widening influence is the guarantee of the future 

 vitality of physical chemistry. 



THE INFLUENCE OF INVESTIGATIONS ON THE ELECTRICAL 

 PROPERTIES OF GASES ON OUR CONCEPTIONS OF THE 

 STRUCTURE OF MATTER. 



By Sir J. J. Thomson, O.M., Pres. R.S. 



A LI> workers in science owe much to \ai ure, 1 

 ■^*- and so I am glad to comply with the request i 

 of its Editor to write a few words on the progress 

 of some branch of physics in the fifty years since 

 Nature was started. I shall confine myself to 

 the effect which results obtained by investigations | 

 NO. 2610, VOL. 104] 



on the electrical properties of gases have had on 

 our conceptions of the structure of matter and the 

 potentiality of further applications of these results 

 to increase our knowledge of physical and chemical 

 problems. In these investigations we study atoms 

 and molecules when they are charged with electri- 



