NATURE 



121 



THURSDAY, JUNE 7, 1900. 



MODERN PHYSICAL CHEMISTRY. 



The Theory of Electrolytic Dissociation. By H. C. 



Jones. Pp. xii + 289. (New York : The Macmillan 



Company. London : Macmillan and Co., Ltd., 1900.) 



T^HE theory of electrolytic dissociation is only some 



■^ fifteen years old, but in that short time its growth 



has been very great, and its suggestiveness most marked. 



We gladly welcome a volume on the subject by one who 



has himself done much to promote its advance, and to 



render more secure some of the positions it has taken. 



The author's preface explains his object ; he has been 

 asked from time to time where an account of the newer 

 developments of physical chemistry is to be found. 

 Original memoirs are not always accessible to a student, 

 and in many cases explanation is wanted, and further 

 development of an argument or line of thought may prove 

 helpful ; and so Mr. Jones has given us a book based in 

 the main directly on the work of van 't Hoff, Ostwald, 

 Arrhenius and the others who have made the theory, but 

 in which the numerous developments of Arrhenius' 

 original idea are skilfully brought together, and the bear- 

 ing of the theory on phenomena, apparently widely diverse, 

 is clearly shown. The plan of the book is in the main a 

 good one. The first chapter is devoted to the earlier 

 physical chemistry with the object of showing its relation 

 to that which was to follow ; accounts are given of 

 Kopp's work on the boiling points of liquids and on mole- 

 cular volumes, of the researches of Lorentz, Gladstone 

 and Dale, Le Bel and van 't HofF and Perkin on optical 

 properties. The investigations of Favre and Silbermann, 

 Berthelot and Julius Thomsen into thermal chemistry, 

 the electrolytic work of Faraday and Clausius, Hittorf 

 and Kohlrausch, are described ; and an interesting and 

 important discussion of the development of chemical 

 dynamics and chemical statics concludes the chapter. 



In the explanation of Guldberg and Waages' Law of 

 Mass Action, there is a vague and somewhat unsatis- 

 factory use of the wox^ force j we are told that 

 "if we represent the active masses of two substances by 

 m and //, and the coefficient depending on the nature of 

 the substance, &c., by c, the force of the chemical reaction 

 is expressed by mncP 



" Force " has no meaning used in this connection, the 

 mass of compounds, or the number of molecules of com- 

 pounds produced, can clearly be put equal to nine, and 

 the condition of equilibrium will be reached when this 

 mass is equal to the mass of matter combining to form 

 the original substance. There is an obvious misprint on 

 page 62 ; k'lk is clearly written for c'jc of page 61. 



In Chapter ii. we are introduced to the main subject of 

 the book. An account is given of van 't Hoffs original 

 paper "The Role of Osmotic Pressure in the Analogy 

 between Solutions and Gases " {Zeitschrift fiir physikal- 

 ische Chemie, i. p. 481), and the grounds for believing 

 that in certain solutions the osmotic pressure conforms 

 exactly to the three gaseous laws of Boyle, Gay Lussac 

 and Avogadro are stated. Attention is here drawn to the 

 large class of compounds, all the acids, all the bases, and 

 all the salts, which form exceptions to the above state- 

 ment. For these the law is no longer PV = RT, but 

 NO. 1597, VOL. 62] 



PV = ?RT, where / is a coefficient always greater thai> 

 unity, to which a meaning is given when we consider 

 the work of Arrhenius. 



According to this the molecules in an electrolyte, or 

 some of them, are dissociated into ions. The electro- 

 lytic effects depend on the dissociated or active mole- 

 cules ; let these be « in number, and suppose each is- 

 divided into k parts ; suppose also that there are tn mole- 

 cules remaining inactive or undissociated, then the total 

 number of molecules is ;« + //, the number of inactive 

 molecules and ions is m + kn, and the value of van 't 

 Hoff's coefficient / is shown to be (w -I- kn)l{in + n). 



Arrhenius' theory of electrolysis is an extension of that 

 of Clausius. Clausius had shown that in an electrolyte 

 it was necessary to suppose some molecules of the dis- 

 solved salt were broken up into ions ; Arrhenius explained 

 how to determine from observations on osmotic pressure 

 the number of such molecules in a given solution. 



From this we are led on to two interesting chapters — 

 " Evidence for the Theory " and " Applications of the 

 Theory"; the evidence which the author has accumu- 

 lated is most valuable, while the fertility and resource- 

 fulness of the theory are strikingly shown. The book 

 will be very useful ; at the same time, in one respect, it is 

 open to criticism of some importance. A student not 

 unnaturally asks, What is osmotic pressure due to ? Why^ 

 under certain circumstances, does liquid run into a closed 

 vessel apparently against the pressure ? What is the 

 mechanism by which such a process is managed ? It 

 may be answered. We do not know ! The author may 

 fairly wish to use language independent of any molecular 

 theory, and not bind himself down in any way ; it is 

 enough for many purposes, it may be said, to know that 

 there is a definite pressure in such a solution without 

 inquiring how that pressure is caused. At the same time 

 it is impossible to avoid alluding to molecular impacts 

 and the like ; there is no evidence that Mr. Jones does- 

 wish to avoid it, and in places, e.g. p. 95 and elsewhere,, 

 he refers to the modern kinetic theory of gases, and we 

 think— this is the criticism— that it would have made the 

 book clearer if he had based his explanation throughout 

 on the extension of that theory to liquids. When an 

 ordinary experiment for measuring osmotic pressure is 

 started with a solution inside a vessel closed with a semi- 

 permeable membrane, the number of water molecules 

 which strike a unit of area of the interior surface in any 

 given time is less than the number striking the same 

 area on the outside ; thus more molecules of water 

 enter the space than leave it ; the molecules of the 

 salt cannot pass the membrane, hence the pressure 

 inside increases. The tendency is both for the water 

 and the dissolved substance to distribute themselves 

 uniformly ; the pressure inside is due to the impacts 

 (i) of the water, (2) of the dissolved substance that 

 outside arises from the impacts of the water only ; 

 ultimately the pressures due to the water balance, and 

 the excess of pressure inside measures the effect due to 

 the impacts of the dissolved substance. 



The whole is merely an example of the first proposition 

 of Mr. Jones' third chapter. The physical properties of 

 completely dissociated substances should be additive. 

 This is all implied in the book ; it might with advan- 

 tage be stated more precisely. 



G 



