January 13, 1898] 



NA TURE 



243 



ELECTRO-CHEMISTR Y. 

 The Elements of Electro-Chemistry, treated Expert- 

 mentally. By Dr. Robert Liipke. Translated from 

 the second, revised and enlarged edition by M. M. 

 Pattison Muir, M.A. Pp. xv + 223. (London : H. 

 Grevel and Co., 1897.) 



THE parts of electro-chemistry dealt with in this 

 book are the electrolysis of liquids, the theory of 

 solutions and of osmotic pressure, and the theory of the 

 galvanic battery. We gather from the preface that the 

 main object of the book is to give a description of a 

 number of e.\periments which illustrate the most im- 

 portant laws of the subject. We think that the author 

 has been successful in this respect, and that the book 

 will prove useful to teachers of physical chemistry. For 

 the experiments described in this book are essentially 

 lecture experiments ; they are arranged not so much with 

 the object of getting the greatest possible accuracy, but 

 by means of simple apparatus to give striking illus- 

 trations of the points under discussion in the time 

 available in a lecture. The experiments are described 

 in sufficient detail to enable any one to repeat them 

 without difficulty, and seem well adapted for the purpose 

 for which they are intended. When, however, we leave 

 the description of the experiments and come to the 

 conclusions which the author draws from them, the book 

 seems to us to be much less satisfactory ; the statements 

 are sometimes obscure, and occasionally erroneous. In 

 the chapter on Faraday's laws of electrolysis the text 

 seems to imply that the amount of salt electrolysed 

 by a given quantity of electricity depends upon the time 

 the electricity takes to pass through the electrolyte. The 

 statements are on pp. 34 and 35, and occur in the 

 definition of what the author means by the expression 

 "electrochemical equivalent." This is quite different 

 from the ordinary meaning of the term electro- 

 chemical equivalent of a substance. In this book the 

 electrochemical equivalent is not, as hitherto, the number i 

 of grammes of the substance separated by the passage 

 through the electrolyte of one unity of electricity, but 

 the number of units of electricity required to separate 

 the formula weight in grammes of the substance. We 

 think that nothing but confusion can come from using 

 the old expression in this new sense. This, however, is 

 not the worst ; for in defining this quantity the author 

 says "the electrochemical equivalent — that is to say, 

 the number of coulombs which causes the separation in 

 one second [the italics are ours] of that fraction of the 

 atomic weight of the metal or of the formula weight of 

 an anion group which corresponds with a single valency." 

 Of course the allusion to one second is quite misleading ; 

 the statement would have been equally true if one cen- 

 tury had been substituted for one second. The point is 

 unfortunately emphasised by the translator, who has a 

 footnote to the effect, " the statement in the text may be 

 put thus : the electrochemical equivalent is the number 

 of coulombs that causes the separation of a gramme 

 equivalent of a metal or anion group in one second." To 

 make matters worse the following numerical example is 

 given : 



"Supposing that 20 c.c. hydrogen and 10 c.c. o.xygen 

 were obtained in an apparatus for electrolysis of water in 

 three minutes, this is equal to the production of '1667 c.c. 



NO. 1472, VOL. 57] 



explosive gas per second, and therefore the quantity of 

 electricity was •i667/*i74 ; that is "958 coulomb." 



We have devoted some space to this point because it 

 is one about which students are apt to fall into error, 

 even when it is not suggested to them by the text-books. 

 It is only fair, however, to say that the other chapters are 

 much better than the one on Faraday's law. The chapter 

 on the resistance of electrolytes is clear : it would have 

 been more complete if it had contained an account of 

 Mr. Whetham's experiments on the velocity of the ions. 

 By a misprint the student, when determining the resist- 

 ance of an electrolyte, is directed to adjust the resist- 

 ances until the telephone sounds. In the chapters on 

 osmotic pressure we have a clear account of the methods 

 of preparing the semi-permeable membranes of ferro- 

 cyanide of copper. In connection with this subject we 

 may remark that it would be interesting to have brought 

 together all the various experiments made on the freezing 

 and boiling points of organic solutions, so as to see how 

 great, if any, are the divergencies from the strict accuracy 

 of the law that substances in solution exert, as osmotic 

 pressure, the same pressure as that which they would 

 exert if they were to occupy as gases the same volume 

 at the same temperature. 



A large portion of the latter part of the book is taken 

 up with the discussion of the theory of the galvanic cell ; 

 this is generally clear, and the experiments as usual are 

 good and well described. It would be well, however, to 

 discriminate in the theory of the potential difference 

 between a metal and a solution of the salt of the metal 

 between what is derivable from the theory of osmotic 

 pressure and what is the result of further assumption. 

 The theory shows that tt this potential difference is 

 expressed by an equation of the form 

 ■n — - aQ log ft> -H C 

 where 6 is the absolute temperature, a a constant, w a 

 quantity proportional to the osmotic pressure of the 

 action in the solution, and C a quantity of which all that 

 is known about it is that it is independent of the strength 

 of the solution, to write the expression in the form 



p 



IT = aQ log — 



as is done in the text, is to assume that this constant is 

 proportional to the absolute temperature, an assumption 

 which ought to be justified by a series of experiments at 

 different temperatures. We have no hesitation in recom- 

 mending the book to teachers of physics, though we think 

 it a somewhat dangerous book to put in the hands of a 

 student at the commencement of his studies in electro- 

 chemistry. 



OUR BOOK SHELF. 

 The Foundations of Scientific A sericulture. By Samuel 



Cooke, M.A., F.I.C., F.G.S. Pp. ix -I- 268. (London : 



Longmans, 1897.) 

 The author is a Professor in the College of Sciences at 

 Poona ; his book contains the matter of a short course 

 of lectures which he has been in the habit of delivering 

 to a mixed class of students. In the course of 260 pages 

 he treats of meteorology, mineralogy, rocks, soil, plant 

 morphology, Indian crops, manures, implements used in 

 cultivation, and mensuration. There is besides a scheme 

 of qualitative analysis, a glossary of terms, and 129 



