692 



NATURE 



[January 27, 192 1 



The British Journal Photographic Almanac and 

 Photographer's Daily Companion, 192 1. Edited 

 by George E. Brown. Pp. 840. (London : 

 Henry Greenwood and Co., Ltd., n.d.) Price 

 2s. net. 

 The sixtieth issue of this welcome annual appears 

 only a week or two later than in pre-war time, 

 and the edition is increased from 25,000 to 30,000. 

 This indicates a gradual progress towards normal 

 conditions. On the other hand, the volume is about 

 seventy pages fewer than last year, and the 

 price is increased. The obvious reticence of ad- 

 vertisers with regard to quoting prices, which we 

 remarked on a year ago, still impresses one, 

 though perhaps less strongly. The editor con- 

 tributes a lengthy article on general photographic 

 procedure which cannot fail to be of assistance 

 to beginners. The usual " Epitome of Progress " 

 is an excellent summary, extending to nearly 100 

 pages, of the notable events, business items, legal 

 matters, novelties in apparatus and equipment 

 (including raw materials used in photography), 

 and new methods or modifications of them. After 

 the extensive collection of formulae follows "A 

 History in Brief of Photographic and Photo- 

 mechanical Processes," giving the year and some- 

 times the month and day of the chief events, be- 

 ginning with Thomas Wedgwood's experiments, 

 published by Davy in 1802. Of the other new 

 matter, we are particularly glad to see that the 

 editor has given a table which he calls "Corre- 

 sponding Focal Powers and Focal Lengths." The 

 focal powers are given in diopters, and the corre- 

 sponding focal lengths in centimetres and in 

 inches. Many lens problems are so very much 

 more simple when calculated in diopters instead 

 of in focal lengths that we hope this table will 

 be extended in next year's issue, and that there 

 will be added to it a few simple instructions as 

 to its use. C. J. 



Physiology and Biochemistry in Modern Medicine. 

 By Prof. J. J. R. MacLeod, assisted by Roy G. 

 Pearce, A. C. Redfield, and N. B. Taylor, and 

 by Others. Third edition. Pp. xxxii + 992 -(- 9 

 plates. (London : Henry Kimpton, 1920.) 

 Price 42s. net. 

 A NOTICE of an earlier edition of this work ap- 

 peared in Nature of December 18, 1919 (p. 389). 

 That a new edition should be required in a year's 

 time shows that the book has been found to meet 

 the purpose for which it was written. The oppor- 

 tunity has been taken to recast the section on 

 the nervous system, which has been excellently 

 done by Dr. Redfield adding to it an account of 

 the fundamental principles of the physiology of 

 muscle and nerve. These changes will add to 

 the value of the book to those for whom it is 

 primarily intended, particularly to the medical 

 man who wishes to apply advances in physi- 

 ology to his clinical practice. Recent work 

 on such questions as the effects of deficient oxygen 

 supply, on "vitamins," on the capillary circulation, 

 and on wound shock has been duly incorporated. 

 A good account of the problem of the carriage of 

 NO. 2674, VOL. 106] 



oxygen and carbon dioxide in the blood, still 

 a disputed one, will also be found. Although the 

 price of the book seems rather high, it may 

 reasonably be held that good value is obtained. 

 It might be worth consideration, however, 

 whether the omission of some of the coloured 

 plates would not enable a wider circulation to be 

 ensured by a lower price. The further such know- 

 ledge as that contained in the book is spread, the 

 better will it be for the advance of medical science 

 and practice. W. M. B. 



Letters to the Editor. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



The Passivity of Metals. 



Chromium, iron, cobalt, nickel, copper, and bismuth 

 are said to exhibit the phenomenon of passivity. These 

 metals have electrode potentials varying from about 

 Fe=+oo6 to Bi=— 067 volt — that is, they are not 

 very electro-positive. They also all exhibit dual 

 valencies. This suggests that passivity may be due 

 to an electrical double layer on the surface of the 

 metal, especially when it is remembered that an anode 

 of iron becomes passive in nitric acid of sufficient 

 concentration. 



The chief theories of passivity assume that a lajier 

 of oxide, nitride (St. Edme, Comptes rendus, vol. lii., 

 p. 930, 1861), or gas is formed on the surface. 

 St. Edme's view is founded upon the fact that am- 

 monia is formed when passive iron is heated in dry 

 hydrogen. But Finkelstein {Zeit. phys. Chem.y 

 vol. xxxix., p. 91, 1901), from the results of his 

 investigation of the polarisation capacity of passive 

 iron, concludes that there can be no opaque layer on 

 the surface, and he thinks that passive iron is ferric 

 iron, whereas ordinary iron (active) is ferrous iron. 



It is here sue^ested that passivity is produced by a 

 layer of nitric acid or of nitrate ions firmly adhering 

 to the surface of the metal. This view is not incom- 

 patible with either St. Edme's or Finkelstein's results. 

 In fact, it seems as if there is considerable similarity 

 between the surface of a passive metal and that of the i, 

 disperse phase in a metal sol. ^ 



A consideration of the chemical forces yields the 

 following model of the polarisation effects at the 

 surface of a metal in an aqueous solution of an elec- 

 trolyte, say iron in dilute nitric acid : 



3H(a)- 

 3NO3 



-^ H(a)H 

 0(a) 



-B 



AB is the cross-section of a surface drawn between 

 the liquid and the metal. Below AB is the Helmholtz 

 electrical double laj'er, which we may conveniently 

 regard as due to a layer of positive metal ions, and a 

 layer of their valency electrons, two of which ions and 

 their associated electrons are shown in the figure. 

 The ions are represented in the ferric state, but this 

 is not essential to the argument. Above AB, in the 

 liquid phase, is a layer of molecules, which are 

 IK>larise<l at a given moment in a regular surface 



