April 



1913] 



NATURE 



109 



following description (which is unaccompanied by 

 any illustration) : — "The aecidium is found below 

 the cortex of a stem or the epidermis of a leaf " ; 

 and shares his bewilderment on reading, in the 

 paragraph dealing with cultural methods, the 

 sentence: — "Artificial solutions . . . ought to 

 contain the substances present in the diseased 

 specimens." 



Although " mykoplasm " is mentioned, no refer- 

 ence is made to the discovery of " specialisation of 

 parasitism " by Eriksson and others. This leads 

 the author wrongly to assume that the hop and 

 pea mildews spread from their numerous wild 

 host-plants to the cultivated plants in question. 

 The view expressed that Nectria ditissima is 

 always the secondary and not the primary cause 

 of apple-" canker " was disproved some time ago. 

 By a slip Fusicladium dendriticum is stated to 

 attack pears; and it may be pointed out that 

 Alternaria is not an Oomycete, Hypochnus is not 

 an Ascomycete, and that green vitriol is not 

 copper sulphite. 



The illustrations are bad, some extraordinarily 

 so — e.g. those of corn-mildew, vine-mildew, apple- 

 scab, and Orobanche. The last-named is certainly 

 worthless ; six botanists (all familiar with the 

 plant), to whom it was shown by the reviewer, all 

 failed to recognise the plant. 



The best we can say is that the descriptions of 

 the various fungi mentioned are clear, and the 

 book is very free from misprints. E. S. S. 



Technical School Organisation and Teaching. By 

 I C. Hamilton. With a preface by G. Udny Yule. 

 Pp. xii+178. (London: George Routledge and 

 Sons, Ltd., 1913.) Price 2s. 6d. net. 

 The great and rapid changes which have recently 

 taken place in the organisation of technical educa- 

 tion in evening schools have (says Mr. Yule) 

 created a demand for a new series of text-books 

 specially adapted to the new circumstances. The 

 present volume is issued as a general introduction 

 to the series. Its aim is to define the proper scope 

 and function of evening school work, to discuss 

 the organisation necessary to make that work 

 effective, and to provide — especially for those who, 

 without previous training or experience, become 

 instructors in evening schools — a simple exposition 

 of the chief principles of teaching. It is evident 

 that the author has excellent qualifications for 

 performing his task. In his introductory section 

 he shows so clear a grasp of the problems of 

 evening school work, so sane a view of its possi- 

 bilities and of the part it should play in a national 

 scheme of education, that he gains at once the 

 confidence of his readers. The same lucidity, 

 liberality, and practical good sense characterise 

 the subsequent section on the arrangement of 

 courses, the details of administration and the func- 

 tions of examinations. The final sections show a 

 refreshingly sound appreciation of the principles 

 of method, and much skill in applying them to 

 the special problems of the technical teacher. It is 

 orobable that these 120 pages will prove the most 

 helpful and informative part of a thoroughly useful 

 book. T. P. N. 



NO. 2 266, VOL. 91] 



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.'] 



An Attempted Photochemical "Resolution" of Silver. 



The recent correspondence between Prof. Schuster 

 and Mr. Soddy in these columns suggests the placing 

 upon record of a bold but unsuccessful attempt to 

 split up the element silver which 1 made some years 

 ago with one of my students. In these days, when 

 tentative speculations are figuring- so largely in the 

 scientific world for positive knowledge, it may be 

 necessary to point out that the research was prompted 

 by no theoretical views concerning the compound 

 nature of silver. But while there was no special 

 a priori reason for suspecting the elemental character 

 of that metal it was well known as a fact that its 

 chloride, &c, on exposure to light only underwent a 

 limited decomposition — i.e. that the photo-reduction 

 ceased when a certain minute proportion of " photo- 

 salt " had been formed. The consideration of this 

 property of the silver halides suggested the inter- 

 pretation (purely hypothetical!) that the "element" 

 contained a constituent (say, o-silver) of which the 

 chloride was sensitive to light, and another (say, 0- 

 silver) less sensitive or insensitive to light. From 

 this it followed that if, after exposure and complete 

 saturation with "photo-salt," the unchanged chloride 

 could be separated from the photo-reduction product, 

 the latter, on reconversion into chloride, should 

 furnish a salt very highly sensitive as compared with 

 the main portion of unchanged chloride. 



For various reasons the research was never com- 

 pleted, chief among these reasons being the difficulty 

 of effectively separating the minute trace of photo- 

 reduction product from the large excess of unchanged 

 chloride. Notwithstanding our failure, the experi- 

 ment might be worth repeating under more favour- 

 able conditions since the relative sensitiveness to light 

 of two specimens of silver chloride — the hypothetical 

 a and modifications — could be easily detected and 

 possible measured. Even if a negative result is 

 obtained it would seem worth the expenditure of time 

 and trouble in order to set at rest the question raised 

 by the hvpothesis. Should the result be positive it is 

 needless to point out that the discovery might have 

 important practical bearings upon photographic pro- 

 cesses. It may be worth nientioning that in the 

 course of our experiments it was found that a boiling 

 saturated solution of aniline hydrochloride was a good 

 solvent for silver chloride. What is wanted, how- 

 ever, for the present purpose is some inorganic sol- 

 vent which dissolves the unchanged silver chloride at 

 ordinarv temperatures more freely than the metallic 

 chlorides (lithium, &c.) hitherto used for this purpose. 

 Reagents like thiosulphates, cyanides, &c, which 

 form salts with and freely dissolve silver chloride, 

 anpear to decompose too much of the "photo-salt" 

 to be of use in such an inquirv. The bearing of the 

 present discussion upon our abortive attempt to "re- 

 solve " silver is sufficientlv obvious — is the darkening 

 of a silver salt under the influence of light a case of 

 "physical analvsis"? R. Meldola. 



Bournemouth, March 22. 



Dana's Proof of Darwin's Theory of Coral Reefs. 



In connection with Prof. Davis's paper on Dana's 

 proof of Darwin's theory of coral reefs, which ap- 

 peared in Nature on February 6, it is interesting to 

 point out that land valleys which extend beneath the 



