September 2, 1922] 



NA TURE 



307 



Dr. Scott Robertson's experiments were carried out 

 on several different farms in various parts of Essex 

 during the years 1915-20, and were designed to test 

 the relative fertilising value of Bessemer and open 

 hearth basic slag, and of mineral phosphates, on 

 permanent grassland cut for hay. The results of these 

 experiments form one of the most important contribu- 

 tions which have been made in recent years to the 

 Gterature of phosphatic manures, and their publica- 

 tion in book form is thus very welcome, ft was 

 found that on heavy soils of the London Clay and 

 Boulder Clay the improvement effected by rock phos- 

 phates compared favourably with that due to high- 

 soluble basic slags, especially in a wet season when the 

 hay harvest was late, and on sour soils. The low-soluble 

 fluorspar slags were definitely inferior, though still 

 effecting a considerable improvement. At two of the 

 centres where the experiments were carried out there 

 was no response to phosphatic manuring, and the 

 author produces evidence that this is due to the 

 operation of another limiting factor, probably deficiency 

 of potash. This point is of interest in connexion with 

 the fact, well known to agriculturists, that basic slag 

 is not invariably effective on all grassland. It is quite 

 likely that some, at least, of these failures are due to a 

 similar cause. 



Dr. Robertson did not restrict his work to the 

 determination of the yields of hay on his plots. He 

 presents interesting data on the botanical composition 

 of the herbage, and on the effect of the phosphatic 

 manures on such soil factors as moisture content, 

 temperature, total nitrogen and nitrate content, 

 bacterial numbers, and acidity, all of which, together 

 with climate conditions, are considered in relation to 

 their possible influence on the yields obtained. 



Needless to say. the book is well printed and pro- 

 duced, but the price is high for a small volume and 

 will certainly react unfavourably on its sale. This is 

 regrettable, for it deserves a wide circulation among all 

 interested in the improvement of our grasslands and 

 in the country's agricultural production. 



II. J- P. 



Our Bookshelf. 



(1) Potash. By Sydney J. Johnstone. New edition 

 revised and enlarged. (Imperial Institute. Mono- 

 graphs on Mineral Resources, with special reference 

 to the British Empire.) Pp. x + 122. (London: 

 John Murray, 1922.) 6s. net. 



(2) Oil Shales. By Dr. H. B. Cronshaw. (Ibid.) Pp. 

 x + 80. (London: John Murra\, 192 1.) 5s.net. 



(1) Mr. S. J. Johnstone's monograph on sources of 

 potash is the most useful summary that has been pro- 

 duced since that written by Messrs. H. S. Gale and 

 W. B. Hicks for the Geological Survey of the United 



NO. 2757, VOL. I ioj 



States (" Potash in 1917," published 1919). It has 

 no index, but ends with an excellent bibliography, 

 arranged in the sequence of references to the papers in 

 the text. The author deals with all commercial sources 

 of potash, including (p. 112) the product styled Karroo 

 ash, a residue from the ignition of the sheep-dung used 

 as fuel in the Karroo region of S. Africa. The attention 

 now given to alunite is well reflected in the summaries 

 on pp. 51 to 60. The methods of treatment are 

 described, and it may be remembered that a research 

 by \V. T. Schaller, the mineralogist, led to the sug- 

 gestion of the simultaneous extraction from alunite 

 of potash and alumina for commercial purposes. The 

 nomenclature in the analyses of products from the 

 Alsatian mines on p. 12 does not agree with that adopted 

 elsewhere in the text, and the use of " kainite," here 

 and on p. 5, as a synonym for " sylvinite " is an obvious 

 error. " Sylvinite " is, of course, a trade-name for 

 a mixture of sylvine and rock-salt. It is surely time 

 that " muriate of potash," as a name for a substance 

 containing no potash, disappeared. On p. 5 the 

 potassium-content of various products is given, calcu- 

 lated as potash, and the German and other salts are 

 quoted as yielding 100 per cent. We believe that 12-4 

 per cent, was the official figure adopted by the German 

 Potash Syndicate in 1921. The account of the occur- 

 rences of the ordinary soluble potassium salts seems the 

 least satisfactory part of the present memoir. What, 

 for instance, is meant (p. n) by " the amount of potash 

 averages 30 per cent, of potassium chloride " in the 

 description of an Alsatian deposit ? 



(2) Dr. Cronshaw's review of oil-shale resources is 

 valuable as a record of attempts to locate such shales 

 by boring in England. Something seems to have gone 

 astray in the account of the Ballycastle coalfield (p. 28), 

 where the Scottish " Broxburn shale " and a place 

 called " Newlygen " are introduced. The description 

 of explorations and results in other countries shows 

 how comparatively successful the industry has been in 

 south-eastern Scotland, though even here the re- 

 fineries are now to be supplied with imported oil. The 

 author provides a good general and local bibliography. 



G. A. J. C. 



Town Theory and Practice. By W. R. Lethaby, G. L. 



Pepler, Sir T. G. Chambers, R. Unwin, and R. L. 



Reiss. Edited, with an Introduction, by C. B. 



Purdom. Pp. 139. (London : Benn Bros., Ltd., 



1921.) 5^. net. 

 Mr. Purdom points out that, notwithstanding the 

 frequent mention of " garden-cities " in the popular 

 press, it is not generally understood that it is a technical 

 term denoting a self - contained area set out upon a 

 definite plan and including within its boundaries all the 

 requisite elements for the life of an independent com- 

 munity, and that Letchworth and Welwyn alone con- 

 form to this definition. This little book on the theory 

 and practice of the garden-city contains five essays 

 which deal with various aspects of the question. Mr. 

 W. R. Lethaby deals with the town itself in an essay of 

 a general character ; Mr. G. L. Pepler describes the 

 town plan, showing how the garden-city endeavours 

 to combine practical utility, the convenience of the 

 workers and business undertakings, and aesthetic and 

 hygienic considerations, and Mr. Raymond Unwin, in 



