8b 



f^^ATURE 



[March 23, 1916 



soil, insect and fungoid pests, the size of farms, 

 and the use of machinery, etc. 



The last section of the book deals with the 

 different wheat-producingf countries. Australia 

 is described first, then the Argentine, and next 

 the United States, which has a larger wheat pro- 

 duction than any other country in the world ; then 

 follows an account of Canada, finally of the Euro- 

 pean and Asiatic wheat-producing countries. 



The illustrations are well chosen and add con- 

 siderably to the value of the book. Altogether 

 it makes a very interesting volume, which we put 

 down with the feeling that the authors have done 

 their work well and produced something that will 

 be of much value to teachers. E. J. R. 



Post-Mortem Methods. By Prof. J. Martin 

 Beattie. Pp. viii + 231. (Cambridge: At the 

 University Press, 1915.) Price 105. 6d. net. 

 It is now generally recognised that the diagnosis 

 and scientific treatment of disease must be based 

 on a sound knowledge of the abnormal conditions 

 present in the various organs and tissues in cases 

 of disease. 



Such knowledge can only be obtained in the 

 post-mortem room, and it is very important that 

 the examinations should be conducted system- 

 atically and by some routine method of procedure. 

 The object of the author of this book has been to 

 set out a definite method of procedure, and such 

 modifications of this procedure which may be 

 demanded by special circumstances. We think 

 that Prof. Beattie has successfully accomplished 

 these aims ; the book is thoroughly practical with- 

 out being too full of detail, and the scheme of 

 examination suggested is a sound one. A chap- 

 ter is included on post-mortem examination for 

 medico-legal purposes, and another on the exam- 

 inations required in the various diseases ; in this 

 reference is made to the principal tropical maladies. 

 Finally, in an appendix a summary is given of the 

 methods employed for the preparation of museum 

 specimens, the preparation of tissues and sections 

 for microscopical examination, and of bacteriologi- 

 cal culture media and stains. The book is illus- 

 trated with eight half-tone plates and some figures 

 in the text. R. T. H. 



The Y ear-Book of the Scientific and Learned 

 Societies of Great Britain and Ireland. Com- 

 piled from official sources. Pp. viii + 351. 

 (London: Charles Griffin and Co., Ltd., 1915.) 

 Price ys. 6d. net. 

 This thirty-second issue of a very useful annual 

 work of reference will be welcomed by many 

 workers in science. We notice the inclusion of 

 several new societies, and these additions serve 

 to increase the value of the year-book. The 

 particulars given about the British Association 

 run to some eighteen pages, but they refer to the 

 Australian meeting of August, 1914, no account 

 of the proceedings of the Manchester meeting 

 Ikst September being included, though the par- 

 ticulars have long been available. The volume 

 deserves a place among the reference books in 

 every scierttlfic library. 



NO. 2421, VOL. 97] 



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 Liesegang Phenomenon and Concretionary 

 Structure in Rocks. 



The curious formations illustrated were produced 

 during some experiments made to support a sugges- 

 tion that the Liesegang phenomenon might be attri- 

 buted to adsorption {Science Progress, x., 369, 19 16). 

 The tubes contained 15 c.c. of i per cent, agar gel, in 

 which small quantities of either liver of sulphur or 

 manganese sulphate had been dissolved, and were 

 treated with 10 c.c. of a standard solution of the other 

 reagent. Particularly in the case of the gels contain- 

 ing the polysulphides, the resulting stratification 

 differed from that hitherto observed, in that many of 

 the zones were separated into a number of concre- 

 tions, which in some instances were joined by rods to 

 those of the succeeding zone. The concretions were 



Fig. I. 



Fig. 



all sharply defined; the indistinctness of Fig. 2 is due 

 to their being imbedded in the gel. The peculiar 

 structure may be due to the presence in the gel of 

 small nuclei in the shape, of deposited sulphur, 

 or possibly to the composite character of one 

 of the solutes. The separate spheroids, once started, 

 would gro^v by adsorption in the same way as the 

 solid strata. To determine the exact conditions of 

 their formation requires further investigation, but it 

 should be possible to repeat the experiment with the 

 carbonates of calcium and magnesium. 



The structures appear closely to resemble the con- 

 cretionary limestones described bv Sedgwick (Trans. 

 Geol. Soc. (2), iii., 18.^5), Garwood {Geol. Mag., 

 (3), viii., 1891), Abbott (Q. J. Geol. Soc, lix., 190.1), 

 and others. Indeed, certain specimens, which Mr. 

 Abbott kindly showed me, appeared identical in detail 

 with the formation of Fig. 3. To one who is not a 

 geologist it is difficult, at first sight, to refuse the 

 conclusion that similar causes have been at work in 

 each case. Silicic acid gels are known to occur in 

 nature. A gel may contain as little as i per cent, of 

 silica. Should solutions of calcium and magnesium 

 salts come into contact with a dilute silicic acid gej, 

 containinc: alkali carbonates, under the proper condi- 



