144 



NATURE 



[September 30, 1920 



William Smith: His Maps and Memoirs. By T. 

 Sheppard. Pp. 1114-75-253 + plates. (Hull: 

 A. Brown and Sons, Ltd., 1920.) Price 7s. 6d. 

 Mr. Sheppard has spared no pains in making 

 bibliography attractive. He has reproduced by 

 photography original title-pages and maps ; he has 

 added portraits, and views of William Smith's 

 homes at Midford and at Harkness — the latter 

 from a good oil-painting. By quoting character- 

 istic passages, including "The Geology of Eng- 

 land : Mr. Wm. Smith's Claims," published in 

 1817, he has given a very interesting and effective 

 picture of the man. The rarity of Smith's original 

 works — only 250 copies seem to have been printed 

 of the four parts of " Strata identified by Organized 

 Fossils" — has rendered Mr. Sheppard's collation 

 of various copies a labour of time as well as of 

 pious erudition. The result is a book that will be 

 welcomed In every scientific library. Smith's sec- 

 tions across various English districts are here 

 given in a reduced form, and we are grateful to 

 the Yorkshire Geological Society for undertaking 

 this liberality of illustration when Mr. Sheppard's 

 memoir first appeared in its Proceedings. As 

 the author points out, Messrs. Cruchley of Lon- 

 don, in quite recent years, sold road-maps of 

 English counties reproduced from John Gary's 

 plates (though of course with the addition of rail- 

 ways), and on some at least of these William 

 Smith's geological data still appeared. If the 

 original plates exist, it might be possible to re- 

 construct for libraries Smith's "Geological Atlas," 

 much as it was issued between 1819 and 1824. 



G. A. J. C. 



Practical Histology. By Prof. J. N. Langley. 



Third edition. Pp. vili -1-320. (Cambridge : W. 



Heffer and Sons, Ltd., 1920.) Price los. 6d. net. 

 Prof. J. N. Laxgley's work is a laboratory 

 manual containing full directions for students 

 undergoing a course of practical histology. 

 Though primarily a book of histological methods, 

 a description is given of the appearances which 

 should be found after the instructions regarding 

 each preparation have been carried out, but the 

 volume does not aim at being a descriptive hist- 

 ology, and contains no illustrations beyond those 

 of apparatus. Provision is made for the instruc- 

 tion of both junior and senior students, the large 

 type in general representing the course of practical 

 histology for elementary students in Cambridge. 

 Histological methods vary somewhat in different 

 schools, but whatever the general procedure may 

 be, this volume is full of useful information, and 

 may be turned to in confidence for assistance in 

 the preparation of any histological specimen. 



Prof. Langley, in his preface, comments on the 

 desirability of histology being taught by the 

 physiologist, seeing that minute structure and 

 function are so bound together. The plea is a 

 good one, but the point of view of the pathologist 

 must not be neglected, for the medical student 

 rarely arrives at a full appreciation of the value of 

 histologv until he has studied the pathological 

 alterations which occur in the normal tissues. 

 NO. 2657, VOL. 106] 



Letters to the Editor. 



[The Editor does not hold himself responsible lor opinions 

 expressed by his correspondents. Seither can he undertake to 

 return, or to correspond vnth the ii/riters of, refected manu- 

 scripts Intended for this or any other part of NATURE. No 

 notice is taken of anonymous communications.] 



The Separation of the Isotopes of Mercury. 



We h.'ive Ix'cn .successful in .-ichieving a partial 

 separation of the isotopes of mercury by evaporating 

 mercury at low pressure and condensing the evaporated 

 atoms on a cooled surface. The rate of evaporation 

 of the isotopes being inversely proportional to the 

 square root of their atomic weight, and practically 

 everv atom leaving the liquid being condensed on the 

 highly cooled surface, a partial separation of the 

 isotopes of mercury was to be expected. 



By using the pyknometer method the following 

 numbers have been obtained in one set of e.xperiments 

 for the density of the condensed, and in another set 

 for that of the residual, mercury, when taking the 

 density of ordinary mercury as unity : 



Condensed mercury ... 0-999980 

 Residual mercury ... 1-000031 



The apparatus contained 40 c.c. of mercury. In the 

 first set of experiments about one-seventh of the mer- 

 cury was evaporated and the density of the condensed 

 part determined ; in the second set about three-fourths 

 of the mercury was evaporated and the residual por- 

 tion examined. .After the separation everv portion 

 was distilled again several times in the ordinary way 

 and the density measured after each distillation. No 

 di (Terence was found between these measurements, 

 the error of measurement of density being less than 

 one in a million. J. N. Bronsted. 



G. Hevesv. 



Physico-Chcmical Laboratory of the Poly- 

 technic High School, Copenhagen, 

 September 23. 



The British Association. 



The leading article in Natlkk of September 16 

 directs attention to a matter which must have exer- 

 cised the thought 6f most men of science. There is 

 certainly a widespread feeling that the British .•Asso- 

 ciation might be better occupied than in listening to 

 papers on special subjects, often given before very 

 small groups of people. Speaking of my own section, 

 that of Physiology, it has become more and more 

 difficult to get promises of communications of this 

 kind, and discussions on questions of interest at the 

 time have been arranged. I am inclined to think, 

 however, that the addresses of presidents of sections 

 are useful when they present general asp)ects of the 

 science which would be inappropriate in papers pub- 

 lished in the proceedings of learned societies or in 

 journals and describing original discoveries. The dis- 

 cussions would undoubtedly be of more value if the 

 practice of joint meetings of several sections were more 

 extensively made use of than is the case at present, 

 since it is becoming less and less possible for a worker 

 in any one branch of science to acquaint himself with 

 advances in other branches, although these mav have 

 a very material bearing on his own work. There can 

 be no doubt that the more he knows of other sciences 

 the better equipped is the worker in anv particular 

 branch. If the Association were able to remove some 

 of the dangers of the excessive specialisation into 

 which modern science seems to be drifting, it would 

 be a function useful to men of science themselves. I 

 am inclined to think that the reading of original 

 papers, and probably also discussions on subjects of 

 interest, to one section only might well be given up. 



But, however this may be. I thoroughlv agree that 

 the chief function of the British -Association is to 



