274 



NATURE 



[January 17, 1901 



directed to those minerals which are peculiar to 

 Belgium. 



Many of the figures will disappoint the modern reader 

 on account of the indifferent printing ; but among the 

 illustrations he will find several useful diagrams which 

 are not the familiar figures common to all the text-books, 

 for example, the projection which shows the migration of 

 the indicatrix axes with change of composition in the 

 plagioclase felspars. 



The authors have succeeded in producing within a 

 small compass a fairly comprehensive yet lucid treatise 

 on the principles of mineralogy and the chief mineral 

 species, which may safely be recommended to the student 

 in England as well as in Belgium. 



The Essentials of Practical Bacteriolos^y : an Elementary 

 Laboratory Book for Students and Practitioners. By 

 H. J. Curtis, B.S. and M.D. Lond., F.R.C.S. (London : 

 Longmans, Green and Co., 1900.) 



This book consists of a series of lessons upon practical 

 bacteriology, mainly for a course of study required for 

 the Diploma of Public Health. Commencing with the 

 preparation of nutrient media, it passes on to the sys- 

 tematic study of, first, certain typical non-pathogenic 

 bacteria, then to the moulds, including ringworm and 

 allied forms, the account of which is much fuller than 

 usual, and, lastly, to the pathogenic organisms. Fer- 

 mentation and the beer yeasts are referred to, the malaria 

 parasites, the Amoeba coli, and the supposed cancer 

 organisms are described, and the methods employed for 

 the examination of air, water, &c., and for testing disin- 

 fectants are given. The practical details described seem 

 to be fairly complete and accurate, and the book is 

 copiously illustrated, many of the illustrations of cultures 

 being extremely good. The Bacillus enteritidis sporogenes 

 of Klein is not mentioned, though it is a capital organism 

 for class work. The method of freeing cultures for the 

 " Widal" reaction from clumps by filtration is attributed 

 to Symmers, but is mentioned in Hewlett's " Manual of 

 Bacteriology." The paraffin method of embedding de- 

 scribed is needlessly complicated. These and a i^vi 

 other: omissions and errors will doubtless be corrected 

 should another edition be called for. 



What is Heat? and What is Electricity? By F. 

 Hovenden. Pp. xvi -I- 329. (London : Chapman and 

 Hall, Ltd., 1900.) 



Mr. Hovenden has set himself the modest task of 

 overthrowing, in the space of about 300 pages, all exist- 

 ing physical tenets, and substituting in their place a 

 remarkable theory of his own. In this effort he has not 

 succeeded, except, apparently, to his own complete 

 satisfaction. In the first part of the book the author 

 quotes freely from Maxwell and others, and endeavours 

 to prove that their reasoning is fallacious. His arguments 

 only show that he does not understand what he quotes, 

 and that he has not appreciated the most elementary 

 principles of the subject, such, for example, as the 

 difference between mass and weight. Having, as he con- 

 siders, sufficiently disposed of the views held by modern 

 men of science, Mr. Hovenden proceeds to the eluci- 

 dation of his own theory. It is impossible to regard this 

 part of the book seriously, Mr. Hovenden's deductions 

 from experiments being altogether too extravagantly 

 absurd. It is interesting to note that his treatment of 

 the subject is throughout entirely qualitative ; we venture 

 to think that in no single instance would Mr. Hovenden's 

 explanations stand the test of quantitative examination. 

 If modern theory is to be disproved, it will not be by 

 such writings as this. The least one can expect of its 

 opponents is that they should properly understand the 

 fundamental conceptions involved, and this Mr. 

 Hovenden cannot be said to have attempted to do. 



NO. 1629, VOL. 63] 



LETTERS TO THE EDITOR. 



\The Editor does not hold himself responsible for opinions eX' 

 pressed 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.\ 



On a Proof of Traction-Elasticity of Liquids, 



I HAVE read with much interest the note of Mr. T. J. Baker, 

 on a surface-tension experiment (Nature, No. 1600, June 

 28, 1900). The author describes, with photographic illustra- 

 tions, a phenomenon at first observed by Savart (1833), and 

 later studied by Hagen, Tyndall, J. Plateau, Boussinesq and 

 myself, but in all these studies, as in Mr. Baker's note, no other 

 force than surface tension is supposed to produce the different 

 phases of the phenomenon. Therefore I resumed the subject 

 two years ago ^ and endeavoured to explain the consecutive 

 phases by proving that in this experiment there arises always 

 some elasticity of traction, not only in both superficial layers, 

 but even in the whole mass of the sheet. 



For example, if the velocity of the jet is extremely high, the 

 liquid is suddenly compressed by the shock against the disc ; 

 but on account of the perfect elasticity of the liquid, there is 

 no sensible loss of vis viva, and the little expansion is performed 

 in a very minute fraction of a second, during which the liquid 

 is quickly projected in all directions parallel with the plane of 

 the disc, and forms a sheet ; as long as the intermolecular dis- 

 tances do not increase, the only retarding forces are the surface- 

 tensions of both faces of the sheet ; therefore the central part of 

 the latter is even and transparent. But soon, by the stretching- 

 out of the sheet, all molecules separate from each other, ex- 

 tremely little indeed, but enough to produce suddenly strong 

 resistances ; then each coming layer strikes against a retarded 

 one, and so are formed circular strips from which many drops con- 

 stantly part. Besides, as the elasticity cannot be the same in all 

 points of a circular strip, some radial strips are also produced in 

 the sheet, from whose broken edge very many little drops are 

 continually thrown. 



On diminishing the rate of outflow, the production of interior 

 elasticity of traction becomes also smaller, and therefore the 

 transparent portion of the sheet increases gradually ; but the 

 edge sinks slowly, and soon closes inwards and reaches the 

 vertical piece supporting the disc. The surface-tension of both 

 faces of the sheet is not the only force which drags in the water 

 radially ; for by the action of gravity the sheet can be compared 

 with a membrane of india-rubber, that is to say, all portions are 

 distended, not only in the superficial layers, but even in the 

 interior mass. 



It is easy to show that the distension of falling particles is all 

 the greater as the velocity is smaller. Therefore the elasticity 

 of traction produced by gravity increases in the proportion that 

 the movement slackens. 



We can now understand why the motion of the liquid in the 

 vicinity of the summit of the closed figure becomes more and 

 more difficult, until the figure rises above the plane of the disc, 

 afterwards falls again and reforms a closed figure of smaller 

 breadth. 



With a still slower stream of water, the figure begins to 

 oscillate vertically, just because the force of gravity draws it 

 down, while the elastic force of traction pulls it up. 



Ghent, January 2. G. van der Mensbrugghe. 



Mathematics and Biology. 



In the interesting address of Prof. Howes published in 

 Nature of December 10 occur the following words : — 



" On this basis there are now being pushed forward attempts 

 to apply statistical, experimental and mathematical tests to the 

 study ot vital phenomena. All honour to those who are making 

 them, for it is cert-ain there are phases of life capable of mathe- 

 matical treatment, but the mystery of life can never be thus 

 solved ; and, concerning the objection to the observational 

 method, with confirmation and generalisation, and rejection of 

 the non-confirmable, our non-mathematical procedure is scientific. 

 Huxley has long ago said of mathematics that what you get out 

 of the machine depends entirely upon what you put into it." 



1 " Sur Us nombreux effets de I'Slasticite dts liguides," 3™'' Communica- 

 tion {Bitll. de I' Acad. Roy. de Belgique, 3n>«-.s6rie, vol. xx.wi., p. 281, 

 1898.) 



