NA TURE 



[May 31, 1906 



OUK BOOK SHELF. 

 Ihc Elements of Geology. By Prof. W. H. Norton. 



Pp. X + 461. (Boston, New York, and London : 



Ginn and Co., n.d.) Price 6s. 6d. 

 This is a further addition to the well written and 

 well printed introductions to physical geography and 

 geology which have been produced of late years for 

 American schools. \\"e do not quite agree with the 

 author as to the novelty of the arrangement of his 

 material, but it is certainly effective, and the ques- 

 tions attached to many of the illustrations are such as 

 will draw out the reasoning powers of the pupil. 

 ■Chemical and mineralogical considerations are kept 

 in the background, and rocks are very broadly dealt 

 with, as when syenite is defined (p. 274) as consisting 

 of "feldspar and mica," and diorite as being "still 

 less siliceous, composed of hornblende and feldspar — 

 the latter mineral being of different variety from the 

 feldspar of granite and syenite." 



The book, however, hardly suffers from this, as 

 •e.xplanations are brought in at the proper points, and 

 a certain chemical knowledge seems to be presup- 

 posed. Zoological definitions are given from time to 

 time in the stratigraphical portion, but we still think 

 that the studv of geology is held in too light estim- 

 ation when it is thus regarded as elementary and 

 independent, and not to be preceded by an outline of 

 other branches of natural history. The geographical 

 •studv of surface-features may, of course, be linked 

 with geological considerations quite early in the curri- 

 culum of a school, and the portion of Prof. Norton's 

 book that deals with the shaping of the earth's surface 

 strikes us as especially admirable. It is confessedly 

 and worthily based on the methods of Prof. W. M. 

 Davis, who contributes a note of introduction. 



The illustrations, moreover, are well selected 

 throughout, and, where they have been borrowed, 

 acknowledgment is made in most cases of the source. 

 Mr. \\'elch's famous pot-hole in Glenariff, which was 

 introduced to geologists in an English work in 1895, 

 does dutv here for the fourth or fifth time ; but the 

 numerous photographs of .'American scenery will prove 

 of special interest to readers on this side of the 

 .Atlantic. The book is modern and very carefully 

 thought out. On p. 246 volcanic phenomena are 

 "extra illustrated" bv the ruins of St. Pierre; on 

 p. 306 the latest views are expressed on the gneisses 

 of the " fundamental " Archaean complex; the Meso- 

 zoic reptiles receive attractive treatment on pp. 385- 

 392; while on p. 446 a "restored" head of Pithe- 

 canthropus finds a place among relics of primitive 

 man. This last instance errs, however, in showing 

 much that is prophetic of future discoveries. Prof. 

 Norton still translates roches vioutonnies as " sheep 

 "backs," but the mention of this detail is onlv a tribute 

 to his general accuracy. G. A. J. C. 



Letters from the Dead to the Dead. By Oliver 

 Lector. Pp. loi. (London : Bernard Ouaritch, 

 1905.) Price 6s. net. 

 The fact that the letters attributed to Bacon, Shake- 

 speare, Napier, Henry Briggs, and Guy Fawkes have 

 dates attached to them about the years 1904 and 1905 

 should convince most readers that the book ought to 

 "be regarded in the light of a practical joke, in fact, a 

 " take off " of the antique. .As another example we 

 may take the " Notes to Henrv Briggs's letter," in 

 particular the supposed proof (p. 75) that Napier's 

 true base is the reciprocal of e and not e itself. .As 

 the difference depends entirely on whether log sin 45° 

 has a plus or minus sign attached to it, it is interest- 

 ing to speculate on how many readers will be de- 

 •ceived by what is, after all, a somewhat clever hoax. 



NO. 1909, vol.. 74] 



Atlas calorie des Planies et des Animaux des Cotes 

 de France. By Dr. M. Langeron. Translated and 

 adapted from P. Kuckuck's work. Pp. vii + 67; 

 witli 24 coloured plates. (Paris : J. B. Bailliere ec 

 Fils, 1906.) Price 7.50 francs. 

 This attractive publication is an adaptation, so far 

 as the marine flora and fauna of France are con- 

 cerned, of the well-known work of M. Kuckuck. It 

 consists of three parts, dealing respectively with the 

 phanerogams living in the neighbourhood of the sea, 

 marine algie, and marine fauna. The book is in- 

 tended for the use of young students with a love of 

 natural historv, and should prove a useful companion 

 during seaside holidays. Tlie platen arc instructive- 

 and well produccft 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspotidents. 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.] 



Osmotic Pressure. 



.At Prof. .Arrnslrong's request, published in Nature of 

 May 24 (p. 79), 1 willingly summarise the electrical evidence 

 on which the theory of ionic dissociation seems to me to 

 rest, though a full discussion of that evidence would, I 

 fear, be too long for a letter in this place. Perhaps 

 Prof. Armstrong will allow me to refer him to two papers 

 in which I have written more fully what follows ; one 

 paper is in the Philosophical Magazine for February, 1903, 

 and the other in the Electro-Chemist for July of the same 

 year. 



it will, I think, be admitted that we must accept the 

 general view of electrolysis which we owe to Faraday and 

 Kohlrausch, and imagine that opposite parts of the electro- 

 lyte move in opposite directions through the liquid. Such 

 a view seems necessary to explain the appearance of the 

 products of chemical change at the electrodes only, and 

 mav be verified by direct visua' observation, as in the 

 experimental measurements of the velocity with which 

 those parts travel. It need involve no further assumption 

 if, for convenience, we agree to call the moving parts 

 ions. Controversy begins when we attempt to explain 

 how and by what mechanism the ions move. The dissoci- 

 ation theory represents the ions as free from each other 

 (though probably combined with the solvent) during that 

 fraction of their lives in which they are concerned actively 

 in conveying the current. Prof. .Armstrong, on the other 

 hand, rejects the idea of any kind of permanent or semi- 

 permanent dissociation, and holds that the electrolyte exists 

 in solution as combined molecules of, let us say, potassium 

 chloride. On the latter theory the mobihty of the ions 

 must be secured by some mechanism which involves a series 

 of interchanges between the opposite parts of the molecules. 

 Grotthus pictured the molecules arranged as the links of a 

 chain extending all the distance from one electrode to the 

 other. This particular hypothesis would be exempt from 

 the following criticism, but I suppose it would not be upheld 

 by anyone at the present day. It conflicts too clearly with 

 our knowledge of other phenomena. From our modern 

 kinetic point of view, we should regard the molecules as 

 in continual irregular motion, and suppose the ionic inter- 

 changes to occur at the instants when two molecules come 

 within each others' sphere of influence. 



Such, I imagine, is the alternative to the theory of ionic 

 dissociation ; let us trace its consequences. The greater 

 the frequency with which molecular collisions occur, the 

 more often can ionic interchanges take place, and the faster 

 will the ions work their way through the liquid. Thus 

 the ionic velocities will depend upon and increase with 

 the frequency of collision — a frequency which, on the 

 kinetic theory, varies as the square of the concentration. 

 Thus, on the hypothesis we are considering, the velocities 

 of the ions will increase with the concentration, and be 



