OCTOHKK 15, 1S96J 



NA TURE 



571 



It is not possible, within tlic narrow limits of a small 

 text-book, to present an adcc.|uate picture of the tribes 

 and orders of the Cryptogams, and this is especially true 

 for the Algif and Fungi. Hence the treatment accorded 

 to the last-mentioned groups is of necessity somewhat 

 sketchy. But, nevertheless, the author has managed to 

 include a considerable amount of the most important 

 information respecting them, illustrated in many cases 

 by copies of Kny's admirable IWindltafeln. We notice, 

 however, that the familiar drawing, after Sachs, of the 

 structure of the mushroom is reproduced, in which the 

 basidia are represented as bearing only two spores, 

 although in the text the normal number is correctly given 

 as four. It seems high time that this figure disappeared 

 from our text-books ; its chief function at present is to 

 show how difficult a matter it is to get rid of a fiction 

 which has once managed to pass itself oft" as a genuine 

 fact. Whilst we are on the subject of illustrations, we 

 cannot forbear to remark on the surprising group of Moss- 

 antheridia represented in Fig. 860. No doubt in future 

 editions the author will replace this by a more adequate 

 drawing. Both the X'ascular Cryptogams and the 

 Angiosperms are, on the whole, admirably treated, but 

 the (iymnosperms hardly receive the recognition due to 

 their important position ; we \enture to think that the 

 artificial key on p. 180 might well have been omitted. 



In dealing with the Phanerogams the classification of 

 Bentham and Hooker is adhered to, and much valuable 

 information is given as to the uses and g^eographical dis 

 tribution of the plants comprised in the various Orders. 



But it is the physiological pa it of the book which im- 

 presses us most favourably. The student will find the 

 most important facts and principles of this branch of the 

 science most clearly and suggestively put before him. 

 Nutrition is especially well handled ; and it is not neces- 

 sary to add that the chapters on reserve materials and 

 ferments form a most \aluablc epitome of our knowledge 

 respecting them, since the author is well known as a 

 distinguished investigator in connection with these 

 matters. 



The book is, altogether, one of the best of our English 

 intermediate text-books, and it is certainly one which no 

 student ought to neglect. 



Wool Dyeing. Part i. By Walter M. Gardner, F.C.S. 



Pp. 108. (Manchester : John Heywood.) 

 This little book is a reprint of a series of articles con- 

 tributed to the Textile Recorder. In spite of the title, 

 the subject of dyeing is not dealt with, being reserved 

 for parts ii. and iii., and only the operations previous to 

 dyeing are treated of in the (jiesent volume. The divi- 

 sions of this subject are : (i) The Wool Fibre ; (2) Wool 

 Scouring ; (3) Wool Bleaching ; '4) Water for Technical 

 Purposes. The treatment, although not exhaustive, is 

 fairly thorough and quite up-to-date, and no important 

 feature of these subjects, eitlior chemical or mechanical, 

 is left untouched. The book can hardly be described as 

 attractix e reading for an outsider ; but it will doubtless 

 prove useful to teachers and students in technical classes, 

 and should be helpful also to those engaged in the dye- 

 ing industry — happily a growing number — who wish to 

 understand the principles underlying the operations they 

 conduct, and who may be led by it to the study of some 

 more exhaustive work. 



It is to be regretted that in appearance the book is 

 hardly worthy of its subject-matter ; the paper employed 

 has a very uninviting aspect, and the few illustrations are 

 of little value. This being the case, we must demur at 

 the author's claim to cheapness. One more complaint : 

 to those whose chemical knowledge is but slight, one or 

 two misprints may cause perplexity, and we can imagine 

 a student inquiring in bewiUlerment why the hardness of 

 water should be expressed in terms of the CaCOi {sic) 

 contained therein. 



NO. 1407, VOL. 54] 



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. '\ 



Osmotic Pressure. 



In the October number of the Philosophical Magazine will be 

 found an interesting paper, by Prof. Poynting, w hich explains the 

 phenomena of the osmotic pressure of solutions by the hypothesis 

 of chemical combination between the solvent and the dissolved 

 matter. I wish to direct the attention of your readers to one 

 point in the paper, and to a development of it which seems to me 

 to be worthy of notice. Any successful theory of solution must 

 explain the fact that the osmotic pressure ol)e)S the usual laws 

 of gaseous pressure — those of Bojle and Avogadro — and, more- 

 over, has the same absolute value as that of the pressure which 

 the dissolved molecules would exert in the gaseous state, when 

 filling a volume equal to that of the solution. It has always 

 been clear that, whatever be the ultimate cause of the osmotic 

 pressure, the gaseous laws must be obeyed by dilute solutions. 

 The molecules of any finely-divided matter must be, in general, 

 out of each other's sphere of influence, so that each will produce 

 its effect independently of the rest. But this is all that is neces- 

 sary for Boyle's law and Avogadro's law to hold, so that these, 

 as well as the mere existence of osmotic pressure, are explained 

 by chemical combination just as well as by molecular bombard- 

 ment. On the other hand, no good reason has been hitherto 

 given why chemical forces should be so adjusted that the osmotic 

 pressure of the dissolved molecules should have the same abso- 

 lute value as that of the pressure which the same number of 

 gaseous molecules would exert when filling an equal volume. 



Prof. Poynting supposes that each molecule of dissolved matter 

 combines with the solvent to form unstable compounds, which 

 continually exchange constituents. The molecules of solvent 

 thus combined will be less energetic than the molecules of pure 

 solvent, and thus ma)- be unable to evaporate. Nevertheless, 

 since they are always being liberated and re-combined, they will 

 still be effective in retaining molecules of vapour condensing on 

 the surface. The vapour pressure will, therefore, be reduced, 

 and it follows that, if we make the additional assumption that 

 one molecule of the dissolved substance unites with one molecule 

 of the solvent, the fractional diminution of vapour pressure will 

 be the same as that calculated from the osmotic pressure given 

 by \'an 't Hoff's law. We can, of course, working b.rckward 

 from this result, show that the osmotic pressure will have the 

 gaseous value. If one dissolved molecule combines with two or 

 three solvent molecules, the osmotic pressure would have double 

 or treljlf its normal value. Although he does not explicitly 

 say so. I fancy that Prof Poynting means to suggest this as a 

 cause of the abnormally great osmotic pressures shown by solu- 

 tions of metallic salts and other electrolytes, and thus to do 

 away with what he calls " the difficulties of the dissociation 

 hypothesis." 



' Now the evidence in favour of the view that the opposite ions 

 of an electrolyte are dissociated from e.ich other is enormously 

 strong, though there is no reason to suppose that the ions are 

 not united to the solvent. Some of the chief points in favour of 

 their freedom may shortly be summarised as follows: (l) The 

 fact that the electrical conductivity of a dilute solution is propor- 

 tional to its concenuation ; whereas, if the ions moved forward 

 by taking advantage of collisions between the dissolved mole- 

 cules and consequent rearrangements, it would vary as some 

 power of the concentration higher than the first. (2) The con- 

 firmation of the values given by Kohlrausch as the specific 

 velocities of the ions, the velocity of each ion being, in dilute 

 solution, independent of the nature of the other ion present. 

 (3) The successful calculation by Nernst and Planck of the 

 coefficients of diffusion of electrolytes, and of the contact 

 differences of potential between their solutions on the hypothesis 

 that the ions migrate independently of each other. 



Thus we canriot li.ghtly give up the idea that the ions are free 

 from each other, and' it seems to me that a very simple extension 

 of Prof Poynting's theory will enable us to retain that view. 



We have only to suppose that, in the case of electrolytes, the 

 dissolved molecules are resolved into their ions, and that each 

 ion so produced unites with one solvent molecule, or, at all 

 events, destroys the mobility of one solvent molecule. A simple 



