July 26, 1906] 



NA TURE 



295 



lo the highest. This is an ingenious compromise 

 between two methods each of which has something 

 to recommend it. The early chapters, that deal with 

 the functions of animals, the modern conception of 

 protoplasm, th(> elements of structure, reproduction, 

 the evolution of sex, and heredity are particularly good. 

 The chapter on paUeontology is, owing lo the neces- 

 sary limitations, far loo short for the subject, but a 

 table makes clear the order in which the different 

 classes of animals appeared upon the earth. When we 

 come to the body of the book we notice, as in the 

 opening chapters, the remarkable clearness of the 

 style; and though morphology is in no way neglected, 

 yet some room is always found for the description of 

 (he habits of the animals in question. For instance, 

 Ihi re are some eight pages devoted to the habits and 

 functions of birds, their modes of flight, their court- 

 ship, their nests, moulting, diet, migrations. 



One or two minor points may now be mentioned 

 that seem to be open to criticism. Plants and animals. 

 Prof. Thomson says, " represent the divergent 

 branches of a V-shaped tree of life." But pl.inls 

 originated before animals; the nature of their food 

 proves this beyond a doubt, .\nimals we must look 

 upon as a branch from the primitive ve.'relable stem. 

 The .account of the HydromedusK would be much 

 biller for an illustration — a figure of a hydroid with 

 the Medusa of the alternating generation or of Tubu- 

 laria with its Actinula. Such additions would, of 

 course, increase the bulk of the book, but the figure 

 of a frog (p. 560) is superfluous, since everyone knows 

 what a frog is like, .^gain, the process of natural 

 selection is easily intelligible without Fig. -^78. 



F. W. H. 



Animal Heroes; being the Histories of a Cat, a Dot;, 

 II I'ifieon, a Lynx, two Wolves, and a Reindeer. 

 By R. T. Seton. Pp. 362 ; illustrated. (London : 

 .Archibald Constable and Co., Ltd., 1906.) Price 

 6s. net. 



Mr. Seton has always something fresh and interest- 

 ing to tell his readers, and in the present beautifully 

 illustrated volume breaks new ground in attempting to 

 reveal some aspects of the strenuous side of the lives 

 of animals, both wild and domesticated. Every one of 

 the stories, we are told — although of course amplified 

 and set out with the picturesque surroundings the 

 author knows so well how to ix)rtray — is founded on 

 the actual life of some individual bird or quadruped ; 

 the biography of the lyn.x being based on the author's 

 own backwood experiences. Where all is so good, 

 fresh, and entertaining, it seems almost invidious to 

 select one portion of the book for special commenda- 

 tion. To our thinking, however, the almost pathetic 

 story of ",\rnaux," the homing-pigeon, is far ahead 

 of the rest in sustained interest ; but some may prefer 

 the history of the tame wolf, while to others, again, 

 the narrative of the wild reindeer may appeal more 

 strongly. Alike to voung and old the book may be 

 heartily commended as an excellent example of the 

 best style of animal biography. R. L. 



Some Facts about the WeatJier. By William Marriott. 



Pp. 32. (London : Edward Stanford, ir)o6.) 



Price '6d. 

 This pamphlel supplies just the information about 

 meteorological phenomena likely to be useful to the 

 general public. The instruments in use in climato- 

 jogical stations are enumerated, and the determining 

 factors of climate are explained in order. The 

 booklet should be the means of stimulating interest 

 in llic scientific study of weather. 



NO. I917, VOL. 74] 



LETTERS TO THE EDITOR. 



[The Editor aoes 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 'Satvre. 

 No notice is taken of anonymous communicalions.\ 



Osmotic Pressure. 



Prof. Kaulesberg's letter published in Nature on 

 July 5 shows that, as so often happens, the controversy 

 about osmotic pressure is based on a difference in meaning 

 assigned to that term. 



We may adopt what now appears to be Prof. Kahlen- 

 berg's conception, and regard the osmotic pressure of a 

 solution as a real experimental pressure reached with some 

 actual membrane in certain practical conditions. Such a 

 definition gives us a conception of great interest and 

 importance, especially from a physiological point of view. 

 But unfortunately it has no bearing on ihe thermodynamic 

 theory of solution — or the allied theories of fusion and 

 evaporation — which apparently Prof. Kahlenberg still 

 wishes to attack by its means, after he has insisted that 

 " the formation of crystals from a solution, or the concen- 

 tration of a solution by evaporation are not osmotic 

 processes." Of course they are not osmotic processes in 

 Prof. Kahlenberg's sense of the term. But the theory of 

 fusion and evaporation, which, as I pointed out in my 

 letter published on May 31, has been verified experimentally 

 in the case of the depression of the freezing point 10 an 

 accuracy of nearly one in a thousand, depends on the 

 hypothetical separation of solvent by some ideal and perfect 

 semi-permeable process. 



It is such considerations as these that demand the other 

 conception of osmotic pressure, which, suggested no doubt 

 by Pfeffer's experiments on osmosis, has now, in accord- 

 ance with the usual course of development of the concepts 

 of physical science, come to possess an ideal significance, 

 towards which the actual experimentally measured quantity 

 can but tend as the experimental conditions approach the 

 ideal state postulated in the theoretical definition. 



Defining osmotic pressure as the hydrostatic pressure 

 needed to keep a solution in equilibrium with its solvent 

 across an ideally perfect semi-permeable membrane, we 

 obtain a conception, possibly of less chemical and physio- 

 logical importance, which nevertheless enables us to 

 develop a thermodynamic theory of solution ; and this 

 theory has been verified experimentally in cases where we 

 have reason to suppose that the actual conditions approach 

 the ideal. 



I have found that this confusion of ideas as to the con- 

 ception of osmotic pressure has occasioned trouble in other 

 cases. It would be well if a new name could be applied 

 to osmotic pressure when used in one or other of its mean- 

 ings ; but I suppose that each side in the controversy would 

 insist on the rights of possession and customary usage. 

 Hence I would suggest that, at the cost of some complexity 

 of nomenclature, one of the two meanings should be 

 emphasised as " experimental osmotic pressure " and the 

 other as " thermodynamic osmotic pressure." 



Prof. Kahlenberg remarks that " in creating the theory 

 of electrolytic dissociation, the actual phenomena of electro- 

 Ivsis have played a minor part," and wishes thus to 

 invalidate my statement that " the theory rests on electrical 

 evidence, and by such evidence it must be tried." I can 

 hardly believe that Prof. Kahlenberg would wish seriously 

 to commit himself to the opinion that the historical train 

 of ideas by which a given hypothesis may have been 

 reached necessarily supplies the only (or even the best) 

 logical basis for its support. We do not always doubt the 

 stability of our houses because it has been necessary or 

 convenient to remove some of the scaffolding used in their 

 construction. 



It is true that the abnormally great osnwtic pressures 

 and freezing-point depressions of electrolytic solutions 

 originally suggested that the molecules of their solutes 

 were dissociated ; but such observations clearly can give no 

 information on the electrical state of the dissociated struc- 

 tures. A valid test for an electric ion must depend on soirc 

 electrical property, such as motion in .-in elertrir field. 



