May 13, 1897] 



NATURE 



29 



adapted to give to the young student a clear idea of the 

 leading structural features distinctive of the different 

 groups of animals. The figures are well selected, well 

 drawn, and well coloured, and are of a size sufficiently 

 large to display the structure of each type of animal. 

 The letter-press is written in exact and yet such popular 

 language as to be easily understood by the most un- 

 scientific. 



.Although the author has seen fit to conceal his identity, 

 certain peculiarities in spelling {e.s;. " armored " and 

 " centipeds ") suggest that he is an American. It would 

 have been better if a little more attention had been given 

 to proof-reading, and we should not then have met with 

 *' chitin " on one sheet, and " chitine " on the ne.xt, while 

 certain errors in punctuation would have been avoided. 

 It seems a pity to allude to the argonaut as the nautilus, 

 and a figure of the pearly nautilus ought certainly to have 

 been introduced. We fancy, too, that the common gaper 

 {Mya) will be somewhat unfamiliar to English students 

 imder its American title of" clam." R. L, 



A Guide to the Fossil Invertebrates and Plants in the 

 Department of Geology and Pala:ontology in the British 

 Museum {Natural History). Pp. xvi -[-158. (Printed 

 by order of the Trustees, 1897.) 

 The guide-books prepared by officials of the Natural 

 History Museum at South Kensington, to interest visitors 

 in the collections under their charge, are models of what 

 guide-books should be ; they are concise in text, often w ell 

 illustrated, and marvellously cheap ; and the persons who 

 digest them obtain a liberal education on the subjects 

 with which they deal. In this new Guide, prepared under 

 the direction of Dr. Henry Woodward, the fossil inverte- 

 lirates and plants represented by specimens and drawings 

 in the Natural History Museum are described ; the 

 characteristics of the living organism, as well as of the 

 parts found in a fossil state, being placed before the 

 reader. With this Guide in his hand, the student of 

 geology and palaeontology will be able to derive the 

 fullest advantage from the admirably-arranged geological 

 record at South Kensington. 



Report on the Causes and Pre%'ention of Smoke from 

 Manufacturing Chimneys. By Dr. Harvey Littlejohn, 

 M.A., M.B., B.Sc, Medical Officer of Health. Pp. 51. 

 (Sheffield : Wm. Townsend and Son, 1897.) 

 Dr. Littlkjohn drew up this report, upon the subject 

 of the smoke nuisance in Sheffield, at the request of the 

 Health Committee. He gives a short account of the 

 past history of the subject, which occupied the attention 

 of a Select Committee of the House of Commons so far 

 back as 1819. Sheffield has an unenviable notoriety for 

 smoke, owing, of course, to the fact that a large number 

 of its manufactures depend almost wholly upon the 

 combustion of coal. Dr. Littlejohn suggests that further 

 restrictions be imposed on the amount of smoke emitted 

 by steam-boiler furnaces, but no special form of apparatus 

 for preventing excessive smoke is recommended, the 

 opinion being that greater care and attention in firing 

 would considerably lessen the nuisance. 



Birds of Our Islands. By F. A. Fulcher. Pp. 368. 



(London : Andrew Melrose.) 

 With the multitude of readable books which now 

 €xist on British birds, it is almost a reproach to be with- 

 out a knowledge of bird-life. In this dainty volume the 

 characteristics and habits of birds, and the curiosities of 

 bird-land, are pleasingly described. The book is not an 

 exhaustive treatise, but a collection of word-pictures 

 drawn by the author in various parts of the British 

 Isles. It is simple-worded ; nevertheless, it is instructive, 

 and it will lead its readers to look about them so as to see 

 for themselves how interesting are the works of nature. 

 The book would be a very acceptable present for a boy 

 with a taste for natural history. 



NO. 1437, VOL. 56 j 



LETTERS TO THE ED J TOR. 

 {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 coinmunications.'\ 



The Theory of Dissociation into Ions. 



I AM glad that Mr. Dampier Whetham has noticed the two 

 experiments which I adduced against the present theories of 

 osmotic action and dissociation respectively. The force of the 

 first of these experiments he admits, but, as regards the .second, 

 I fear that he can hardly have realised the true results of the 

 experiment, or else I do not realise the meaning of the explana- 

 tion which he offers of it. 



The experiment was : That when a mixture, represented by 

 I00II.3O-1-H0SO4, is put into excess of acetic acid, the lowering 

 of thefreezing-point of the latter shows that the mixture contains 

 less than loi acting units, instead of more, as would be the case 

 if the H2SO4 molecule was dissociated into ions. The actual 

 number of acting units indicated was about 70. (I have only 

 an abstract of the paper by me : it will be found in the Berichie, 

 24, p. 1579.) Mr. Whetham's explanation is that the acetic acid 

 takes the water away from the sulphuric acid, and this latter 

 goes into solution as such in the acetic acid, and in this solvent 

 it is undissociated. But even if this were so— and a determina- 

 tion of the conductivity of the complex solution should tell us at 

 once whether it is, or not — we should still have our loi acting 

 units (lOoHoO and H.,SO.i) in the acetic acid, or, even if the 

 sulphuric acid molecules combined with each other to foim 

 complexes, we should have, at any rate, something more than 

 100 units ; whereas, as a matter of fact, we find only 70. 

 Complete recombination of ions, and complete polymerisation of 

 the sulphuric acid is quite incapable of explaining the reduction 

 of the number of acting units present. 



To quote some actual values : 16 "8 molecules of water lower 

 the freezing-point of 100 molecules of acetic acid 7 "32° ; 0-097 

 of a molecule of sulphuric acid lowers the freezing-point of 100 

 molecules of acetic acid 0'038° ; the two together should lower 

 the freezing-point of acetic acid 7 "358° if they acted on it inde- 

 pendently of each other, but the actual lowering which they 

 produce is only 7 "03° ; therefore, they do not act independently 

 of each other. The two together have even less action than the 

 water only. 



As an alternative explanation, Mr. Whetham suggests that 

 "dissociation of the ions hom each other does not forbid the 

 assumption that the ions are linked with one or more solvent 

 molecules." Quite true : but when a theory can only explain 

 observed facts by driving us to assumption of the existence of 

 such compounds as Hx-HoO and S04^H.,0, I venture to think 

 that that theory must be somewhat shaky. 



Harpenden, May I. Si'ENXER Pickering. 



I AM very glad that Mr. Pickering has given further details 

 of his experiment. From his former letter I did not gather 

 that the number of acting units indicated by the freezing-point 

 of the solution of lOoHjO -f- HjSO, in acetic acid was as low 

 as now appears. The result is most interesting, and seems 

 to me to furnish strong evidence for the modification of the 

 dissociation theory for which I am contending, under the belief 

 that, in spite of the last paragraph of Mr. Pickering's present 

 letter, it furnishes the best explanation of all the facts. 

 Had the number of acting units indicated been nearer 

 100— say 90, or more — it would have been possible to 

 explain the experiment in the first way which I suggested, 

 for the freezing-point of a soltltion of water in acetic acid shows 

 that some of the solute molecules are polymers of H.^O (Raoult's 

 value for the molecular depression is 33-0, as compared with 

 38-8 found from Van 't HofTs formula, which agrees well with 

 Raoult's values for other substances). This would reduce the 

 number of acting units in the case of the mixed solution also, 

 and even complete dissociation of the sulphuric acid would be 

 insufficient to bring that number up again to 100. 



This explanation, however, seems to me to be entirely upset 

 by the result that the lowering of freezing-point produced by a 

 mixture of water and sulphuric acid is actually less than that 

 produced by the water alone. Certainly, as Mr. Pickering says, 

 the water and sulphuric acid " do not act independently of each 

 other"— at least, when dissolved in acetic acid. I do not 

 think it quite logically follows that they are combined when 



