76 



NATURE 



[May 26, 1892 



the Fucaceae, but non-sexual sporanges containing zoo- 

 spores similar to those of the Laminar iacece. Mr. E. A. 

 L. Batters describes an interesting new genus of perforat- 

 ing marine Algae, Conchocelts, belonging to the order 

 Porphyracece, which forms pink stains on empty shells, 

 especially those of Mya trttticata and Solen vagina. 

 Miss Ethel S. Barton describes malformations produced 

 in two seaweeds, Ascophyllum nodosum and Desmarestia 

 aculeata, by the attacks respectively of a new species of 

 Nematode, Tylenchus fucicolus, somewhat similar to that 

 which produces the well-known "galls" of Vaucheria, 

 and of an undetermined Copepod. The editor himself has 

 two papers, one on a fossil Alga belonging to the genus 

 Caulerpa, from the Oolite (Kimmeridge clay of Dorset- 

 shire), a new species, which he names C. Carrtithersii ; 

 and one on the genus of marine Algse, Dictyospharia^ the 

 position of which he retains among the Valoniacece, near 

 to Valonia and Anadyomene. The present number is 

 illustrated by eight well-executed plates, most of them 

 coloured. A. W. B. 



ISve Stock. By Prof. Wrightson. (London : Cassell, 



1892.) 

 This is the third of Cassell's series of agricultural text- 

 books, and though hardly equal to other writings of Prof. 

 Wrightson, will be found useful as a reader in elementary 

 classes. 



The illustrations are well done, and the text is pretty 

 clear, except perhaps on pp. 52-53, in a paragraph upon the 

 " effect of food on milk." Here it is said that 



"The quantity of milk is therefore in some degree 

 dependent on liberal feeding. The quality of the milk is 

 much less easily controlled, and it is doubtful if any 

 special feeding will materially alter the percentage of 

 butter-fats or cream in milk." 



Then, at the end of the paragraph we have — 



" Watery foods, such as silage, grass, grains, and dis- 

 tillery wash, increase the quantity of milk, but lower 

 the quality, and in town dairies, where a large amount 

 of milk is the principal object, they are much employed." 



This paragraph is contradictory and confusing, for 

 Prof. Wrightson himself admits that the quality of milk 

 may be lowered by using watery foods, and we are 

 decidedly of opinion that it may be increased by means 

 of rich, oily foods. 



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. ] 



Lord Kelvin's Test Case on the Maxwell-B jltzmann 

 Law. 



In his recent communication to the Royal Society, of a case 

 disproving the Boltzmann law, Lord Kelvin seems to have over- 

 looked an important consideration. 



It is well known that in an atmosphere near the earth, under 

 conductive (not convective) equilibrium of temperature, the mean 

 kinetic energy {i.e., the temperature) would be uniform not- 

 withstanding the attraction of the earth, which causes each 

 molecule to move more rapidly at the lower end of its path than 

 at the upper end. This is due to the effect of gravity in sifting 

 out the less rapidly moving particles, preventing them from 

 reaching the upper layers, so that, of the particles in any one 

 layer which reach a higher layer, the great proportion are those 

 which move rapidly in the lower layer. Thus there will be 

 fewer particles in the upper layer, bat the mean kinetic energy 

 of a particle will be the same in both. 



Applying these considerations to Lord Kelvin's example, it 

 appears that the C particle, when going rapidly, will penetrate 

 a considerable distance into the re.^ion of the repulsive force, 

 while, when going slowly, it will only penetrate a short distance. 

 Thus the duration of a slow flight might be much shorter than 



NO. I 178, VOL. 46] 



that of a quick one (with a force varying directly as the dis- 

 tance, the durations would be equal). It is quite different with 

 the A particle, which moves uniformly to the end of the tube and 

 back again. The duration of a slow flight will be long and of a 

 quick flight short, being always inversely proportional to the 

 velocity. Again, it appears evident that the chances for C 

 having a great or small initial velocity at B are exactly the 

 same as those for A. Hence, if we compare the velocities of 

 A and C at an instant arbitrarily chosen, the probability of 

 our happening on a time when A is moving slowly may be 

 less than that of our happening on a time when C is moving 

 slowly, and we cannot conclude that the mean kinetic energy 

 of A is greater than that of C ; indeed, a comparison of this 

 case with that of the atmosphere, would lead us to expect that 

 the mean kinetic energies of A and C would be equal. 



There are cases in which the Boltzmann-Maxweil distribution 

 does not hold. For instance, the case of a large particle con- 

 fined at the end of a tube, with numerous small particles bom- 

 barding it. The mean kinetic energy of the large particle 

 will depend on the range of its motion in the tube. This 

 example would suggest the conclusion that in such cases a< 

 gases in contact with solids and liquids, where the molecules 

 of the latter are so confined by molecular forces as to approxi- 

 mate to the condition of the large particle at the end of the 

 tube, the conditions of temperature equilibrium can hardly be 

 determined by the Boltzmann-Maxweil law. 



40 Trinity College, Dublin. Edw. P. Culverwell. 



Poincare's Thermodynamics. 



Rentkant a Paris apres une assez longue absence, je prends 

 seulement connaissance de la derniere lettre de M. Tait. Je ne 

 veux pas continuer une discussion qui ne saurait se prolonger 

 sans degenerer en une simple logomachie. II resulte en effet dcs 

 debats que M. Tait n'attribue pas le meme sens que moi a cer- 

 taines expressions, et en particulier au mot force electromotrice. 

 II me semble seulement, puisque c'etait mon livre qu'il criti- 

 quait, que c'etait a lui d'adopter mon langage, qui est d'ailleurs 

 celui de tout le monde. Je m'anelerai done la, quoiqu'il 

 arrive. 



Je suis pourtant oblige d'insister sur un point, parce que je ne 

 veux pas laisser suspecter ma bonne foi. M. Tait a ecrit : 

 " Nothing is said, in this connection, about Joule's experiments." 

 En ne tenant pas compte de ces mots "in this connection," j'aurais 

 denature sa pensee. Ces mots ne m'avaient pas echappe. lis 

 signifient, si je ne me trompe : " dans ses rapports avec la deter- 

 mination de la temperature absolue." Et c'est pourquoi, apres 

 avoir rappele que j'avais decrit ces experiences a la page 164, j'ai 

 ajoute que j'avais explique a la page 169 comment elles per- 

 mettent de determiner la temperature absolue. 



PoiNCARt. 



[I need scarcely say that I never dreamt of doubting the good 

 faith of M. Poincare. What I did (and still do) doubt is my 

 having made my meaning clear to him. For I cannot see how 

 such a discussion could degenerate into a mere war of words. 

 So far as I understand myself, I have been dealing mainly with 

 the validity of certain modes of estai)lishing physical laws, 7iot 

 with the mere terms employed in describing the experimental 

 facts on which they are founded. — P. G. T.] 



Land and Freshwater Shells Peculiar to the 

 British Isles, 



There cannot be any reasonable doubt that the inland Mollusca 

 of Britain present some peculiar features, but it is surprising, con- 

 sidering the amount of attention that has been devoted to them, 

 how little exact knowledge we have of this subject. This want 

 of knowledge is doubtless due to two principal causes — first, 

 that so many conchologists consider varieties, and especially 

 slight varieties, to be of little or no importance ; and secondly, 

 because those who study our native shells are, as a rule, but ill 

 acquainted with foreign species and varieties. The publication 

 of a list of supposed peculiar forms in the new edition of Dr. 

 Wallace's "Island Life," will, it is hoped, direct attention to 

 this matter. Although this list is more or less provisional, and 

 will doubtless require much alteration as time goes on, I antici- 

 pate that the number of forms actually peculiar to our islands, 

 when fully ascertained, will considerably exceed eighty-three, 

 the number at present listed. On the other hand, no doubt, 

 several at present in the list will have to be eventually struck 

 out. 



