5^^. 



NATURE 



[June 24, 1920 



A Junior Course of Practical Zoology. By the 

 late Prof. A. Milnes Marshall and the late 

 Dr. C. Herbert Hurst. Ninth edition, revised 

 by Prof. F. W. Gamble. Pp. xxxvi + 5i7. 

 (London : John Murray, 1920.) Price 12s. net. 

 The principal change in the new edition of this 

 admirable and well-established text-book is the 

 substitution of Dipylidium caninum for Taenia as 

 an example of a tapeworm. This change has no 

 doubt been determined largely by the common 

 occurrence of Dipylidium and the consequent 

 facility in obtaining sufficient material — especially 

 scolices — for class purposes. The account is illus- 

 trated by a page of good figures, but there is an 

 error in the magnification given for Fig. 3. From 

 the point of view of the organs of the segment, 

 Dipylidium — ^with two sets of reproductive organs 

 in each segment, and the uterus subdivided into 

 capsules in the mature segment — is not so good 

 as Taenia as a type for study by junior students, 

 and for the convenience, of those teachers who 

 prefer the latter type a brief description of the 

 organs of the segment of Taenia might be added 

 at the end of the account, together with Figs. 4 

 and 6 on p. 47 of the previous edition. 



In spite of the care with which the book has 

 been edited, a few slips have escaped attention — 

 e.g. on p. 12 "Monocystis belongs to . . . the 

 Sporozoa or Gregarines," as if these two terms 

 — one relating to a class, and the other to an 

 order in the class — were synonymous ; there is 

 the loose statement on p. 33 that in Obelia some 

 of the buds "have no mouth and become 

 medusae " ; and the amount of acid given in the 

 formula for acid alcohol is incorrect owing to 

 the omission of a decimal point. 



Letters to the Editor. 



[The Editor does 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 Nature. No notice is 

 taken of anonymous communications.} 



The Separation of the Isotopes of Chlorine. 



Mr. Chapman's conclusion (Nature, June 17, 

 p. 487) that the isotopes of chlorine should on certain 

 assumptions be capable of separation by chemical 

 means, is clearly wrong, unless there is something 

 not stated in the reasoning to prevent it being applied 

 to the case of a "pure" element, such as, for 

 example, according- to the results of Aston, fluorine 

 actually is. Denoting an entirely imaginary difference 

 between two kinds of fluorine atoms by F and F', 

 the reasoning seems to lead in this case to the 

 obviously absurd result that these two kinds of iden- 

 tical atoms with a purely imaginary difference must 

 be capable of separation by chemical means. 



The error appears to be' in the equilibrium equa- 

 tion (i) 



[ci,][cig=[ci.ciT. 



Mr. Chapman does not show how he deduces this, 

 and it is of interest to know whether the error is due 

 to a slip in the apoHcation of the theory of chemical 

 equilibria to the case or to a fundamental flaw in 

 that theory. In the present rase, if the isotopes are 

 NO. 2643, VOL. 105] 



assumed, to be chemically identical and the distribu- 

 tion of the two kinds of atoms in the molecules due 

 to pure chance, then if n is' the fraction of CI atoms 

 and {i~n) that of the CI' atoms, the fraction of 

 CI2 molecules is n.^, of the Cl'2 molecules (i — n)^, and 

 of the CI. CI' molecules 2n(i— n). This gives 



[cy[ci',]=i[ci.ciT, 



which is in accord with the ratio 9 to i to 6, stated 

 by Merton and Hartley for the case n=o-75 (Nature, 

 March 25, p. 104), and with Mr. Chapman's own 

 equation (iv) deduced from the assumption that the 

 isotopes are non-separable. Frederick Soddy. 



A Possible Cause for the Diamagnetism of Bohr's 

 Paramagnetic Hydrogen Atom. 



One of the difficulties which confronts Bohr's 

 structure of the hydrogen atom is the fact that 

 hydrogen, on his hypothesis, should be paramagnetic,, 

 whereas it is, like the majority of the simple gases, 

 diamagnetic. Experiments on the magnetism of gases 

 have, however, always been made above absolute zero,, 

 and the atoms must therefore be in motion ; and if 

 this motion involves vibrations and rotations of the 

 atom as well as translatory movements, then it is 

 possible for the atom to appear diamagnetic, although 

 it may be inherently paramagnetic. In a paper on 

 "The Mean Magnetic Moment and Mean Energy of a 

 Vibrating Magnet" (Mems. Manchester Lit. and Phil. 

 Soc, vol. Ivii., 1913, No. 4) I considered in a simple 

 case how such an effect might arise if a magnet 

 were in a uniform field and free from the influence 

 of neighbouring magnets. In these circumstances^ 

 when the vibrations exceed 130° on either side of 

 the position of rest, or if the vibrations pass into 

 rotations, then the magnet will appear to be dia- 

 magnetic^ because the average time during which the 

 positive and negative poles are in the diamagnetic 

 position is longer than the average time during which 

 they are in the paramagnetic position. 



Honda (Phys. Rev., Ser. 2, xiii., 1919) has recently 

 examined at length the effect of all the possible rota- 

 tional movements of a magnet in his kinetic theory 

 of magnetism, and, with certain assumptions as to 

 the shape of the atom, comes to the same conclusion. 



Applying this result to a paramagnetic atom, it is 

 possible that such an atom, in virtue of its motion, 

 may appear to be diamagnetic, and the fact that 

 hydrogen is diamagnetic may be quite consistent with 

 Bohr's tJaramagnetic model of the atom. 



The kind of diamagnetism here considered, which 

 may be called pseudo-diamagnetism, differs from that 

 due to induced electric currents in the atom, which 

 may be regarded as true diamagnetism. Pseudo- 

 diamagnetism will be subject to variation with 

 changes of temperature and with the state of aggrega- 

 tion of the atoms, while true diamagnetism is probably 

 independent of these. 



If the diamagnetism of hydrogen should be found 

 to change at a verv low temperature and in a very 

 strong field, it would show that the diamaj?netism of 

 this gas was probably an effect of the motion of its 

 atoms, and such a result would indirectly help to 

 confirm Bohr's view of the structure of the atom by 

 removing a difficulty. J. R. Ashworth. 



Rochdale, June 8. 



A Stalked Parapineal Vesicle in the Ostrich. 



The ostrich chick on hatching displavs an oval, 

 dark-coloured, bare patch towards the hind part of the 

 head. Later, it tends to be hidden by the thick 

 growth of hair-like feathers which cover the head 



