638 



NATURE 



[October 24, 1907 



which this result is stated in " Thermodynamics " (§§ 124, 

 126, 156)? 



It cannot be deduced from the laws of thermodynamics 

 or the definitions of a perfect gas {§ 124). These leave 

 the change of entropy in the form of an undetermined 

 constant. 



It must necessarily be based entirely on experimental 

 evidence (§§ 126, 156). It is in all probability approxim- 

 ately true for actual gases, but of this the experimental 

 physicist is the only competent judge. .As applied to 

 " perfect gases " it should be regarded, in common with 

 Boyle's law, as one of the " definitions of a perfect gas," 

 a definition selected partly on account of its simplicity 

 and partly on account of its approximate agreement with 

 the properties of actual gases (§ 156). 



.An irreversible transformation does not, ipso facto, 

 imply a gain of entropy. Unless a compensating trans- 

 formation exists (§§ 50, 51), and unless the final result 

 involves nothing more than a loss of available energy, we 

 have no justification for applying the methods of thermo- 

 dynamic analysis. If diffused gases could never be 

 separated, we should have an instance in point ; but do 

 such exceptions exist? 



Mr. Burbury asks why should different gases behave 

 differently from different portions of the same gas? This 

 question must be decided by the experimental physicist, 

 subject to some further condition, e.g. that the gases are 

 in the presence of a liquid which dissolves one of them 

 or of a membrane which is permeable to one of them only. 

 In other words, the matter resolves itself into the question. 

 Why should the conditions of equilibrium of a gas in 

 such circumstances depend on its partial pressure instead 

 of on the total pressure of the mixture? 



If the experimental physicist had told me that the total 

 pressure, and not the partial pressure, was the determin- 

 ing factor, I should have asserted that no entropy was 

 gained by diffusion, and should have written zero as the 

 value of my constant C. 



But then we should have no vapour of water in our 

 atmosphere unless the temperature rose above the boiling 

 point of water. These, generally speaking, are the views 

 which the book was intended to convey ; but may I direct 

 attention to the large number of open questions in thermo- 

 dynamics that have hitherto only received scanty attention 

 in the hands of mathematical physicists? 



G. H. Bryan. 



In the passage in my review to which Prof. Bryan 

 takes exception I had in my mind his definition of avail- 

 able energy at p. 35 and p. 43 : — " The available energy 

 of a system under given conditions is the quantity of 

 energy which tinder these conditions can be converted into 

 work"; and in the same passage the conditions are also 

 spoken of as " external " conditions. Let the system 

 consist of two gases occupying equal halves of a cylinder, 

 both at the same temperature and at pressure p, separated 

 by a piston impervious to either, and the whole surrounded 

 by air at the same pressure p. It seems to me to be 

 impossible under those conditions to convert any of the 

 energy of the system into work ; but if it can be' done, it 

 must be possible to explain how. The context of p. 125 

 does not seem to me to explain it. 



S. H. BURBURV. 



The Nomenclatur"" of Radio-activity. 



The name " ionium " which Dr. Boltwood proposes 

 for the new radio-active element, of which he announces 

 the discovery in N.vture of October 10, is open to serious 

 objections. I do not mean merely linguistic objections — 

 it is too late to consider them ; beside such a hvbrid as 

 " ionisation " the philological barbarity suggested by Dr. 

 Bollwood is insignificant; but it is a first principle of 

 scientific nomenclature that a name should connote some 

 of the distinctive properties of the thing named. A 

 thoroughly satisfactory system for naming radio-active 

 elements has not been put forward, but that adopted by 

 Prof. Rutherford in designating the members of the scries 

 descended from radium is at least better than none. 



According to this system, the products arising 

 successively from the disintegration of a radio-active 

 element are denoted by the name of that element followed 



NO. 1982, VOL. 76] 



by the letters X, A, B, C, &c. The principle of this plan 

 has been adopted by universal consent in the nomenclature 

 of the products of radium, thorium, and actinium, but for 

 historical reasons slight divergences from the simplest 

 form of the system have been permitted. Only one dis- 

 integration product of uranium (other than the radium 

 series) has been known hitherto ; its name, uranium X, 

 is in accordance with Prof. Rutherford's nomenclature. 

 Dr. Boltwood now announces the discovery of a descendant 

 of uranium subsequent to uranium X ; it appears to me 

 desirable that this product should be known as uranium A, 

 and should not be given any purely fanciful and meaning- 

 less name such as its discoverer suggests. 



Norman R. Campbell. 

 Trinity College, Cambridge, October 12. 



On Correlation and the Methods of Modern Statistics. 



In my last letter (October 3, p. 566) I ventured to 

 express the modest hope that " an astronomer may be 

 permitted to dissent from these applications of modern 

 statistical methods." Prof. Pearson refuses the desired 

 permission with such warmth of language and wealth of 

 argument that I find it difficult to make a suitable renewal 

 of the request. Perhaps I may be allowed to confine my 

 reply to the point of most general interest. 



With regard to the supposed relation between magnitude 

 and colour. Prof. Pearson wishes " to say a strong word " 

 about my criticism of a conclusion respecting the bulk of 

 the lucid stars, which I said was based on a record in 

 which the white stars had no frequency. I have re-read 

 Miss Gibson's paper, and am unable to see that my 

 criticism in any way misrepresents the facts. 



In section (3) of her paper Miss Gibson discusses the 

 relation between magnitude and colour, basing her results 

 upon the Cape list of 159 stars, from which all stars less 

 coloured than deep yellow are excluded. 



In section (4) .she fits various types of frequency curve 

 to statistics of counts of the lucid stars. In this part of 

 the paper there is no mention whatever of colour or 

 spectral type. 



At the end of section (4) we have the conclusion, to 

 which I ventured to take exception : — " Thus we have the 

 suggestion, even if it be only of the vaguest kind, that 

 the bulk of the lucid stars may belong to a separate 

 universe within which magnitude is not mainly deter- 

 mined by parallax or distance, but is more closely associ- 

 ated with colour, and thus probably with chemical or 

 physical condition." The phrase " hut is more closely 

 associated with colour " is undoubtedly there. If it does 

 not arise from section (3), its origin is " wrop up in 

 mystery"; if it does arise from section (3) my criticism 

 was not so unjustifiable as Prof. Pearson would with 

 strong words call upon the reader to believe. 



Arthur R. Hinks. 



Cambridge Observatory, October 18. 



New Zealand Birds. 



During the past twenty-five or thirty years many re- 

 ports have been published in regard to the extinction of 

 New Zealand birds, and an impression has gone abroad 

 that our avifauna, with its striking peculiarities and its 

 wealth of interest to ornithologists, will soon be lost. 

 Some time ago, when I was inquiring into the results of 

 the acclimatisation of English birds, I had thousands of 

 circulars distributed in all parts of the colony, and on 

 those circulars I placed questions dealing with the position 

 of the native birds, ^^'hen the circulars were returned to 

 me I found that every native bird was accounted for, in 

 some cases in many different districts. 



I feel, therefore, that I am able to sound a brighter 

 note than has been sounded by most writers on New 

 Zealand ornithology. From personal observations, I can 

 say that several species the extinction of which was 

 announced twenty years ago arc fairly plentiful, and are 

 increasing. I may mention specially the stitch-bird 

 (Pogonornis cincta), the bell-bird (.inihornis melanura). 

 the North Island robin (Jl/i>o australis), and the tui 

 {Prosthemadera novae zealandiae). 



I do not know of a single New Zealand bird which we 

 can sav with any degree of certainty has become extinct 

 since European occupation of the country, except perhaps 



