6io 



NA TURE 



October i8, 1906 



from the conjugant, and only in the first days of the con- 

 jugating fit is there an approach to a very sl-ight secondary 

 mode at the conjugating type value. This may correspond 

 in the first days to a very small percentage of individuals 

 in a '■ conjugating mood " among the non-conjugants. 

 The skewness, however, as measured by Dr. Pearl's 

 numbers, although slight, is the oihcv way, showing that 

 the non-conjugant population might best be conceived as 

 a distribution ii'anting a portion about the conjugant type, 

 and not as a population with an addition on that side, 

 as it must be in the case of a mixture of non-conjugants 

 and potential conjugants. Taking the variability of (a) all 

 populations in w-hich there were no conjugants, (b) popula- 

 tions of non-conjugants in which conjugants appear, and 

 (c) populations entirely consisting of conjugants, we have 

 the three numbers 8-7, S-6, and 7-9, which suffice to show 

 that non-conjugants in a conjugating population are' prac- 

 tically identical in variability with non-conjugants in a 

 non-conjugating population, i.e. the potential conjugate is 

 a very small proportion of the population, conjugation 

 talving place rapidly after the conjugating phase is reached. 



I am sorry to have to reply to Mr. Lister in this fashion. 

 I fear, to use his own phrase, he will still " go away 

 unedified " from " the biometric side of the church." But 

 the time has come when vague insinuations based on no 

 complete study of biometry must be replaced bv some 

 attempt to understand before criticism is passed, .\bove 

 all, in a case like the present, a total disregard of the 

 contents of Dr. Pearl's memoir and a suggestion that he 

 has made errors and overlooked difficulties, which he has 

 actually dealt with at every turn, is not to the credit of 

 the critic. A man who has spent years in studying Para- 

 maecia, and made thousands of measurements after much 

 consideration of the difficulties, may reasonably expect a 

 different type of criticism from another who clearlv has 

 attempted no such series of measurements, and whose 

 authority for c.v cathedra utterances may therefore be well 

 called into question. Dr. Pearl's full paper is now in 

 type, and I do not think his reputation will suffer when 

 the paper is tested against the a priori criticisms which 

 Mr. Lister has passed upon it. Karl Pearson". 



Biometric Laboratory, University College, London, 

 October 12. 



Radium and Geology. 



In Nature of October 11 (p. 585) two letters appear on 

 this subject, in reply to which a few words mav perhaps 

 usefully be said. Mr. Fisher's principal point Is that if 

 the earth's internal heat is maintained by radium, there 

 is no room left for that shrinkage of the globe bv cooling 

 which some geological theories require. I think that the 

 difficulty is only apparent. The duration of radium, it is 

 generally agreed, is limited to a few thousand vears. 

 The supply must be in some way maintained, or there 

 could be no radium on the earth now. Writers on radio- 

 activity are generally agreed that the radium supplv is 

 kept up by the spontaneous change of uranium into radium. 



Since radium is found in ordinary rocks, we must, on 

 the received theory, suppose that uranium also exists in 

 these rocks. It may be objected that uranium is never 

 entered as one of the constituents found by chemical 

 analysis. But, since the quantity to be expected is only 

 of the order of i/ioooth of i per cent., this is not sur- 

 prising. It might be possible, by very special methods, to 

 detect uranium in granite, but I think in any case we may 

 feel confident that it is there. 



Everything depends on this initial supply of uranium. 

 It gradually passes into radium, and, after that, into some 

 inert form. The supply of uranium cannot last for ever. 

 Its gradual diminution must involve the cooling and 

 shrinkage of the globe. 



It may perhaps be thought in these circumstances 

 illegitimate to equate the escape of heat per second from 

 the earth to the supply generated by radium in that time. 

 There is reason, however, to feel pretty sure that thermal 

 equilibrium is practically established in a time small in 

 comparison with the duration of uranium, so that the rate 

 of change in the amount of the latter can have no appreci- 



NO. IQ2g, VOL. 74] 



able influence on the distribution of temperature in the 

 globe at any moment. 



Mr. Palmer suggests that if the earth's internal heat 

 is due to radium, the moon ought to be internally hot too, 

 and its volcanoes should be active. I discussed the ques- 

 tion of the moon's internal heat in my first paper (Proc. 

 Roy. Soc, A, vol. Ixxvii., p. 472). I quote from that 

 paper : — " It has generally been supposed that the lunar 

 volcanoes are extinct. But that view seems to rest chiefly 

 on an a priori conviction that the moon has no internal 

 heat. As Prof. W. H. Pickering has pointed out, all those 

 observers who have made a special study of the moon 

 have believed in the reality of changes occurring there." 



Even if there were good reason to be sure that the lunar 

 volcanoes were extinct, that would still be inconclusive. 

 For it is believed by many geologists that volcanic action 

 is due to the penetration of surface water to the hot 

 interior of the globe. Thus volcanic inertness may be due, 

 not to the absence of internal heat, but to the absence of 

 surface water. R. J. Strutt. 



The Rusting of Iron. 



In my remarks on "The Rusting of Iron," published 

 in Nature of September 27, I directed attention to the fact 

 that pure hydrogen peroxide solution was rapidly decom- 

 posed by cast-iron, the latter becoming covered with rust. 

 This, I stated, " was, no doubt, due to catalytic action." 



In his friendly criticism of my remarks. Dr. Gerald T. 

 Moody writes, in Nature of October 4, " that the metal 

 beconies covered with rust in a few minutes, is not, how- 

 ever, to be referred to catalytic action, as Mr. Friend 

 suggests, but is a consequence of the formation of acids 

 by the oxidation of some of the impurities present in the 

 iron, and of the subsequent electrolytic action." 



That acids are formed in the above manner may be re- 

 garded as certain. These attack the iron, forming minute 

 quantities of salts, which are decomposed by the oxygen of 

 the peroxide, yielding rust, and liberating the acid, which 

 can now attack more iron. In this way a small quantity 

 of acid may be instrumental in oxidising a large quantity 

 of iron. In other words, the acid is a catalyser, and the 

 reaction is analogous to the rusting of pure iron in the 

 presence of carbonic acid, oxvgen, and water. The par- 

 ticular acid or acids which will cause this catalytic action 

 must depend, of course, on the sample of iron used. 



For the same reason " the intensity of action will be 

 determined by the amount of acid formed on the surface 

 of each particular sample of metal, when in contact with 

 the peroxide." It is thus unnecessary to assume an 

 electrolytic action, as Dr. Moody suggests. This is sup- 

 ported by the fact that the same result may be obtained 

 bv employing pure iron, and commercial hydrogen peroxide, 

 which invariably contains hydrochloric acid and other 

 impurities, as Dr. Moody has himself pointed out. 



Wiirzburg, October 9.' J. Newton Friend. 



Optical Illusions. 



In your issue for September 27 a description of some 

 optical illusions furnished by revolving fans recalled to 

 my mind a very powerful illusion which I noticed some 

 time ago, but for which I have not been able to furnish 

 a satisfactory explanation. 



.A thaumatrope card (i.e. a card having a cage pictured 

 on its one side and a bird on its other side) was mounted 

 so as to turn round a vertical median axis at a speed of 

 about two revolutions a second. 



When an observer, viewing the rotating card from a 

 distance of 5 feet or more, shuts one of his eyes, the card 

 appears instantly to reverse its direction of rotation. (.At 

 the same time the axis of rotation appears to tilt a little 

 away from the vertical.) On reopening the closed eye the 

 illusion vanishes, and the card again appears to assume its 

 true direction of rotation. 



I showed the illusion to several friends, who all agreed 

 as to its striking perfection. Douglas Carnegie. 



Newcastle-on-Tvne, October 10. 



