October 27, 1898] 



NATURE 



619 



of our solar system which cannot be self-generated, the possi- 

 bility of having evidence of anti-matter in comet tails and 

 coronal streamers, and the idea of potential matter. 



Arthur Schuster. 



Solar Radiation. 



At the conclusion of his British Association lecture on Phos- 

 ]ihorescence,^ Mr. Jackson makes a suggestion with regard to 

 solar radiation which will doubtless receive due attention from 

 those who are interested in solar physics. It is one of especial 

 interest to me because, by an entirely different train of thought, 

 Mr. Jackson has arrived at a possible explanation of the relation 

 between sun-spots and terrestrial magnetic disturbances which is 

 practically identical with a suggestion I have recently put 

 fcuward in a paper on "The cause of the darkness of sun- 

 spots," published in the Astrophysual Journal {k.-\^xA 1897). 



In this paper I attempted to show that absorption by relatively 

 cool material offers no satisfactory explanation of the darkness 

 of sun-spots, and that the spectroscopic evidence is really quite 

 compatible with a relatively high temperature even in the umbra 

 of a spot. 



But in abandoning the absorption hypothesis, one is brought 

 face to face with an apparent contradiction of KirchhofTs law. 

 Thus it is certain from the low mean density of the sun that the 

 interior region under enormous pressures must be vastly hotter 

 than the photosphere. If, therefore, spots are really breaks in the 

 photospheric clouds through which we obtain a glimpse of the 

 interior, why is it that the radiation from them is apparently so 

 much less intense than from the photosphere ? The clouds of con- 

 densed matter may, of course, possess a much higher radiating 

 power than the gaseous mass below them ; but this, according 

 to Kirchhoff's law, should be entirely compensated by the 

 enormous depth of the feebly radiating interior mass. 



To meet this difficulty I suggested that the radiation from the 

 interior, at the transcendent temperatures which must exist 

 even a few thousand miles below the sun's visible surface, may 

 possibly not be apparent as visible light, but may occur in 

 wave- frequencies of a higher order than the known spectrum ; 

 and " may be effective in producing those magnetic disturb- 

 ances which are characteristic of large umbrce." 



Mr. Jackson however, if I have rightly understood him, 

 supposes that it is not so much a question of temperature as of 

 molecular structure that determines the wave-frequency of the 

 radiation ; and he regards the light of the photospheric clouds 

 as a phosphorescent glow induced by undulations of a high 

 order of frequency which are emitted by the simpler uncon- 

 densed materials. The condensed clouds containing more 

 complex molecular groups acting as a screen, and converting the 

 invisible radiant energy of high frequency into ordinary light. 



With regard to this interesting speculation, one would like to 

 know more particularly what is the nature of the evidence on 

 which the idea is based that very simple molecular systems give 

 rise to undulations of high frequency ? There can scarcely be 

 any analogy between the behaviour of matter in highly ex- 

 hausted tubes and under the enormous pressures and tempera- 

 tures which must exist within the photosphere. 



The case of the phosphorescent limes is an exceedingly 

 interesting one ; but is there any ground for the belief that the 

 lime obtained from organic salts, and giving a blue phos- 

 phorescence, is really simpler in molecular structure than a lime 

 which glows red ? J. Evershed. 



Kenley, Surrey, October 14. 



Hibernating Reptilian Embryos. 



Will you allow me space to correct an error that has crept 

 into the account given in the Christchurch Press, and reprinted 

 in the last number of Nature (p. 609), of Prof. Dendy's 

 successful investigation of the development of the egg of the 

 Tuatara lizard, Sphenodon. 



The fact of an embryo hibernating within the egg was not, as 

 stated, unknown among vertebrates, an exact parallel being 

 offered by no less well-known a reptile than the European pond- 

 tortoise {Emys orbicularis). This was first observed in Austria, 

 in the last century, by Marsigli, whose statement has been 

 corroborated by Miram in 1857, eggs laid in his garden at 

 Kieff in May hatching eleven months later, and by Rollinat in 

 1894, the latter author concluding that hatching does not, as a 



rule, take place in France before the twenty-second or twenty- 

 third month after oviposition. 



I need hardly add how pleased I feel at the result of Prof. 

 Dendy's investigations showing the close resemblance which 

 the development of Sphenodon bears to that of the tortoises, 

 since I believe to have been the first systematist to follow 

 Cope (1885) in placing the Rhynchocephalia in close proximity 

 to the Chelonia with the remark : " The affinities of the 

 Rhynchocephalia to the Chelonia are at least as great as to the 

 Lacertilia" (Cat. Chelon., 1889, p. i). 



G. A. BOULENGER. 



British Museum (Natural History), October 23. 



NO. 



1 Nature, October 6, p. 562. 

 513. VOL. 58] 



Organic Variations and their Interpretation. 



I SHOULD be glad if Mr. Cunningham would tell us upon 

 what evidence he founds his opinion that, in crabs, "it is 

 certain that the number of ecdyses depend on age, not on size." 



This assumption lies at the base of Mr. Cunningham's 

 criticism of Prof. Weldon's arguments ; but, even apart from 

 that, the matter is one of such general biological interest that 

 I hope he will respond to an invitation to substantiate a view 

 which to me, at any rate, is altogether novel. I have always 

 understood that exuviation was a phenomenon essentially con- 

 nected with the process of growth in Crustacea rather than with 

 the mere passage of time, and it is needless for me to remind 

 Mr. Cunningham of the familiar facts and published statements 

 which support this generally accepted view. Will Mr. Cunning- 

 ham, on the other hand, tell us how many cases of exuviation, 

 unaccompanied by growth, he has observed among Crustacea ? 



Unless Mr. Cunningham can revolutionise the present state 

 of knowledge on this subject, his criticism, based on the 

 greater relative growth of young crabs in 1893 than in 1895 and 

 1898 (which in itself is probable enough), falls to the ground ; 

 for he admits that "change in the proportions of a crab occurs 

 only at the ecdysis." In assuming that, on the whole, similarity 

 of size in young shore-crabs indicates an equal number of moults, 

 Prof. Weldon appears to me to be quite in accord with our 

 present knowledge of the subject. Certainly— to modify Mr. 

 Cunningham's phrase — the frequency of exuviation in different 

 Carcini corresponds much more closely with their relative 

 growth than with the periods of time occupied. 



Plymouth, October 22. Walter Garstang. 



Wall Mirages. 



Mr. R. W. Wood, who describes a mirage on city pave- 

 ments, in Nature of October 20 (p. 596), may like to refer to 

 the second volume of Nature (p. 337, August 25, 1870), where 

 he will find an account of mirages seen by looking closely along 

 a wall, which was exposed to a hot afternoon sun. The mirage 

 must be very common, and needs only looking for. Mr. Wood's 

 interesting letter may lead others to photograph this curious 

 phenomenon in our own country. A wall will be easier to deal 

 with than a pavement. C. T. Whitmell. 



Leeds, October 22. 



A White Sea. 



I have received several letters respecting this phenomenon 

 (see p. 496), and have distributed the samples of water to two 

 gentlemen who were desirous of examining it. Will you per- 

 mit me to say that subsequently I received another application 

 from a bacteriologist on the continent, and that the letter was 

 unfortunately lost before complete perusal. Hence my failure 

 to acknowledge its receipt. James W. Barrett. 



22 Cavendish Square. 



SURFUSION IN METALS AND ALLOYS > 



THE author points out that metals and alloys may be 

 maintained in a fluid state at temperatures which are 

 many degrees below their true freezing points, and states 

 that this fact has been but little studied. As regards 

 salts, the question of surfusion has recently received 

 much attention. Ostwald {Zeit.fiir Physikal. Chem., 1 897, 

 vol. xxii. p. 3j has shown, as the result of an investigation 



1 "Surfusion in Metals and Alloys." By Prof. W. C. Roberts-Austen, 

 C.B., F.R.S. (Abstract of a paper read at the Royal Society, May 26. 



