Janixrv 1, 1901. 



KNOWLEDGE. 



Hcttrrs. 



[The Editors do not hold themselves responsible for the opinioug 

 or statements of correspondents.] 



IS HUMAN LIFE rOSSIBLE UN OTHER PLANETS? 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — I have just tome acro.ss a jnissaije iii Grant 

 Allen's little l>iiok, " Tbe Eritish Barbarians," in which he 

 states that •' i>lanetosco|iists " are agreed that there is no 

 form of livin^r ereature iu any of the planets that can he 

 called •' human," because the j>hysieal conditions are in all 

 probability so different from those on this planet that the 

 life which probably does exist there, as liere, will have 

 followed a very different course of development or 

 evolution. 



I should be very glad if you could inform me in 

 Knowledge whether this statement of the views of those 

 who have made a special study of the planets is correct. 

 I had a itotion that it was considered that the physical 

 conditions of ilars (e.g., so far as they can be known or 

 guessed at) are so like those of this earth as to make it 

 prob;i.ble that human beings may be living there. 



Of course I know tliat a scientific man would be very 

 reluctant to commit himself to an opinion on a question 

 for the solution of which there is so little evidence avail- 

 able as there is for this question of possible human life 

 elsewhere than on this earth ; and for that vei-y reason 

 I thought Grant Allen's statement was too definite and 

 sweeping. 



Churchstantou Rectorv, Akxold D. Taylor. 



Honiton, October Slst, 1900. 



PHOTOGRAPHIC SEARCH FUR THE LEONIDS. 



Mr. J. Maclair iJoraston watched for the Leonids on 

 November 14th, from I2h. to 14h. ; -5 meteors were seen — 

 3 of them being Taurids, and 2 probably Leo Minorids. 

 "The result was that no true Leonids appeared." Mr. 

 Boraston also exposed a plate from 13h. 15m. until I4h. 

 The star trails are not, however, true curves, but show 

 slight irregularities, for which Mr. Boraston enquires the 

 cause. 



We have examined the photograph scut, and the 

 irregularities are manifestly due to small motions of the 

 camera.— E. W. M. 



CONSTITUENTS OF THE SUN. 



"Nemo" enquires whether the experiments of Prof. 

 Janssen, conducted at the summit of Mont Blanc, has 

 not proved that oxygen is wanting iu the suu. 



No ; Prof. Janssen showed that the !',reat absorption 

 bands given by the oxygen iu our atinosjihere are not 

 also given by alisorptiou iu the solar atmosjihere. But 

 the temperatures of the atmospheres of the two bodies are 

 so entirely different that this is no criterion. It simply 

 shows that oxygen does not exist iu the sun at terrestrial 

 temperatures. 



" Nemo " also finds the prominences an extremely 

 striking feature in the dark lines of calcium, as seen on a 

 photograph of the solar spectrum, and asks " Is it not 

 then safe to suppose that they are either wholly oi' partially 

 due to incandescent calcium vapour instead of hydrogen, 

 as it is usually stated ? " 



Certainly they are jiartially due to calcium, even largely 

 so. But their striking relief on the H and K lines is due 

 to the great breadth and darkness ot the corresponding 

 Fraunhofer lines of the solar spectrum. They are bril- 

 liantly seen on the hydrogen lines, notwithstanding that 

 the dark lines of hydrogen by no means ftiraish so good a 

 background for them. — E. W. M. 



jfloticcs of Boofes. 



'• AT.Mosi'iir.iuc H.voiATioN. " A Koscarch comiucted at the 

 AUeHbeny Observatory ami ,-it Providence, U.I., by Prof. Frank 

 W. Very.— 'I'hi.s research took its origin in a proljlcni suggested 

 l>y Prof. Cleveland Alilio to Prof. \'ery in IH'.n, as to whether 

 absorption is the absohitc inverse. of radiation for n.ascs. It 

 deals with a subject intimately connected witli that which Air. 

 "W. E. Wilson takes up in his Daramona Observations, lately 

 published, but whereas Mr. Wilson has to do with the radiation 

 from gases at the enormous temperature of the sun's surface, 

 Prof. Very tries to determine the absolute radiation in calories 

 from a unit mass of gas at given temperature and density and at 

 ordinary temperatures, not when burning, not when electrified, 

 but when simply heated. The difficulties of the problem arc 

 very gieat, for, as Prof. Very says, " The investigator here is 

 dealing with the invisil)Ie and the evanescent. In an optical 

 apparatus a little stray liglit immediately attracts attention, and 

 we proceed to trace it to its source with our eyes o])en. In our 

 study of feeble invisible radiations, on the other hand, we grope 

 in the dark, and only succeed in eliminating the unwelcome 

 extraneous rays after innumerable trials and errors.'' It is quite 

 impo.»sible here to give any account of the methods of research 

 or of all the results obtained ; we must confine ourselves to but 

 a few points which appear to bear directly on .solar physics. 

 As an absorbent of terrestrial r.adiation, arjucous va])OHr is' very 

 much more efficient than any other atmospheric ingredient ; 

 but as radiators when in large mas.ses, the substances composing 

 the atmosphere do not differ so widely as might be supposed, 

 since the facility with which a highly radiative vapour parts 

 with its heat is largely annulled by self absorption in its deep 

 layers of its own radiations, and since in gases heat is transferred 

 from molecule to molecule with the greatest ease, it is the feebly 

 radiative molecules which act as radiators, excejit iu the com- 

 paratively tluu outer layers. The depth of gas which gives 

 ma.ximum radiation at short range is an insignificant quality 

 compared with atmospheric dimensions, and radiation from 

 either the atmosphere of the earth or the solar chromo- 

 sphere is a superficial phenomenon, even when the masses of 

 heated gas measure thousands of miles iu thickness. The 

 fineness of the chromospheric lines in the solar spectrum, 

 although the shifts of the Fraunhofer lines indicate pressures of 

 many atmospheres at the base of the chromosphere, is a sufficient 

 demonstration that only the outer layers radiate. Prof. Very 

 also confirms and extends Air. Evershed's results on the heating 

 of iodine and its kindred vapours, that the light emitted by these 

 glowing vapours appears to give a ])erfectly continuous spectrum 

 while tile corresponding absorptiou-spectra are selective. "Thus 

 there is no such close relation between emission and absori)tion 

 as isimphed by Kirchotf's law of radiating bodies. There seems, 

 however, to be a general relation between the total absorbing 

 and radiating power for the visible rays.'' These points seem 

 to have a very clo.se bearing on the ([uestion of the relation 

 between the Fraunhofer and " Flash " spectra. 



•'TiiK Kiddle ok tiii'. U.nivkusk." By Ernst Haeckel. 

 English edition. (Rationalist Press Association.) (is. net. — 

 However unwiUing we may bo to accejitsoine of the conclusions 

 arrived at iu this book, it is impossible not to admire the man 

 who has written it. Towards the close of a life devoted to 

 science he has, in "The Riddle of the Univer.se,'' summed up in 

 popular form his views on all the great questions that ad'ect 

 mankind. His wealth of accumnlated knowledge enables him 

 to sweep from one great problem to another with astonishing 

 ease and power. He first briefly summarises the evidence of 

 evolution. Comparative anatomy and physiology alike forbid 

 us to separate man from the apes. Embryology puts it beyond 

 a doubt that men have been by gradual stages evolved from a 

 simple type of organism. After an enthusiastic appreciation of 

 the work of Darwin, he discusses the " Nature of the Soul, " 

 writing as a convinced monist, to whom dualism in any form is 

 an abomination. The soul cannot exist apart from the body, 

 nor the boily apart from the soul. " Athanatism,' the im- 

 mortality of the soul, is inconsistent with the very principle of 

 monism. The individual perishes, but according to the "law of 

 substance " matter is eternal. The chemical law of substance 

 must be us^ociated wiih the phy.siciil law, the conservation of 



