March 23, 1893] 



NA TURE 



487 



slowly be effected in the condition of the surface through earth- 

 falls and landslips" . . "until all the more striking and abrupt 

 •irregularities have disappeared from their action." 



Now it is precisely at cliff faces and steep slopes that we should 

 best see the real colour, if any, of the superficial strata, and what 

 do we find ? Wherever we turn, from pole to pole, there is an 

 entire absence of colour ; they are white and at times as brilliant 

 as "new fallen snow." If we scan the vast cliffs of the 

 "Apennines," say at sunset, for hundreds of miles, rising to 

 Sooo or 10,000 feet, with peaks up to 20,000, they are white, 

 seen in sunshine. 



If we examine the cliffs of the Sinus Iridium highlands, the 

 huge array round Mare Crisium, or indeed anywhere else, it is 

 the same, and without a doubt it demonstrates to us that the 

 outer strata on the moon are of the same white material all 

 over the globe. Precisely where degradation is most certain, and 

 where the true colour of the strata would be distinctly visible, 

 there we find the most extraordinary and invariable whiteness 

 for a thickness of at least two or three miles. 



A remarkable feature of the case is that, as a rule, all cliffs 

 are much whiter than the general surface around them. In the 

 raised ramparts of craters and walled plains, it is well known 

 that the outer, and more gradual slopes, are invjtriably darker 

 than the steep inner cliffs facing the enclosure. In Aristarchus, 

 Theophilus, and such like rings, at sunrise, this is very conspi- 

 cuous, especially in photographs, and it is not easy to account 

 for this peculiar feature (evidently the result of disintegration 

 and removal of the surface by gravitation) except by the supposi- 

 tion that the outer surface all over (and excepting rays and 

 nimbi) is snow stained by meteoric dust. " It is well known 

 that the fall of meteoric dust on our earth is very considerable, 

 and estimated by Dr. Kleiberg, of St. Petersburg, at about 

 11,435 tons per annum. It has been found on all our ocean 

 bottoms, and on our polar snows, where it is soon overlaid or 

 removed by winds. On the moon, however (where there is no 

 wind and now no snowfall), it could accumulate for many 

 thousands of years, at least on levels, and so stain them very 

 perceptibly.' 



Undoubtedly we see the true colour of the surface layers at 

 the cliff faces, but unless the outer surface were stained in some 

 way their bright contrail would be impossible. 



Hence I take it that the outer layer of the surface all over, for 

 at least one or two miles in thickness, is formed of snow, 

 stained outside by a deposit of meteoric dust, the accumulation 

 of many thousands of years, the removal of which, by gravita- 

 tion, at cliffs causes their brightness, and this would explain the 

 perennial enigma of where all the water has gone. 



At low temperature neither ice nor snow vaporise, even in 

 vacuo, and also that at low temperature ice is a non-viscous 

 solid (like glass) has been experimentally demonstrated by Mr. 

 T. Andrews, F.R.S., and the results laid before the Royal 

 Society ^see Nature, vol. xlii. p. 214). The prevailing white- 

 ness, therefore, of the lunar cliffs and steep inclines would seem 

 to be a powerful argument against a " volcanic" surfacing to our 

 satellite, and a good one in favour of glaciation. 



The question of maximum surface temperature under fourteen 

 <lays' solar heat has undergone a startling change since Lord 

 ■Rosse's classical experiment. The possibility of snow existing 

 on the moon is now admitted by leading astronomers, since the 

 researches of Profs. S. P. Langley and F. W. Very, of the 

 Allegheny Observatory, have demonstrated that the maximum 

 •may be so low ihat the ;«^a« temperature may possibly be below 

 - ioo°C. 



The old volcanic " selenology " is dying ; there is no hope of 

 any xx\o\& progress in it (and that is the great sign of life in all 

 branches of science nowadays) ; it is fossilised. That a "new 

 selenology " is badly wanted is pretty obvious to all who look 

 into the question. The surfacing of our own satellite, one of the 

 •most conspicuous and easily seen objects in the heavens, is still 

 the standing enigma. S. E. Peal. 



Sibsagar, Asam, February 8. 



THE CROONIAN LECTURE. 



MUCH interest has been excited not only among 

 men of science but among the general public by 

 Prof. Virchow's visit to England.* From the moment 



NO. I 22 I, VOL. 47] 



when it was announced that he had agreed to deliver 

 the Croonian Lecture, it was universally felt that it 

 would not do to lose so good an opportunity of doing 

 honour to an illustrious investigator. Prof. Virchow is 

 known, of course, chiefly as a pathologist. He is the 

 founder of the science of pathology in the sense in which 

 : it is now everywhere understood and taught ; and it 

 j would be difficult to form too high an estimate of 

 the value of this part of his labours. But Prof. 

 I Virchow is one of those men of genius who never find in 

 '\ any one department of researcli a sufficient outlet for 

 their energies. In archaeology, anthropology, and ethno- 

 logy he has been for many years one of the foremost 

 workers of the age, and he has brilliantly represented 

 science in the political life of Prussia and the German 

 Empire and in the municipal life of Berlin. As a teacher 

 in the Berhn University, of which he is now Rector 

 Magnificus, he has done much to foster a genuinely 

 scientific spirit among the pupils who have flocked to 

 his class-room ; and as a writer he has command of so 

 pure and attractive a style that he has been able to ex- 

 ercise a wholesome and stimulating influence on the 

 : intellectual life even of classes to whom science does not 

 \ usually make a very strong appeal. Altogether, Prof. 

 1 Virchow's career is one of which Germany has good 

 I reason to be proud. In him she possesses one of those 

 rare and potent thinkers who touch no subject without 

 giving it fresh significance, and who have the secret of 

 I awakening in other minds something of their own en- 

 j thusiasm, independence, and vigour. 



There was so great a demand for tickets that arrange- 

 ments had to be made for the delivery of the Croonian 

 Lecture in the theatre of the University of London ; and 

 [ here — on Thursday, March 16— a crowded audience 

 I listened with the deepest interest to what Prof. Virchow 

 I had to say about the great subject in the development of 

 j which his researches have marked so splendid an era. 

 I In the evening a public dinner was given in his honour at 

 j the Hotel M(ftropole. Lord Kelvin presided, while the 

 [ Presidents of the Royal Colleges of Physicians and 

 j Surgeons acted as vice-chairmen. In proposing the 

 I toast of the evening, Lord Kelvin said he was one of 

 those who had listened with rapt attention that day 

 to the lecture delivered by Prof. Virchow. The mystery 

 he dealt with remained a mystery, but they were con- 

 scious of no feeling of disappointment. Though it was 

 i not for any man to tell them what life was, they had 

 j been brought nearer than ever to the solution of that 

 ! fascinating problem by the researches of Prof. Virchow. 

 j Mr. Huxley, Sir James Paget, and Sir Andrew Clark also 

 \ spoke of Prof. Virchow's magnificent discoveries. 

 I Prof. Virchow, in responding to the toast, ex- 

 j pressed the pleasure he felt in being welcomed by 

 " so large and so illustrious an assembly of the learned 

 I men of England." " Abroad," he said, " he had never 

 i seen anything like it." English men of science do not 

 j often indulge in demonstrations of this kind, and it is 

 I satisfactory to know that when they do try to show what 

 ! they think of a great investigator their achievement does 

 i not fall short of anything done with a like intention in 

 I Paris or Berlin. 



This week Prof. Virchow has been in Cambridge, where 

 he has been received with as much enthusiasm as in 

 j London. On Tuesday the University marked its sense 

 I of the importance of his labours by conferring on him 

 the honorary degree of Doctor in Science The follow- 

 ing is the speech delivered by the Public Orator, Dr. 

 Sandys, in presenting Prof. Virchow for the degree :~ 



Dignissime domine, domine Procancellarie, et tota Aca- 

 demia : — 



Universitatis Berolinensis Rector Magnificus, vir non modo 

 de medicina et salute publica, sed etiam de anlhropologia, de 

 ethnologia, de archaeologia praeclare meritus, vir et sexagesimo 



