Sbptembkb, 1902.] 



KNOWLEDGE 



203 



starlight cannot vary from time to time, and if the liglit 

 of the sky is different on one occasion from that which it 

 is on another, allowance, of course, being made for any- 

 annual variation, due to the Milky Way or some of the 

 brighter constellations being in iin especially favourable 

 position, then this variation in luminosity must be due to 

 some cause other than starlight. Over and above, there- 

 fore, the two very important researches (a) of the relative 

 brightness of different portions of the heavens, and (6) of 

 the total sum of starlight, there will come the question as 

 to whether there is iu addition any variable source of 

 luminosity, and, if so, what are its nature and urigiu, and 

 the laws aud causes of its changes. 



THE "TRIPLE CAVE" IN AQUILA. 



By Dr. Max Wolf. 

 The writer discovered these very interesting dark 

 structures in the Milky Way in Aquila on the 12th of 

 July, 1891, using a Krauz o-inch photographic doublet. 

 Their discovery was published for the lirst time at the 

 meeting of the Astronomische Gesellschaft at Munich in 

 1 892. The liroadest arm of the dark structure appears as 

 if it were the nearest, and the smallest arm as if it were 

 the furthest from the observer, so that it would seem to 

 give a perspective view into space of the heavens in the 

 Milky Way. But this is probably a mere illusion. 



The accompanying plate is a reproduction of part of the 

 original negative made •with the worse of the two Brashear 

 l(3-iuch portrait lenses. The plate taken with the better 

 lens shows a defect iu the centre of the structure, aud is 

 therefore not suitable for reproduction. A bubble iu the 

 glass of the negative from which the present reproduction 

 is taken shows itself as a slight defect, but does not 

 interfere with the rei:)resentation of the dark structure, 

 and the photograph seems therefore good enough for 

 j)ublication. 



The broadest of the three "caves" is the most southerly. 

 The co-ordinates of the middle "cave " are — 



R.A. = 19h. 33m. Dec. = + 10° 35' (18.550). 



Tlie plates were exposed on July 10, 1901, for tliree and 

 three-quarter hours. Scale, one degree equals about 

 seven centimetres. 



Heidelberg. 



[The Editors do not hold themselTes responsible for the opinions 

 or statements of correspoudents.] 



VISIBILITY OF THE CRESCENT OF VENUS. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — I would like to add a word to the discussion 

 raised b_v Mr. Maunder' s interesting remarks on the 

 visibilitv of Venus. I was stationed for some time in the 

 remarkably fine air of Darjeeling, at an elevation of abotit 

 8000 feet. On any clear night when the Pleiades were 

 high up I could see eleven stars without difficulty ; I have 

 glimpsed twelve. The dividing of e Lync in that air was 

 easy, though it is generally not so in England. Aud when- 

 ever Venus was fairly high up I could almost certainly 

 pick her up within 5 minutes with the imaided eye, know- 

 ing only the general direction in which to look, and not 

 using my equatorial as a guide. In this way I have seen 

 her when crescented very near to overhead, and I entirely 

 failed to trace the least vestige of her true shape. With 

 a magnification of 3 the crescent was visible, but no more. 



As to Jupiter's satellites, I cannot help thinking thiit, in 

 a perfectly still clear air, with no glare, the outer area 



would be visible without optical aid if a fine wire were 

 fixed up aud used as an occulting bar — but I have not 

 tried this. J. W. Meares. 



"Writers' Buildings, Calcutta. 



THE SUPPOSED DISCOVERY OF ALUMINIUM 

 2000 YEARS AGO. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — The following paragraph appeared in " Echoes 

 of Science " in the Globe of August 19th, 1898 :— 



" The first discoverer of aluminium had the reward of genius. 

 Pliny tells us that in the reign of Tiberius (-11 D.c. to 37 A.D.) a 

 worker in metals presented a beautiful metal cup resembling silver, 

 but lighter, to the Emperor, who questioned him, and learned that 

 he had extracted the new metal from clay. The secret, he said, 

 was known but to himself and the gods. The sage Tiberius, 

 rellecting that if this metal could be made from earth it wovdd 

 lower the price of silver and gold, decapitated the artificer in order 

 that his secret might remain with the gods, and so deprived the 

 world of a most usefiJ metal for eighteen centuries." 



It would interest many readers of Knowledge if some 

 student of Pliny would state where in his works the 

 account is to be foimd, and furnish a translation thereof. 



That the metal of which the cup was formed was really 

 aluminium appears possible from four circumstances : — 

 CI) It was obtained from clay; (2) it resembled silver; 

 (3) it was lighter than silver; (4) it was capable of 

 being wrought into a vessel. 



The question naturally occurs, How did the metallurgist 

 of the first century obtain a metal, which in the twentieth 

 century can only be extracted by processes totally unknown 

 to the ancients, and which it is inconceivable that any 

 individual worker among them could have accidentally hit 

 upon ? Modern methods of in-ocuring aluminium are : 

 (1) Chemical, involving the use of sodium or potassium ; 

 and (2) electrical. The contemporary of Tiberius cannot 

 have knowingly isolated sodium or potassium, and then 

 applied it to separate aluminium, though it is possible tliat 

 sodium or jiotassium may have been liberated from some 

 ingredient of a mixture in his retort or crucible, only to 

 oust aluminium from some other ingredient. That his 

 process was electrical is quite out of the cjuestion. 



If Pliny's account actually refers to aluminium there 

 must be some method of separating it which modern 

 chemists have failed to light on, but which lay not very 

 far outside of the range of ancient metallurgical knowledge. 

 Cannot it be rediscovered by some chemist possessing a 

 wide knowledge of the science, and who is at the same 

 time well acquainted with the methods (many of them 

 forgotten) adopted by ancient and mediaeval workers ? 

 Such a process would probably go very far towards 

 cheapening that most useful metal. 



3rd July, 1902. John T. Kemp. 



Bvttt's^ #vnt«jologttal Notes. 



Conducted by W. P. Pycraft, a.l.s., p.z.s., m.b.o.it. 



Birds in Captivity. — The Zoologist for July contains 

 a really valuable and extremely interesting article by Dr. 

 A. G. Butler, in which ho recounts his avicultural 

 experiences for tbe past twenty years. He surveys in 

 turn the postures, bowings, and dances of birds when 

 courting, their songs, nest-building, sexual characters, and 

 habits in confinement. It is difllcult, from the mass of 

 information which Dr. Butler has brought together, to 

 select items for special comment, but his remarks on the 

 precocious development of some birds seem particularly 

 interesting. Thus he writes : " Some of the little Ploceid 



