544 



KNOWLEDGE & SCIENTIFIC NEWS. 



[September, 1906. 



Central Asia would be overwlielmed by the sea. So far as 

 the races of mankind are concerned this niiffht prove of 

 immense benefit, and those whose ingenuity has suggested 

 the flooding of the Sahara might turn their attention with 

 greater benefit to tliis part of the Czar's dominions. So 

 long as tlie Straits of GibraUar remained open, a constant 

 current would pour down from the Atlantic, and many of 

 the salt steppes of Asia would be inundated. The subject is 

 an interesting one for s|)CcuIation, antl one may well re- 

 member that what has been in past geological times is 

 quite jiossible of repetition. 



ORNITHOLOGICAL. 



PHYSICAL. 



By W. P. PvcRAFT, A.L.S., F.Z.S., M.B.O.U., &c. 

 The Breeding of Humming Birds. 



Mr. Collingwood Ingram, in the August number of the 

 Aviciiltin-al Magtiziiie, gives a most interesting account of the 

 breeding of two species of humming birds in Trinidad — 

 'J'roihilus colubyis and Florisiiga atia. His remarks, however, 

 are mainly confined to a pair which had built on the bracket 

 of a lamp hanging over the door of Lady Northcote's resi- 

 dence. Thougli the lamp contained a powerful electric light 

 which was frequently turned on during the night so as to shine 

 full into the nest, the birds yet elected to stay, although when 

 sitting the hen was but a few inches from the globe ! The 

 more easily to keep a watch on the progress of the family a 

 mirror was arranged over the lamp. 



From Lady Northcote's notes it would appear that incuba- 

 tion took at least seventeen days, instead of the normal ten or 

 twelve. This extended period is, probably, as Mr. Ingram 

 suggests, due to the many alarms which the bird suffered when 

 sitting. 



Breeding of Rheas in the Zoological 

 Gardens. 



Once again the Rheas in the gardens of the Zoological 

 Spciety have brought off nestlings ; and all who are interested 

 in the problems of the coloration of nestlings should take the 

 first opportunity of inspecting these youngsters. 



Unfortunately they cannot be compared, side by side, with 

 the young of Darwin's Rhea, for the nestlings of these two 

 species differ very markedly, the latter being very conspicu- 

 ously striped, having a longer and more silky down, and a 

 dull grey median stripe along the underside of the body. 



Gadwall Breeding in Scotland 



Mr. H. B. Marshall, in the Field, July 28, records the fact 

 that a pair of tiadwall have succeeded in bringing off a brood in a 

 small lake at Hroughton, Peeblesshire — the first record of such 

 an occurrence in Scotland, where the Gadwall hitherto has 

 occurred only as a winter visitor. 



Pelican and Spoonbill in Norfolk. 



Mr. J. H. Gurney writes to the Fuld, August 4, to say that 

 a Pelican was seen on Breydon Water, Great Yarmouth, by 

 Mr. .\rthur Pat^rson, a well known and experienced naturalist. 

 About four hours after its arrival it was joined by a Spoonbill. 

 The Pelican stayed till the next day, when, early in the morn- 

 ing it made oft in the direction of the sea. Another Spoonbill, 

 by the way, appears to have been seen on Breydon Water on 

 April 2S of this year. 



Though Mr. Gurney thinks the visit of this Pelican may be 

 that of a wild bird, he suggests that it may also have escaped 

 from some private collection. But the loss of such a bird 

 should soon be known. 



Rare Variety of the Jackdaw^. 



Ararevariety of the Jackdaw is described in the i'"/i7i/,July 28, 

 by Mr. H. Beveridge, of Sprouston, N.B. This bird, whichhe 

 shot on July 15, had the head bright chestnut; neck, light 

 buff ; back and breast, bright chestnut, the back slightly 

 mottled with black. The wing and tail feathers had a bronze 

 sheen, but were otherwise black. " He is an old cock bird," 

 he remarks, " in full plumage, and in every other detail is 

 exactly like other Jackdaws. One thing I noticed, whenever 

 he approached his neighbours they invariably hunted him 

 away." 



By Alfred W. Porter, B.Sc. 



The Radiation from a Welsbach Mantle- 



Oxr. ol ihc iiuisl inli iTsruig if |kt1i.l|)s not tlic must popidar 

 discussions at the recent Hrilish Association nulling was 

 on the above subject. It was led off by a spirited challeng- 

 ing paper by Mr. J. Swinburne (who was unfortunately 

 absent), in 'which a sketch was given of the various 

 suggestions which have been made from time to time to 

 explain the high efiiciency of a Welsb.ach m;mllc, and the 

 arguments for or against them were outlined. Dr. II. 

 Rubens, of Charlottenburg (whose recent exiierimeiUs h.-ive 

 already been sketched in these columns), followed with an 

 account of his experiments, and also of a demonstration of 

 one of them. Light from an electric lantern is focussed on 

 a cold Welsbach mantle so as to brilliantly illuminate it, 

 and the reflected light is then focussed on a screen, where 

 it forms an image of the mantle. A cell containing copper 

 sulphate solution is interposed so as to isolate the blue rays. 

 If now the mantle be made luminous by means of gas from 

 the burner in the usual way, the reflected image on the 

 screen is much less bright than before, instead of being 

 brighter as might have been expected. The explanation is 

 that at the higher temperature the mantle is a much worse 

 reflector for blue rays than when cold. When a red cell is 

 interposed, the effect on the screen is a little greater when 

 the inanlle is hot than when it is cold. When the mantle 

 is cold, its reflecting power is comparatively large, and 

 hence its radiating power is small for blue light. It is by 

 no means a full radiator for such light. But when the 

 mantle is heated its reflecting power diminishes; its radia- 

 ting power for blue rays must therefore increase. This 

 tffect is not obtained for red light, and by inference we may 

 suppose that for infra-red radiation there is no increase of 

 radiating power when the mantle is hot. The bearinT of 

 the experiment on the behaviour of a Welsbach mantle is 

 obvious. Such a mantle is nearly white when cold ; this 

 being so, we might expect it to be a poor radiator of all 

 kinds of radiation ; it is from a black body that most radia- 

 tion is expected ; so much so that the perfect radiator is 

 usually (though rather curiously) known as " a perfectly 

 black body." But radiating power for the more luminous 

 radiations increases with temperature ; hence at any high 

 temperature the proportional amount of luminous radiation 

 is greater than at any low temperature. Thus the mantle 

 remains at high temperatures a bad radiator for most radia- 

 tions, and is therefore able to reach a high temperature 

 when inmiersed in a flame. Its great luminous efficiency 

 is to be traced, therefore, to the combined action of this high 

 temperature and the increased radiating power for luminous 

 radiations. In the discussion which followed, there appeared 

 to be a diversity of opinion as to what Dr. Rubens' experi- 

 ments proved. One speaker congratulated Dr. Rubens on 

 having demolished Mr. Swinburne's contentions ; another 

 considered that Rubens and Swinburne were in practical 

 agreement (although Swinburne did not lay enough em- 

 phasis on the existence of selective radiation). With the 

 latter opinion Dr. Rubens concurred. The outcome of the 

 discussion is to show that to account for the efficiency of 

 the mantle, there is no need to invoke the operation of 

 chemical changes or phosphorescence. Dr. Rubens, when 

 questioned, was not disposed to deny the presence of such 

 additional phenomena. They may be present even in the 

 case of a thoria-ceria mantle, with which his experiments 

 were mainly concerned ; in the case of others they may be 

 present to a greater degree. But he considered the evidence 

 wholly against attributing the main phenomena to these 

 causes. The difference between the opposing views may be 

 brought out by indicating that according to the tempera- 

 ture explanation, the total radiation from the thoria and ceria 

 is equal to the sum of the radiations at the nctnnl tempera- 

 ture'of the mantle which would be emitted by the separate 

 thoria and ceria at the same temperature; while on the 

 chemical hypothesis a radiation appears as the result of com- 

 bining the thoria and ceria which would not be present If 

 these materials were separate. 



The discussion, which was of a somewhat recondite 



