March 3, 1898] 



NATURE 



427 



The history of volcanic action in the past, and the conditions 

 under which it is continued now in the polar regions ; whether 

 in high latitudes vulcanism, either in its internal magmas or 

 superficial eruptions, manifests peculiarities not observable 

 nearer to the equator ; what is the nature of the volcanic pro- 

 ducts now ejected at the surface; whether a definite sequence 

 can be established from the eruptions of still active volcanoes 

 back into those of earlier geological periods in Antarctic lands ; 

 and whether among the older sheets leaf- beds or other intercal- 

 ations may be traceable, indicating the prolongation of a well- 

 developed terrestrial flora towards the south pole. 



The influence of the Antarctic climate upon the rocks exposed 

 to its action ; the effects of contact with ice and snow upon 

 streams of lava ; the result of the seaward creep of the ice-cap 

 in regard to any lava-sheets intercalated in the ice. It is con- 

 ceivable that portions of lava-streams might be broken ofi" by 

 the onward motion of the ice which they overspread, and might 

 thus be carried out to sea, intercalated in or capping ice-bergs- 



The physics of Antarctic ice in regard to the history of the Ice 

 Age in northern Europe and America. 



Antarctic Fauna. 



Although an ardent advocate of Antarctic exploration, Mr. 

 Sclater acknowledged that, as regards the higher vertebrates, 

 with which he was most conversant, there was little chance of 

 the discovery of new forms of animal life in the South Polar 

 continent. The Antarctic mammals and birds (of the latter 

 of which about twenty species were known) were exclusively 

 of marine forms. Not a single land-mammal or land-bird had 

 been yet obtained in Antarctica. As regards the class of 

 fishes and the marine invertebrates, the case was quite different, 

 and great discoveries might be anticipated in these groups, 

 where very little had yet been done. The most promising 

 zoological subject of Antarctic exploration seemed to him, how- 

 ever, to be the further investigation of ihe extinct fauna. The 

 few fossil remains already obtained indicated the former exist- 

 ence in the South Polar area of a very different climate from 

 that which now prevailed there, and further researches on this 

 point might lead to most important results. 



Prof. D'Arcy \V. Thompson said that all we knew of the deep- 

 sea life of the Antarctic came from eight hauls of the dredge, 

 which hauls were, by common consent of the naturalists of the 

 Challenger, the most productive of the whole cruise. The 

 fauna of every ocean urgently demanded further exploration, 

 for we knew now no more about the fauna of the deep-sea than 

 was known a hundred years ago of the fauna of the shore. But 

 the circumpolar fauna of the South, at the meeting of all the 

 great oceans, presented problems of peculiar importance. He 

 considered Dr. Murray's theory of a " bipolar fauna," closely 

 akin both in the Arctic and Antarctic, as not proven ; but he 

 believed that there were many remarkable cases of continuous 

 distribution, especially along the cold waters of the Western 

 American coast from the Antarctic into the North Pacific, and 

 even to Japan. If the "bipolar hypothesis" were broken 

 down, Antarctic exploration would lead to new generalisations, 

 not less interesting, to take its place. 



Admiral Sir William Wharton said that an Antarctic Expedition 

 must be under naval discipline. He hoped that such an expedi- 

 tion would not be far off', and he felt sure there would be a rush 

 of officers and men to join it. 



Sir John Evans, in briefly summing up the discussion, said it 

 had maintained a high level, and that the meeting had been pro- 

 longed to an unprecedented hour in the Royal Society. All were 

 agreed as to the immense advantages of an expedition, and he 

 was sure it would find a warm advocate in the Hydrographer to 

 the Admiralty. 



ON THE ABSORPTION OF LIGHT BY 

 FL UORESCING ' BODIES} 

 A/TR. JOHN BURKE has recently given to the Royal 

 -'■■*• Society of London (see Nature, vol. Ivi. p. 261) the 

 result of some experiments which afford an important indication 

 of the mode of action of bodies during fluorescence, and which 

 may lead to a clearer conception of Kirchhoff"'s law on the 

 equality of the emissive and absorptive powers of bodies 



The following is one form of Mr. Burke's experiment : — A 



1 Translation of a paper, by Prof. C. E. Guillaume, in \h& Revue GetUrale 

 lies Sciences, December 15, 1897. 



NO. 1479, VOL. 57] 



;. I / 



/ s 



// 



B 



photographic plate, i' (Fig. l), is adjusted before two equal 

 cubes of uranium glass, A and B, placed so that the light 

 emanating from B is obliged to pass through A before reaching 

 the photographic plate i'. 



The source of light, s, rich in the ultra-violet, illuminates the 

 cubes by rays parallel to the plate, which is screened from the 

 direct action of the source. An image is first formed by letting 

 the exciting rays act on the two cubes simultaneously. The 

 plate is then displaced, and a second image is produced by 

 illuminating each of the cubes separately, each for the same 

 length of time as in the first experiment. 



The result is that on development the resultant impression of 

 the two separate effects is always much more intense than that 

 of the first due to the two conjointly. 



The simplest explanation of this curious phenomenon is to 

 suppose that the cube A absorbs'^the light emitted by the cube 

 B more strongly when it is in a state of fluorescence than when 

 it is screened from the exciting 

 source. At first sight this pro- 

 perty of fluorescent bodies ap- 

 pears to be a direct consequence 

 of Kirchhoff"'s law, all luminous 

 bodies absorbing the radiations 

 which they are capable of 

 emitting. But on looking at the 

 matter more closely we find that 

 this law, which includes so 

 many facts, does not directly 

 apply to the phenomenon dis- 

 covered by Mr. Burke. This 

 law states, in general, that all 

 bodies, at a given temperalitre, 

 have an emissive power and an 

 absorptive power which are equal 

 for each kind of radiation they 

 emit. But we see here a class 

 of bodies which, without having 

 their temperature visibly altered, 

 have their absorptive power 

 changed, in consequence ol 

 the fact that, by a cau.se ap- 

 parently different from an ele- 

 vation of temperature, they 

 emit, momentarily, and under the action of an external source, 

 radiations which are extinguished at the same time as the 

 excitation itself. By this excitation the molecule is not per- 

 manently altered, and it does not become susceptible of 

 vibrating in unison with the light to which it had attained ; but 

 if, by an external cause, it is given this vibratory movement, 

 then, and only then, it becomes a resonator for the radiations 

 identically the same as that which it emits. 



A familiar illustration will give us a more vivid conception of 

 the mechanism of the phenomenon. Let us suppose a sound 

 wave to approach a fixed tuning-fork of another pitch : the 

 wave will pass on unabsorbed. But if we force the tuning-fork 

 in such a way as to make it emit a note identical with that 

 of the wave which approaches it, then it will behave as a 

 resonator and will evidently become absorbent to the passing 

 wave. The tuning-fork is thus capable of absorbing the vibra- 

 tory energy which reaches it, not merely when the latter corre- 

 sponds to its natural period of vibration, but also when it possesses 

 a period identical with that of the forced vibration that is 

 momentarily imparted to it. 



It is probable, similarly, that the fluorescent molecules are 

 excited momentarily to a forced vibration, and become, /^r an 

 instant, susceptible of absorbing the vibrations of the .same 

 period. 



It will be found, perhaps, that the familiar statement of 

 Kirchhoff^'s law will apply, on comparing the small number of 

 fluorescent molecules in the uranium glass, to any molecules 

 whatsoever which have been raised to a fictitious temperature 

 corresponding to their vibratory state. This extension of the 

 notion of temperature has already been suggested with regard to 

 various luminous phenomena other than that of incandescence, 

 but it had merely led, up to the present, to the heaping up of 

 difficulties without arriving at anything conclusive. 



It seems to me far simpler to suppress the notion of tempera- 

 ture altogether in Kirchhoff"'s law, which is far too general to l)e 

 limited by a conception that ought to have a precise and definite 

 signification. 



Fig. I. 



