5o6 



NA TURE 



[September 22, 1904 



Is Selenium Radio-active ? 



It occurred to me recently that a possible method of 

 deciding between the two hypotheses which have been 

 brought forward to explain radio-activity, namely, that of 

 atomic degradation (Rutherford and Soddy, Ramsay, &c.) 

 and that of molecular change (Armstrong and Lowry, Proc. 

 Roy. Soc, 1903), lay in attempting to realise radio-activity 

 in the case of an element well known to undergo molecular 

 change readily, but with an atomic weight small enough 

 to exclude the probability of an atomic instability such as 

 is assumed for radium and thorium. Such an element is 

 selenium (at. wt. 79), which suggested itself to me as a 

 suitable material to experiment with because, under the 

 influence of light, it undergoes a remarkable alteration in 

 its electrical resistance and E.M.F. of contact, suggesting 

 an allotropic change of an altogether unusual character. 

 As this change, whatever be its real nature, occurs almost 

 instantaneously (Bellati and Romanese, Atti R. 1st. Veneto, 

 1881 ; Maiorana, Atti del Lincei, 1894 and 1896), it seemed 

 possible that the rapidity of the intermolecular vibration 

 might be sufficient to cause a radiation similar to that of 

 radium and thorium which, by " ionising " the selenium, 

 would render it conducting. In order to ascertain whether 

 such a hypothetical radiation could be detected photo- 

 graphically, I exposed a piece of selenium, placed on a 

 photographic plate, wrapped in three thicknesses of black 

 glazed paper, during thirty-six hours to the bright sunshine 

 of July. On developing the plate a distinct black stain on 

 a background of clear glass indicated the position the 

 selenium had occupied. The first experiment was made 

 with ordinary vitreous selenium, and the stain, although 

 distinct, was not very pronounced. A second experiment 

 with freshly prepared " metallic " selenium, obtained by 

 heating the vitreous variety at 190° for half an hour and 

 then cooling very gradually to the ordinary temperature, 

 gave a much more intense stain on the negative. 



I should have hesitated before publishing these experi- 

 ments in their present form had 1 not, since they were 

 made, seen a paper in the Physikalische Zeitschrift of 

 August, 25 in which similar results are recorded by 

 J. J. Taudin Chabot. This observer, approaching the sub- 

 ject from a different direction, has also been led to the 

 conclusion that selenium in a selenium " cell " is feebly 

 radio-active to the extent of emitting radiations capable of 

 passing through black paper and affecting a photographic 

 plate. It may, of course, be urged that the stain on the 

 plate is due to selenium vapour penetrating the paper in 

 which the plate is enclosed. I am therefore making experi- 

 ments in which this possibility is excluded. Further, I 

 intend studying the behaviour of sulphur, as it is already 

 known that the other members of the selenium family, 

 namely, oxygen and tellurium, can in certain circumstances 

 give rise to radio-active phenomena. \V. A. D.^vis. 



City and Guilds of London Institute, Central 

 Technical College, S.W. 



Rare Moths in England. 



Mrs. TnoM.\s'.s letter in your issue of September 8 is 

 interesting, both as affording definite proof that Deilephila 

 livornica does sometimes breed in this country,- and as rather 

 tending to support the view that this species enters the 

 country from abroad — for the proximity of Warmwell to the 

 sea is certainly suggestive. 



Apropos to rare moths, it may be of interest to state that 

 I recently took a specimen of Dciopcia pulchella on the cliff 

 here. This moth used to be so rare that Newman wrote : — 

 " Mr. Doubleday has a single specimen taken at Epping, 

 and we believe there are two or three other British speci- 

 mens in different cabinets." .Since that was written the 

 number of English specimens has, of course, been increased, 

 but I believe that the insect is still considered a rarity ; and 

 the scarce occurrence of a weak-flying moth like this, which 

 one can hardly suppose could cross the Channel, and which 

 has been found so far inland as at Epping, is a greater 

 puzzle than the rarity, but occasional comparative 

 abundance, of a strong-flying hawk moth. 



F. H. Perrvcoste. 



Higher Shute Cottage, Polperro, R.S.O. Cornwall, 

 September 12. 



NO. 182 1, VOL. 70] 



THE HEART OF SKYE.' 



"nPHIS volume of detailed rock-description, raising 

 •*■ in its successive chapters questions of profound 

 interest in philosophic g'cology, proves that the Geo- 

 logical Survey of the United Kingdom is confident 

 that the scientific spirit should permeate its public 

 worl^:. None of the roclcs dealt with possesses at pre- 

 sent an economic value ; most of the area is untraversed 

 by roads, and the exposures are not to be sought in 

 quarries, but in rain-swept uplands, or high on 

 desolate mountain-walls. Yet no detail is regarded 

 as unimportant; the surveyor, for months together, 

 leads a life as hard and remote as that of an Alaskan 

 pioneer ; and the result is a book in which the daily 

 difficulties are concealed, while an array of facts is 

 given to us, any one of which may help observation 

 in other and more favoured lands. 



Few lands, however, are more favoured in scenery, 

 and the defects of Skye are mainly meteorological. 

 The very audacity with which the black Cuillins rise 

 from the edge of the Atlantic seems a temptation to 

 the summer-storms. Yet the glaciated surfaces allow 

 of small decay, and Mr. Harker directs our attention 

 to the remarkable freshness of the rocks. Despite the 

 absence of maps, the crystalline core of the island, with 

 its striking scenic contrasts, long ago attracted 

 geologists, and has been again described, in all its 

 picturesqueness, by Sir .'\rchlbald Geikie in our own 

 day. We well remember the seductive map, on the 

 scale of four miles to an inch, with which we ourselves 

 tramped across the boggy grasslands, or w-andered in 

 midnight prodigality above the Sound of Sleat; and 

 Mr. Harker complains that, even now, names and 

 details are not always exactly placed on the sheets 

 issued by the Ordnance Survey. The work that he has 

 now done, and the manner of it, will be honoured by 

 all who know the island. 



The view that the crystalline central masses repre- 

 sent the core of the volcano from which the abundant 

 plateau-basalts flowed appears to be now untenable, 

 although some kind of a volcano, and a fairly large 

 one, may have existed on the crown of the great lacco- 

 litic dome. Certain earlier volcanic vents, moreover, 

 provide curious evidence that gabbro and granite 

 existed, in a consolidated form, down below, while 

 the plateau-basalts were being extruded. In a very 

 remarkable passage (p. 24), the author shows that 

 even the sequence of these rocks was the same as that 

 now known to exist in the central laccolitic area. The 

 acid tuffs of Skye containing granite (p. 20), the basic 

 tuffs crowded with fragments of gabbro (p. 21), go 

 far as evidence of the continuity of lava-types with 

 holocrystalline representatives below. It is interesting 

 to remember that, when Prof. Judd wrote his well 

 known papers, this natural-history view of igneous 

 masses was very far from general acceptance ; and 

 the fact that he strove for it so successfully has been 

 obscured by the subsequent controversy as to the local 

 sequence in the Hebrides. 



Mr. Harker, in dealing with the basalt plateaux 

 (pp. 29, 239, and 43S), shows excellently how large a 

 part is played by the abundant intrusive sills. Tliese 

 form, indeed, on weathering, most of the terraced 

 structure that observers were formerly apt to attribute 

 to successive lava-flows. It is remarkable (p. 12) that 

 the dykes or vents can so rarely be traced into the 

 lava-sheets to which they gave rise; but this is a 

 common complaint in all areas of copious and long- 

 continued activity. The temporary theory of fissure- 



1 "The Tertiary Igneous Rock-; of Skye." By Alfred Harker, M.A., 

 F.R.S., with notes bj; C. T. Clough, M.A., F.G.S. Pp. xii -f- 482, 

 (Mercoirs of the Geological Sui^ey of the Uniteti Kingdom. Glasgow : 

 Printed for His Majesty's Stationery Office by James Hedderwick and 

 Sons, 1904.) Price 9^. 



