3 8o 



NA TURE 



[September 16, 1922 



smaller for thorium. Both crystallise in the face 

 centred cubic lattice. If the interatomic distances 

 may be taken as representing atomic diameters, this 

 means that in the same (or slightly less) volume there 

 are concentrated in the thorium atom thirty-two more 

 electrons than in the cerium atom, the total numbers 

 in the two cases being, respectively, ninety and 

 fifty-eight. 



Thorium is the next to the last element in the 

 periodic table possessed of particular stability. 

 Between the last, uranium, and neodymium in the 

 preceding period, a structural relationship exists 

 similar to that between thorium and cerium. The 

 crystal structures of these elements by the X-ray 

 method have, unfortunately, not been worked out. 

 However, an approximate idea of the relative sizes of 

 the atoms of these substances may be gained by a 

 comparison of their atomic volumes. According to 

 Landolt-Bornstein's " Tabellen," the densities of 

 neodymiumand uranium are6-g6 and 18-7 respectively. 

 Dividing the atomic weights (144-3 an d 2 38"5 re- 

 spectively) by thesenumbersgivesfortheatomic volume 

 of neodymium 207, and for that of uranium 12-8. 

 The corresponding quantities for cerium and thorium 

 are about 20-5 each. It thus appears that in the atoms 

 of uranium there are concentrated in about one-half the 

 volume thirty-two more electrons than in the atoms of 

 neodymium, the total numbers of electrons being, 

 respectively, ninety-two and sixty. 



It is perhaps significant in view of these facts that 

 elements of higher atomic number than uranium are 

 not known to exist, and that most of those of im- 

 mediately lower atomic number are unstable. With 

 increasing nuclear charge the attractive forces exerted 

 by the nucleus on the surrounding electrons con- 

 centrate the latter nearer and nearer toward the 

 centre of the atom. It does not appear improbable 

 that the exceedingly powerful forces, both of attrac- 

 tion and repulsion, which must result from this con- 

 centration may be of sufficient magnitude to assist 

 materially in bringing about those conditions of 

 instability which result in radio-active disintegration. 

 If the large numbers of electrons in the atoms of the 

 radio-active elements be conceived as rotating about 

 the nucleus within the small space which the relatively- 

 small atomic volumes allot to the atoms of these 

 elements, with orbits of different periods, there will 

 evidently come times periodically when numbers of 

 electrons in excess of the average will all be exerting 

 attractive forces on the positive nucleus in the same 

 direction. In such circumstances it is conceivable 

 that a positively charged constituent of the nucleus 

 might be drawn out of its normal equilibrium position 

 and, the local attractive forces which held it in its 

 equilibrium position being overbalanced by the re- 

 pulsive force between this new entity and the positive 

 nucleus acting as a whole, be sent on its path as an 

 a-particle. The rate of decay of the atoms of the 

 elements would then depend on the frequency with 

 which this favourable configuration of electrons, which 

 is just sufficient to exert the critical attractive force, 



( rred. The more stable the nucleus, the greater 



would the numbers of electrons all acting in the same 

 direction need to be. But the greater the concentra- 

 tion required, the less frequently will it occur, other 

 things being equal. Hence, for a more stable nucleus 

 the rate of decay must be less. The rate of decay 

 would thus depend primarily on the stability of the 

 nucleus, and the mechanism suggested would constitute 

 the trigger action by which the actual disintegration 

 was brought about. Robert N. Pease. 



(National Research Fellow in Chemistry). 

 Laboratory of Physical Chemistry, 



Princeton University, Princeton, N.J., 

 August 6. 



NO. 2759, VOL. Iio] 



The Freshwater Winkle. 



I was, recently, fortunate enough to obtain a pair 

 of the yellow-bodied variety of the freshwater winkle 

 (P. contecta) from what I understand was the first 

 consignment to be imported into this country. Un- 

 fortunately the female died, and when I removed it 

 from the aquarium the body fell out of the shell, 

 the snail having apparently been dead a day or two. 

 I then noticed that there was a row of five fully- 

 formed baby snails — about \ inch in diameter — in 

 the gelatinous egg-sac. 



Although I thought there was little possibility of 

 their being alive, I released them with a pair of 

 scissors and placed them in a saucer of water. For 

 twenty-four hours or so there was no sign of life, but, 

 on the second day, I noticed that an operculum was 

 forming on each and that the tentacled head of two 

 of them had been extruded. These were immediately 

 placed in a well-established aquarium, and the follow- 

 ing day the other three were similarly dealt with, 

 they having also become active. All are now feeding 

 upon the conferva? on the sides of the tank and ap- 

 parently doing well. 



I have never heard of such an experiment having 

 met with success, and shall be glad to learn whether 

 the result is new. A. E. Hodge. 



The Effect of a Lead Salt on Lepidopterous Larvae. 



For some time we have been studying the effect of 

 adding various metallic salts to the food of the larvae 

 of Lepidoptera, and, as the results will not be ready for 

 publication for about a year, desire to direct attention 

 to the surprising result of using a salt of lead. When 

 a dozen larvae of S. ocellatus were fed on sprigs of 

 apple which had been treated with lead nitrate it was 

 soon obvious that they were eating more freely and 

 growing more rapidly than the controls ; by the time 

 they were about three-fourths grown they consumed 

 double the daily ration eaten by the latter. There 

 was considerable disease among the controls and in 

 another experimental batch, but those getting lead 

 remained perfectly healthy. and pupated about a 

 fortnight earlier than the controls. The pupas were 

 a very fine lot, the males weighing on the average 

 about 15 per cent, more than the controls, and the 

 moths were large and somewhat peculiarly coloured ; 

 there were too few females for a comparison to be 

 made. Confirmatory results have been obtained with 

 the larva? of other moths. 



This curious result is not without parallel. The 

 herbage near the chimneys of lead-smelting works 

 contains appreciable amounts of lead, and cases of 

 lead poisoning have occurred among sheep ; in 

 Weardale, however, it is a common practice to pasture 

 sheep as near as possible to these chimneys when they 

 are being fattened, as the farmers consider that they 

 fatten much more quickly than on other parts of the 

 moors. F. C. Garrett. 



Hilda Garrett. 



The Pigeon Tick. 



There is a slight error in the statement of L. H. 

 Matthews and A. E). Hobson in Nature of September 2, 

 P- 3 J 3. w 'ith regard to the latest previous record of 

 the pigeon tick Argas reflexus. In 1917 I secured four 

 specimens from the tower of Canterbury Cathedral. 

 At least two living specimens were forwarded to Mr. 

 C. Warburton at the time. 



The Cathedral receives a special cleaning every four 

 years and Argas reflexus is invariably dislodged on 

 these occasions. A. G. Lowndes. 



Marlborough College, September 4. 



