August 21. 19 13] 



NATURE 



035 



brium amount cannot certainly be greater than that 

 equivalent in a activity to 2 mg. of uranium. At 

 present the preparation has reached about one-fourth 

 • •i iis equilibrium value as regards actinium-X. 



In ten years, of the 13-2 mg". of radium in the 

 preparation, 0-053 m S- would have disintegrated. On 

 the assumption that uranium is the primary parent of 

 actinium, Rutherford has calculated that 8 per cent. 

 of the atoms disintegrating must choose the actinium 

 route (" Radioactive Substances," p. 523). So that, if 

 it were formed from radium, the amount of actinium 

 present in the preparation would be 0-0042 mg. But 

 the active deposit from this quantity has an a activity 

 not greater than 2 mg. of uranium. Hence the period 

 of average life of actinium must be at least fifteen 

 million years, the quantity in minerals must be at 

 least 170 grams per ton of uranium, and the a activity 

 of pure actinium in equilibrium could not be greater 

 than 1650 times that of uranium. But a specimen of 

 actinium, prepared and presented to me by Dr. Giesel, 

 must have, judging from a cursory examination, a far 

 greater activity than this, and Mme. Curie (" Radio- 

 activity," I., p. 189) speaks of some actinium prepara- 

 tions as of the order of 100,000 times as active as 

 uranium. All the researches go to show that its 

 actual quantity in minerals is very small, and, if there 

 were anything like 500 times as much actinium as 

 radium in minerals, one would have expected it long 

 ago to have been isolated and its spectrum and chem- 

 ical reactions characterised. So that the experiments 

 appear to .disprove the possibility that actinium can 

 be formed directly from radium. Similar arguments 

 to those above may be used to show that it cannot be 

 a primary radio-element, and its origin remains still 

 a nrystery. In the current number of the Physikalische 

 Zeitschrift (p. 752) Hahn apd Meitner modify my 

 ■original suggestion and suppose that the branching 

 of the uranium series takes place at uranium-X, two 

 simultaneous £-ray changes occurring, which produce 

 two eka-tantalums, one the known short-lived /3-ray- 

 giving product, and the other a still unknown long- 

 lived a-ray-giving parent of actinium, also in group 

 VA. There seems nothing improbable about this. It 

 is almost the only other alternative remaining to be 

 tested, and it should not be difficult to settle by experi- 

 ment. Frederick Soddy. 



Physical Chemistry Laboratory, 



University of Glasgow, August 16. 



Radium and the Evolution of the Earth's Crust. 



Having been away from home, I did not see Mr. 

 Holmes's letter on radium and the evolution of the 

 earth's crust, contained in Nature of June 19, until 

 some weeks after its publication, and thought that the 

 interest in the subject would have so far passed as 

 to make it not worth while referring to, what I believe 

 to be, a considerable misapprehension of the structure 

 of the earth as revealed by earthquakes. Later corre- 

 spondence has shown that interest in the subject has 

 not waned, and as a correct appreciation of what has 

 ■been established regarding the constitution of the 

 interior of the earth seems likely to remove some of 

 the difficulties which have arisen out of the studv of 

 radio-activity, it may be useful to review the results 

 obtained from the study of the transmission of earth- 

 quake waves to long distances. 



To begin with, it must be distinctlv understood that 

 this line of research can tell us nothing, directly, re- 

 garding the chemical composition of the earth, nor 

 can we distinguish between stony and metallic mate- 

 rial ; all that can be established is the rate at which 

 two distinct forms of wave motion are transmitted, 

 and if, at any particular depth, we find a marked 

 •change in these rates of transmission, we may say 



NO. 2286, VOL. 91] 



that it is caused by a change, either in chemical com- 

 position or physical state, of the material through 

 which the waves have travelled. With this 

 premised ; the first great change takes place at, 

 probably, about ten miles or so from the surface, 

 and seems to correspond with the passage from the 

 heterogeneous and fractured rocks of the outermost 

 skin to more homogeneous material. Below this, and 

 to a depth of about 100 miles, it is difficult to say 

 whether any further change takes place ; there are 

 indications of change at about fifty and about one 

 hundred miles, but it is not such as has a great effect 

 on the rate of transmission of the simpler forms of 

 elastic waves, and, as the differences in the time 

 intervals concerned are not of a greater order of mag- 

 nitude than the inevitable uncertainties of observa- 

 tion, it is difficult to be certain of the reality of the 

 supposed alteration. 



Below a depth of about 100 miles there is no 

 evidence of any change until a depth of about 2400 

 miles is reached ; throughout this laver there is a 

 progressive increase of elasticity, but it is gradual and 

 seems to be directly connected with the increase of 

 pressure, with the result that the material, whatever 

 it may be, develops a high degree not only of resist- 

 ance to compression but also of rigidity as against 

 stresses of short duration. At the depth mentioned, 

 or at somewhere between o-6 and 0-5 of the radius, 

 measured from the surface, a very marked and re- 

 markable change in the nature of the material, of 

 which the earth is composed, takes place. The change 

 is rapid, and is characterised by a small decrease in 

 resistance to compression, accompanied by a great 

 reduction, if not the complete disappearance, of 

 rigidity. It is impossible to determine how this 

 change is brought about, but it is very much what 

 would be produced either by passage from the stony 

 shell to the metallic core, of one hypothesis, or from 

 the fluid or solid-fluid to the gaseous state, of another. 



Whatever may be the final interpretation of the 

 distant records of great earthquakes, the important 

 point to be noticed is that the two great changes 

 which they indicate in the constitution of the interior 

 of the earth are, first, at a depth of only a few, prob- 

 ably not more than ten, miles, and, secondly, at 

 about 2000 to 2400 miles from the surface. Between 

 these depths there are suggestions of variations in 

 composition down to a depth of 100 miles or there- 

 abouts, but they seem to be of only minor import- 

 ance, and apart from this no change in physical char- 

 acter, or, presumably, in chemical composition, can 

 be detected. R. D. Oldham. 



8 North Street, Horsham, Sussex, August 15. 



Poroscopy : the Scrutiny of Sweat-pores for 

 Identification. 



At the recent meeting of the British Medical Asso- 

 ciation some attention was directed to a method of 

 criminal identification which has been used at Lyons 

 and elsewhere. A fully illustrated account of it occurs 

 in Les Archives d' Anthropologic criminelle for July, 

 from which, after careful perusal, I cannot find that 

 there is anything in the method that does not come 

 under the scope and practical working of dactylo- 

 graphy. Dr. Locard has shown good reason why we 

 should give more attention than has been usual to 

 small patches of finger-prints, and to seek among the 

 pores for what the ridges are too meagre to supply. 

 Dr. James Scott, at Brighton, rightly describes " poro- 

 scopy " as founded on a study of the " impressions 

 or orifices of the sweat ducts of the finger pulp, instead 

 of the ridges." But pores, the openings of sweat 

 ducts, as printed impressions, cannot be studied quite 



