68 



NA TURE 



[November i8, 1909 



elements with very nearly the same weight might possess 

 very similar properties. This would allow the truth of 

 the following table, in which only three assumptions are 

 made. 



(i) It is possible that two elements of nearly the same 

 atomic weight may occupy the same place in the table. 



(2) The emission of an a-particle is accompanied by the 

 production of an element which occupies the adjacent space 

 of lower atomic weight. 



(3) The emission of a ;3-particle, or a rayless change, 

 may or may not be accompanied by a remove to a space 

 of iower atomic weight. 



In the table the elements which emit o-particles are 

 printed in thick type, the other radio-active elements in 

 italics. 



depends, of course, on the specific physical and chemical 

 properties they possess. These are often none too well 

 defined. The mechanism of a rayless change, or one 

 accompanied by the emission of a ^-particle, may be com- 

 pared with a change of frequent occurrence with organic 

 compounds, the formation of one desmotropic substance 

 from another under the influence of heat. 



It must be remembered that should the conclusions be 

 correct which are drawn from the recent work of Ramsay 

 and Gray on the boiling point and critical constants of 

 radium emanation, and should the atomic weight of 17b 

 be confirmed, not only are the above arguments invalidated, 

 but the whole theory of disintegration put forward by 

 Rutherford and Soddy will require modification. 



University College, London. A. T. Cameron. 



H 



In considering this table, if we assume that Rutherford 

 and Soddy 's theory, that the loss of an a-particle is accom- 

 panied by a corresponding decrease of 4 in the atomic 

 weight, is correct, it seems certainly necessary to confine 

 r.idium and radio-thorium to the same space in the table. 

 They arc both membprs of the barium series. The atomic 

 weight of radium is 2265, while that of radio-thorium 

 must be (232.5 — 4) = 228-5. Similarly the thorium and 

 radium emanations resemble each other so closely that it 

 is legitimate to suppose that they occupy the same space. 

 They condense at almost the same temperature, while their 

 rates of diffusion into other gases are very nearly the same. 

 If one case is admitted, the whole is rendered probable. 

 The evidence with the actinium series is not so positive, 

 but the present arrangement satisfies the known facts. 



It may be pointed out that there are three o-emitting 

 elements between radium emanation and lead, and only 

 three spaces in the table, and two a-emitting elements 

 between thorium emanation and bismuth, with two spaces 

 corresponding ; lead and bismuth were suggested by Ruther- 

 ford and Boltwood as the respective end-products of these 

 series. Again, it may be emphasised that the anomalous 

 existence of the group of rare earth metals, giving a differ- 

 ence of more than 40 in the atomic weight of the elements 

 which precede and follow them, explains the change of the 

 difference between two elements of a vertical series from 

 about 46 to nearly twice that figure, so that no element 

 of the argon series is to be expected between xenon and 

 one with a weight about 220. Except in the two spaces in 

 the vertical series hclow manganese, and possibly in the rare 

 earth series, there is no vacant space in the periodic table 

 between hydrogen and uranium. In this connection it is 

 interesting to recall the suggestion of R. W. Wood 

 (Astrophys. Journ., 1908, vol. xxviii., p. 75), that the 

 green line in the spectrum of the sun's corona is the 

 fluorescent line of some common element, and that the 

 supposed element " coronium " of weight less than hydro- 

 gen does not exist. 



It is possible that other /3-emitting or rayless elements 

 may be discovered. How far these are really elements 

 NO. 2090, VOL. 82] 



Radio-activity and the Rocks. 



In the course of some observations which I have recently 

 made with regard to tlie pleochroic halos sometimes seen 

 round inclusions in various rock-forming minerals, certain 

 points have suggested themselves as possibly of considerable 

 significance. It may be premised that Prof. July's sugges- 

 tion that the halos are due to the a rays emitted by 

 radium appears fully borne out by their remarkable 

 constancy in size and by the fact that they are invariably 

 connected with minerals independently known to be 

 strongly radio-active, that is, comparatively speaking- 

 The only qualification that need be made respecting this 

 view is with regard to the possibility of radio-active sub- 

 stances other than radium producing the observed effects. 



So far as my experience goes, the following minerals are 

 capable of producing halos when enclosed in suitable sub- 

 stances like biotite, cordierite, hornblende, tourmaline, 

 &c., zircon, orthite (allanite), epidote, sphene, and apatite. 

 .All these are siHcates, except apatite, which is a phosphate. 

 The last three are lime compounds, which does not, how- 

 ever, seem to be of any particular import. Zircon contains 

 zirconium, orthite cerium and its allies, and sphene 

 titanium, and it may be noted that orthite always contains 

 thorium in some qifantity, while both zircon and sphene 

 may be expected, from a mineralogical point of view, to 

 contain that element as an impurity. Epidote, being iso- 

 morphous with orthite, and frequently intergrown with it 

 in rocks, the presence of some traces of thorium may also 

 be generally presumed. As regards apatite, it Is obviously 

 significant that the other two phosphates which occasion- 

 ally occur as rock-formers, namely, monazite and xeno- 

 time, always contain thorium in considerable amount ; 

 indeed, monazite owes its commercial value to the constant 

 presence of that element. It would appear, therefore, as 

 if the radio-activity of all the minerals cited might, unless 

 other considerations are opposed to the idea, be fairly 

 attributed to the presence of thorium. 



There is, however, another feature of those minerals 

 which may be of significance. It will be noticed that one 

 or other of them contains all the elements grouped under 



