458 



NATURE 



[August 2, 191 7 



while the inner rings are still in an early stage of 

 development. 



But if now we return to the first curve of develop- 

 ment — that one which takes no account of the spread- 

 ing of the rays— we find a scheme of development 

 which closely coincides with the actual details as ob- 

 served in the process of halo-growth. 



First, we have a solitary ringf, or shell, of ionisation 

 surrounding the nucleus. In its earlier stages it is not 

 easy to photograph. It plainly corresponds with the 

 first conspicuous maximum of the ionisation curve 

 (Fig. 3). This I call the first ring. The rays from 

 U, and U, are chiefly responsible for it. This first 

 ring, accentuated and darkened within, is often found 

 in a succeediner stage of development along with the 

 earliest impression of the outermost ring of all, that 

 due to RaC,. Next, outside the first ring, appears a 

 very delicate and seldom-found ring, which I name 

 the second ring. It corresponds, apparently, with the 

 first notable excrescence oh the downward slope of 

 the curve. By the time this ring has developed, the 

 inner region of th-3 halo has considerably blackened 

 up. Nor have I found this second ring without the 

 presence of a third ring surrounding it, and evidently 

 referable to the next excrescence on the curve. At 

 this stage, too, we find that RaC, has still further 

 registered its effects. 



The stage which succeeds shows the inner detail out 

 ^ to the third ring 



obliterated in the 

 accumulating ion- 

 isation. There is 

 now, therefore, a 

 central pupil sur- 

 rounded by the 

 third ring and out- 

 side all the border 

 due to RaC,. A 

 yet more odvanced 

 stage finds the 

 third ring also 

 swallowed up in 

 the inner darken- 

 ing. This is the 

 stage which in it- 

 self is deceptive as 

 to the true course of development of the halo. 



The successive features of the developing uranium 

 halo have been indicated above the curve of integral 

 ionisation. The features numbered i, 2, a.id 3 are 

 the first, second, and third rings ; 4 shows the limiting 

 position of the radius of the pupil when all is blackened 

 up within ; 5 is the ring due to RaC,. 



An interesting modification of the uranium halo is 

 found in the mica of Countv Carlow. The o rays of 

 U, and Uj, of ionium and of radium, have apparently 

 played no part in its genesis. The parent substance 

 responsible for the halo is the short-lived emanation 

 of radium. This element, and those derived from it, 

 alone take part in its formation. Consequently, only 

 four out of the eight a-ray-expelling substances are 

 concerned in its architecture. As these include the 

 furthest reaching ray — that of RaC, — the outside 

 dimension of the halo is the same as that of the com- 

 plete uranium halo. 



Wben we plot the integral ionisation curve of this 

 halo w'e get an initiating ring of appreciably larger 

 radius than is associated with the beerinning of the 

 uranium halo (Fig. 5). And it is by this larger initiat- 

 ing ring that the new halo is identified. In later 

 stages it is difficult to differentiate it from the uranium 

 halo. 



The mode of origin of the emanation halo is interest- 

 ing. All through the mica in which these halos are 



NO. 2492, VOL. 99] 



Fig. 5. — Integral curve for emanation halo 

 showing position of first ring. 



found there is evidence that radio-active solutions or 

 gases were at one time transported along minute 

 channels or cracks. These channels are bordered with 

 radio-active darkening, showing just such an appear- 

 ance as Rutherford got in the walls of a capillary tube 

 containing condensed emanation of radium. Again, 

 the darkening around the conduit in the mica may 

 often possess the radial dimension of the first ring. 

 Along such conduits we find every now and again 

 a refracting particle which acts as the nucleus of an 

 emanation halo. Apparently the particle has served to 

 condense the emanation or to absorb it, and thus 

 becomes the centre of radiation for a rays given out 

 by substances which are derived from the emanation 

 by further disintegration. Consequently, emanation 

 halos are found developed along such cracks or con- 

 duits, often presenting the appearance of beads upon 

 a string. 



{To be continued.) 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 

 Prof. H. Roger has been elected to succeed the 

 late Prof. Landouzy as dean of the Paris Faculty of 

 Medicine. 



A CHAIR of tropical pathology has been established 

 in the University of Lima, Peru, and Dr. J. Arce 

 has been appointed professor. 



By the will of the late Sir Charles Holcroft a be- 

 quest of 5oooi. is made to the University of Birming- 

 ham, to establish^ Charles Holcroft Research Fund. 



The Museums' Association proposes to hold a con- 

 ference at a town in the midland counties in October 

 next to discuss, among other subjects, local war 

 museums and the Board of Education and museums. 

 Mr. Fisher, President of the Board of Education, has 

 pointed out the necessity for promoting the advance- 

 ment and application of art to industry in a more 

 direct and extensive manner after the war, when indus- 

 trial development will demand all possible aid. This 

 is work that equally concerns many museums. The 

 reorganisation of education, both elementary and 

 secondary, foreshadowed by Mr. Fisher, also calls for 

 the active co-operation of museums, and is the oppor- 

 tunitv for closer co-ordination of their work with the f 

 schools. Both subjects to be considered concern 

 museum committees, as well as curators, and members 

 of museum committees are, therefore, specially invited 

 to attend the proposed conference. 



Following the lead of other educational bodies, the 

 Association of Assistant-masters in Secondary Schools 

 has issued a statement of the educational policy of 

 its members. The aim of education, it is said, shoul<: 

 be to secure the healthy, physical, mental, and moral 

 development of the child, so that he will take his place 

 in the community as an efficient citizen. The nationa] 

 system of education should provide, the policy urges, 

 for the compulsory full-time education of every chiU' 

 up to the age of sixteen years at least. Coritinuec'. 

 secondary education, from the age of sixteen to 

 eighteen," should be preparatory to universitv education 

 or to business or professional life. No child who has 

 shown capacity to profit by a course of secondary 

 education should be refused admission to the schools^ 

 even if the child has to be fed and clothed at the 

 public expense to enable him to attend. No boy should 

 be admitted to a universitv under the age of eighteen. 

 Science should be included in the curriculum of pupils 

 under fourteen years of aee, and it is laid down that 

 every candidate taking the first school examinaticMi 



