1917] Radioactive Haloes 115 



Stonyhurst College Observatory — Eesults of Meteorological, Magnetical and 



Seismological Observations, 1916. 8vo. 1917. 

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Second Series, Vol. XXI. 8vo. 1916. 

 United States, Department of Agriculture — Journal of Agricultural Research, 

 Vol. IX. Nos. 3-7,- 1917. 8vo. 

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 United States Department of the Interior — Geological Survey, Bulletins, 

 Nos. 627, 630, 635, 636, 638, 610 b, d, e ; 641 b, c, d, e ; 645, 649. 

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 Mineral Resources, 1915, Part 1, Nos. 1, 3, 4, 5, 7 ; Part 2, Nos. 12-14, 

 16-17, 19-20. Svo. 1916. 

 Upsala, Boyal Meteorological Observatory — Bulletin, Vol. XLVIII. 4to. 



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 Western Australia, Agent-General — Statistical Abstracts, Aug.-Nov. 1916. 4to. 



WEEKLY EYEXING MEETIXCx, 



Friday, May 11, 1017. 



Sir James Crichtox-Browxe, J.P. M.D. LL.D. D.Sc. F.R.S., 

 Treasurer and Yice-President, in the Chair. 



Professor Johx Joly, M.A. D.Sc. F.Pt.S. 



Radioactive Haloes. 



These minute objects have long been known to petrologists, but 

 their explanation is of recent date. They are formed around 

 particles containing radioactive matter and are caused by an effect of 

 the alpha radiations on certain minerals of the ferro-magnesium 

 group. Radioactive haloes may, therefore, be derived from uranium 

 or thorium, and the series of alpha-ray changes characteristic of the 

 radioactive transformations originated by these parent substances 

 determine the dimension and structure of the halo. It is easy to 

 refei' a well developed halo to one or other of these parent substances. 

 The uranium series of elements gives rise to eight alpha-ray 

 emitting substances, each of special and characteristic range. The 

 thorium series of elements gives rise to seven alpha-rays of charac- 

 teristic range. It is found that when the integral ionization of these 

 rays, as defined by the Bragg curve of ionization for each ray, is 

 determined by the simple process of adding the ordinates of the 

 eight or seven several curves plotted to the same axes of reference, 

 the two resulting curves (i.e. that for the uranium series and that for 

 the thorium series) agree with the features of uranium and thorium 

 haloes. 



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