012 Prof. J. Joly and Mr. A. L. Fletcher : 



effect (whatever its nature may be) is the same for all, the 

 differences in development must simply depend upon the 

 radioactivity of the central originating substance. The 

 larger nuclei are, accordingly,, generally found to be attended 

 with the best developed halos. 



It seems certain that these embryonic halos owe their 

 radial dimensions to the effect of the slower moving « rays; 

 those of ionium having their maximum effect at 0"010 mm. 

 defining the smallest, and the maxima of radium and nranium 

 lying at 0*013 mm. defining the larger. Some allowance 

 for the radius of the nucleus, which very often has been 

 found to be about 0"0022 mm., would refer some of these 

 halos solely to the effects of ionium. But none are ever 1 found 

 of radial dimensions less than can be accounted for by ionium. 

 This seems additional evidence that ionizing « rays of less 

 velocity than those given out by ionium do not exist in the 

 uranium-radium series. 



The influence of the radium and uranium a rays is, pro- 

 bably, an early one ; the latter element being known to give 

 two ol rays upon disintegration. At the radial distance of 

 0*0135,, therefore, three a. rays out of the total eight which 

 are evolved in the entire sequence of changes, exert their 

 greatest effect. It is, therefore, not to be wondered at that 

 the number of embryonic halos having a radius ascribable to 

 these influences is very great, but owing to the uncertain 

 allowance for the nucleus and the gradation of the observed 

 dimensions it is impossible to clearly define those due to any 

 particular ray at this stage of development. In agreement 

 with this, simple halos of varying dimensions somewhat 

 greater than those cited above are found. This, too, is quite 

 in accordance with the Bragg curve. If this curve were 

 prolonged to the right so as to bring in the uranium, and the 

 slower moving ionium, ray, it is evident that while halos 

 would not be expected to show a lesser radius than is due to 

 the ionium ray, the enlarging of the radius under the intense 

 ionization of uranium and radium should be rapid. It should 

 be noted as generally true of the initiation of halos that the 

 first beginning is not from within outwards. The halo 

 suddenly presents itself as a very faint spherical darkening 

 of the mica, which if differentiated at all is darkest at or 

 near the surface. Any greater darkening occasionally ob- 

 served near the nucleus is more or less irregular and fuzzy, is 

 confined generally to small radial distances, and may be due 

 either to the convergent concentration of the rays or to some 

 escape of emanation. This mode of initiation is, of course, 

 in keeping with Bragg's observations. 



