Pleocliroic Halos. 647 



The volume o£ helium is, therefore, only about 3 X 10~ 10 c.c. 

 If this is distributed uniformly throughout a halo having the 

 radius 0*033 mm. the pressure is less than the forty-sixth part 

 of an atmosphere. A nucleus of ten times the mass would 

 still yield a much less amount of helium than would fill the 

 halo-sphere at atmospheric pressure. 



Nothing is so surprising in connexion with the pleochroic 

 halo than the extremely minute effects which suffice to give 

 rise to it. The calculation given above shows that nuclei of 

 the size repeatedly measured contain but 10~ 16 of a gram 

 of radium, even if we suppose them composed of the most 

 radioactive of naturally occurring minerals. This quantity 

 gives off but one « ray in about ten hours. But even smaller 

 nuclei suffice to produce small simple halos whose radioactive 

 character is still quite unmistakable. A series of measure- 

 ments, using a high magnification, gave the following readings 

 in millimetres for the nuclear diameters of embryonic halos 

 previously described as averaging 0*013 mm. in radial 

 dimension :— 0*00195, 0*00243, 0*00144, 0*00243, and for two 

 such embryos showing very faint coronas 0*00292, 000316. 

 Calculating the volume corresponding to the diameter 

 0*002 mm. we have, finally, a quantity of uraninite less than 

 one-sixteenth the amount giving rise to the more developed 

 halos. We are recognizing, therefore, by mere inspection, 

 the presence of considerably less than 10 ~ 17 of a gram of 

 radium ; a quantity expelling about 80 helium atoms in a 

 year. It is to be added that many of the smallest nuclei are 

 probably zircon, which would lead to still lower estimates of 

 the amount of radium involved. We are in some hope that 

 experiments now in progress upon the rate of growth of 

 artificial halos in this biotite may enable us to arrive at an 

 estimate of the time required for the formation of these halos. 



Here we would remark upon an aspect of the study of 

 pleochroic halos which may, in the future, prove of im- 

 portance. We are dealing with quantities of radioactive sub- 

 stances many thousands of times less than can be measured 

 by any known method, and not only so, but the means of 

 discriminating between one radioactive family and another 

 is given to us. Even more, a certain number of the specific 

 « rays can, under favourable conditions, be inferred, and 

 the ranges of some accurately measured. Thus in the 

 uranium-radium halos of the Leinster biotite we might, if 

 starting the research in ignorance of the specific results 

 arrived at by Bragg and his successors, have inferred the 

 presence of the ionium ray : of that very vigorous and pro- 

 bably, therefore, complex group of rays which enlarge the 



