364 
Transactions of the Boyal Society of South Africa. 
It seems a matter of interest to know that very small amounts of a 
radium salt can give rise to such definite changes in colour. A tiny speck 
of the radium salt, weighing less than a milligram and very impure, will 
show a distinct colouration in a capillary tube at the end of 40 days. The 
small amounts of radium present in ordinary rock may thus well be capable 
in geological time of giving rise to the pleochroic halos. 
