AGE OF THE EARTH RAYLEIGH. 255 



With the discovery of radioactive elements the inconceivable hap- 

 pened, and Lord Rayleigh was amongst the first to perceive that the 

 rate of disintegration of uranium might be used to provide the geolo- 

 gist with a trustworthy timekeeper. By his experiments and reason- 

 ing he had not only enlarged our views on the duration of geological 

 time, but also opened the way to other methods of investigation which 

 in the hands of Professor Joly and Doctor Holmes have yielded con- 

 cordant results. 



The a<ie of the earth was thus increased from a mere score of mil- 

 lions to a thousand millions and more, and the geologist who had be- 

 fore been bankrupt in time now found himself suddenly transformed 

 into a capitalist with more millions in the bank that he knew how to 

 dispose of. 



The consequences have been far-reaching ; already some geologists, 

 thus newly enriched, chief among them the brilliant Barrell, whose 

 loss we still deplore, have begun to rebuild their science on a new 

 and magnificent scale, while more cautious people, like myself, too 

 cautious, perhaps, are anxious first of all to make sure that the new 

 clock is not as much too fast as Lord Kelvin's was too slow. Lord 

 Rayleigh does not regard this as inconceivable, but as unlikely. 

 Professor Joly, on the other hand, can not only conceive a source 

 of error, but has obtained evidence which seems to show where it lies. 

 This is furnished by a study of the well-known pleochroic haloes 

 which surround minute uranium- or thorium-bearing crystals in- 

 cluded in the black mica of granite. By a very elegant method of 

 investigation he shows that these furnish estimates of geological time 

 of the same order as those established by Lord Rayleigh and Doctor 

 Holmes; but he does not stop there; he goes further. The haloes 

 consist of a number (seven) of concentric rings due to the bombard- 

 ment of the mica by the a-rays which are emitted by the uranium or 

 the thorium, as the case may be, and their products of disintegration. 

 The outermost of these rings is due to radium C, the innermost to 

 uranium or thorium. From data provided by experiment it is possi- 

 ble to calculate the dimensions of the rings, and in the haloes due to 

 thorium the length of the radii obtained by direct measurement 

 agrees very precisely with that obtained by calculation, and this 

 agreement holds, not for some of the rings only, but for all. A sim- 

 ilar agreement is found for the rings of the uranium haloes with the 

 remarkable exception of the innermost two, due to uranium and its 

 immediate product, ionium. These are larger than they should be; 

 in fact, the length of the radius of the uranium ring as actually 

 observed is one-sixth longer than that predicted by calculation. 

 This shows that when the haloes began to be formed — i. e., in Cale- 

 donian times — the range of the a-rays emitted by the uranium-bear- 



