252 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



Now the alpha particles are stopped by colUsion with other atoms 

 in a short distance (0.03 to 0.04 mm in the mica), and it is found that 

 they are most effective at the end of their flight. Thus they tend to 

 produce a ring of discoloration, if not much overexposed. Moreover, 

 each element gives off particles which have a definite range, or pene- 

 trating power; Holmes gives a table of these ranges.^^ The more 

 radioactive the element, the greater is this range; i. e., the more 

 rapidly it disintegrates. 



If, as we have said, a mineral contains elements disintegrating at 

 different rates, and uranium is composed of two such elements, the 

 older minerals will contain on the average (or in case of isotopes, 

 certainly) more of the less stable, more rapidly disintegrating element 

 and of its products. Henderson has been trying to determine the 

 ages of halos by the relative strength of the rings, especially of one 

 due to a product, known as actinium C, of a uranium isotope to that 

 of a product of the uranium isotope which produces ordinary radium 

 and a shorter-lived element, which makes the outside ring, known as 

 radium C. He has also studied the thorium rings. It has been very 

 difficult to find mica in which the successive rings are distinct and 

 then to split it fine enough to get a section through the center of the 

 sphere of an alteration (which has a diameter of less than 0.04 mm) 

 instead of taking in a good part of the sphere, in which case the separate 

 rings are not visible. Joly ^* thought that the whole diameter was 

 slightly greater in the older halos, but it was only a thousandth of a 

 millimeter or so. The fact that the diameters of these halos are so 

 nearly the same as those of halos now artificially produced is an ex- 

 cellent reason (taken together with the fact that the more rapidly an 

 element sends off helium bullets — alpha particles — the farther they 

 fly) for belie^'ing that the rate of action has not changed substantially. 



To be sure, if the few extra long-range particles were too few to 

 make a perceptible darkening during the time of our present experi- 

 ments but might in a million years, and in the older halos are respon- 

 sible for the outside diameter, then there might be a slight secular 

 quickening in the discharge of the alpha particles which had been 

 thus disguised. 



In order to make any estimates of time, Henderson had to estimate 

 the relative strength of the rings made by the various radioactive 

 elements. This can be done by the microdensitometer, the same 

 kind of instrument which astronomers use in measuring the propor- 

 tions of elements in the spectra that come from the stars, and J. L. 

 Rose, estimated the proportion of isotopes by working upon a fine 

 line structure, that is, comparing the strength of the lines into which 



w Nat. Res. Counc. Bull. 80, ch. 2, pt. 4, p. 160, 1931; Age of the Earth, p. 135, 1937. 



** Joly, J., Pleochroic halos of various geological ages. Proc. Roy. Soc, ser. A, vol. 102, pp. 682-705, 1925. 



