The problem of age determination of a mineral is of great interest both from a geological and physical standpoint. The most widley used method is a radioactive one, based on the lead-uranium ratio as found by chemical analysis of the mineral. There are possible sources of error in this method owing to contamination with common lead and to alteration of the lead. It is the purpose of the present paper to suggest a new method which, although not at present capable of great accuracy, seems to be free from some of these difficulties. It is based on the presence of actinium in pleochroic haloes as disclosed by the new halo photometer discussed in the two preceding papers. The most commonly accepted theory of the origin of the actinium series of radioelements, due to Piccard and Rutherford is that the parent of the series is actinouranium, AcU, an isotope of uranium. This theory has been worked out in detail by von Grosse while Western abd Ruark have further analysed the relevant data. Since AcU is found to have a shorter period than UI, the older the mineral the greater should be the ratio of the amount of AcD lead (Pb
207
) to the amount of RaG lead (Pb
206
).
A New Method of Chemical Analysis
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