Boltwood first drew attention to the "lead-method" of
determining age of a mineral.

Boltwood believed all lead in primary uraninites to be
derived from uranium disintegration, i.e. that

$$
\text{Pb} = \text{RaG} = \frac{\text{U}}{\text{W}_\text{u}} \cdot \text{G} \cdot (\text{e}^{-\lambda_\text{u}} \cdot \text{t})
$$

Richards, Hönigschmid and Fajans showed that ThD is also
a lead isotope.

Hence

$$
\mathrm{ThD} = \frac{\mathrm{Th}}{\mathrm{W}_{\mathrm{th}}} \cdot \mathrm{D} \cdot (\mathrm{e}^{-\lambda_{\mathrm{th}}} \cdot \mathrm{t} - 1) .
$$

Aston mass-spectrographic intensities indicate AcD as
lead isotope of mass 207.

$$
\mathrm{AcD} = \frac{\mathrm{AcU}}{\mathrm{W}_{\mathrm{acu}}} \cdot \mathrm{C} \cdot (\mathrm{e}^{-\lambda_{\mathrm{acu}}} \cdot \mathrm{t} - 1) .
$$

HST.400026.032

Popular formula to calculate age (disregarding all lead of about 207 mass)

$$t_1 = \frac{\text{Pb}}{\text{U} + \text{k} \cdot \text{Th}} \cdot \text{C}$$

k and C are considered constants.

Accurate form of above is

$$t_1 = \frac{\text{Pb}}{\text{U} + \text{k} \cdot \text{Th}} \cdot \left[ \frac{W_u}{\lambda_u} \cdot \frac{1}{A} \right]$$

in which k = a constant but C is a function of the atomic weight of lead in the mineral.

HST.400026.033