Letter from William Huggins, 90 Upper Tulse Hill, S W [South West] London, to the Secretary [Joseph Larmor] Recalling the experiments of [Daniel] Berthelot, which suggested that 'natural amethysts are produced in the depths of the earth by radioactive substances'. Using his findings, Huggins concludes that the colouration of the quartz within his own experiments must be a result of oxides of manganese. 30 May 1907