extent changes may be produced in the earths through oxydation?
506.
Undoubtedly the negation of the above question is confirmed by the circumstance that wherever mineral colours are found, some trace of metal, especially of iron, shows itself; we are thus naturally led to consider how easily iron becomes oxydised, how easily the oxyde of iron assumes different colours, how infinitely divisible it is, and how quickly it communicates its colour. It were to be wished, notwithstanding, that new experiments could be made in regard to the above point, so as either to confirm or remove any doubt.
507.
However this may be, the susceptibility of the earths with regard to colours already existing is very great; aluminous earth is thus particularly distinguished.
508.
In proceeding to consider the