Whether any colour can be excited in the pure earths by the chemical operations of nature and art, without the admixture of metallic oxydes, is an important question, generally, indeed, answered in the negative. It is perhaps connected with the question—to what extent changes may be produced in the earths through oxydation?
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.
However this may be, the susceptibility of the earths with regard to colours already existing is very great; aluminous earth is thus particularly distinguished.
In proceeding to consider the metals, which in the inorganic world have the almost exclusive prerogative of appearing coloured, we find that, in their pure, independent, natural state, they are already distinguished from the pure earths by a tendency to some one colour or other.
While silver approximates most to pure white,—nay, really represents pure white, heightened by metallic splendour,—steel, tin, lead, and so forth, incline towards pale blue-grey; gold, on the other hand, deepens to pure yellow, copper approaches a red hue, which, under certain circumstances, increases almost to bright red, but which again returns to a yellow golden colour when combined with zinc.