Careful choice in the substances to be melted together, good and effective cutting, and careful artistic setting, have gone a long way to reproduce, artificially, the brightness, brilliancy, and colour of the real stone. Chemical analysis shows the sapphire to be pure alumina, as it has shown the diamond to be pure carbon; but it does not account for its colour, which is partly due to an optical effect, and depends upon a peculiar molecular arrangement. This stone possesses the singular property known as _dichroism_—that is, it shines with two colours, blue and red. In a well-cut stone, a red cross often appears in the midst of the sapphire blue. The ruby is also pure alumina, and its vivid red colour, like the blue of the sapphire, is thought by some to be due to a peculiar optical effect. In fact, no chemical analysis has been able to account quite satisfactorily for the red colour of the ruby or the blue colour of the sapphire, for pure alumina is quite white, and the sapphire, as we have seen, shows two colours. This peculiar optical effect noticed in the ruby and sapphire has, strange to say, been accidentally reproduced not long since by a French chemist, M. Sidot, who has been making some experiments on artificial stones. He has produced a kind of glass by melting phosphate of lime at a great heat, and the product possesses the blue colour of the sapphire with the remarkable _dichroism_ before alluded to. The experiment is so curious, that a few lines may be devoted to it here.