It has been maintained that the peculiarities of the opal depend in a great measure upon the quantity of water it contained, and which, mixed mechanically with the silica, varies from three to twenty per cent. But some chemists who have interested themselves in the study of the composition of the mineral do not regard the presence of water as absolutely essential for the development of the varied flashes of color. We will only state in this brief sketch that there is certainly a mystery connected with the part water plays in producing and perfecting the beauty of the opal. This quantity or factor of water varies greatly in the different varieties of opal. Apparently when heat is applied to the mineral the brilliancy of its hues is increased, either from evaporation of its water or some structural change. But if the degree of heat is too great, or its application too prolonged, the hues of the opal vanish and cannot be recalled by human skill. The same results from the effects of heat may be noticed in other gems of greater density and hardness, as the emerald, the topaz, and the tourmaline. It has been maintained that a faded opal may be restored to beauty by immersion for a time in water, with the view of restoring the fancied loss by evaporation; but we fear that the experimentalist will be often disappointed with his results. However, there is one singular variety of the mineral known as the hydrophane, which does not exhibit colors until after it has been immersed for a time in water, and when removed from its bath and becomes dry again its hues vanish. Therefore, we may justly affirm that there is a mystery in connection with the influence of water in producing the color of the opal. The optical properties of this mineral do not afford decisive distinction, and it never crystallizes in regular and definite form like quartz, neither does it exhibit a trace of double refraction. We will say no more at present concerning the composition of this interesting substance except to allude briefly to the experiment of Damour, who found the opal to turn black when sulphuric acid was applied to it; hence he inferred that the substance contained some organic matter, but precisely what he could not determine, although he suspected the presence of bituminous matter. Similar foreign bodies have been detected in many other precious stones. In fact, many of our gems are impaired by impurities; and so generally, that a stone of absolute purity is of rare occurrence. Even the diamond, which is regarded as the emblem of light and purity, is reckoned by microscopists as one of the foulest of gems.