greater than 24° 13'. Diamond refracts light very strongly―the index of refraction for the yellow ray being 2·419, while that of rock crystal is but 1·545; of topaz, 1·621; of white sapphire, 1·75; of phenakite, 1·675; and of white zircon, 2. In the extent to which diamond disperses the several coloured rays into which white light is split, this gem greatly surpasses all others. Its "fire," or the flashing of prismatic hues which characterises this precious stone, is mainly due to this dispersive power.
The specific gravity of the diamond, when transparent and colourless, is of remarkable constancy. When taken in the ordinary way, without the refinement of certain small corrections which are made only for scientific purposes, the best results have lain between the narrow limits of 3·52 and 3·53, at 60° Fahrenheit. The fine colourless Porter Rhodes diamond has the specific gravity 3·523: the smaller but equally fine Gor-do-norr, 3·527. The former stone was found at Kimberley, South Africa, on the 12th of February, 1881, and weighs 474 troy grains: the latter is of Indian origin, and weighs 2131/2 grains. The Star of the South, a Brazilian stone of 2541/2 carats, has the specific gravity of 3·529, according to M. Halphen.
The range of colour of the diamond is extensive; but various hues of yellow, grayish yellow, brown, and straw colour, are the most common. Strongly-coloured diamonds are very rare; but green, blue, and even red stones are known. The celebrated Hope blue diamond, of 441/4 carats, and the Brunswick blue diamond, of 61/2 carats, are both of the same brilliant and steely blue, and may very likely have both been parts of the French blue diamond stolen from the Garde-Meuble, in 1792, and never since seen.
The least valuable diamonds are those which lack brilliancy, or have faint hues of gray, brown, and yellow. The most prized are those which combine brilliancy with decided tints of rose, green, or
