The alumina was precipitated from a solution of pure ammonia—alum, (NH_{4})_{2}SO_{4}.Al_{2}(SO_{4})_{3}.24H_{2}O, in distilled water by the addition of pure ammonia, sufficient chrome-alum also being dissolved with the ammonia-alum to furnish about 2½ per cent. of chromic oxide in the resulting stone. The powder, carefully prepared and purified, was placed, as has been stated above, in the vessel, _D_, and on reaching the flame at the orifice it melted, and fell as a liquid drop, _N_, upon the pedestal, _P_, which was formed of previously fused alumina. This pedestal was attached by a platinum sleeve to an iron rod, _Q_, which was provided with the necessary screw adjustments, _R_ and _S_, for centring and lowering it as the drop grew in size. Great care was exercised to free the powder from any trace of potassium, which, if present, imparted a brownish tinge to the stone. The pressure of the oxygen, low initially both to prevent the pedestal from melting, and to keep the area of the drop in contact with the pedestal as small as possible, because otherwise flaws tended to start on cooling, was gradually increased until the flame reached the critical temperature which kept the top of the drop melted, but not boiling. The supply of powder was at the same time carefully proportioned to the pressure. The pedestal, _P_, was from time to time lowered, and the drop grew in the shape of a pear (Fig. 54), the apex of which was downwards and adhered to the pedestal by a narrow stalk. As soon as the drop reached the maximum size possible with the size of the flame, the gases were sharply and simultaneously cut off. After ten minutes or so the drop was lowered from the chamber, _M_, by the screw, _S_, and when quite cold was removed from the pedestal. Very few changes have been made in the method when adapted to commercial use. Coal-gas has, however, entirely replaced the costly hydrogen, and the hammer is operated by a cam instead of an electromagnet, while, as may be seen from the view of a gem-stone factory (Plate XIV), a number of blowpipes are placed in line so that their cams are worked by the same shaft, _a_. The fire-clay screen, _b_, surrounding the flame is for convenience of removal divided into halves longitudinally, and a small hole is left in front for viewing the stone during growth, a red glass screen, _c_, being provided in front to protect the eyes from the intense glare. Half the fire-clay screen of the blowpipe in the centre of the Plate has been removed to show the arrangement of the interior. The centring and the raising and lowering apparatus, _d_, have been modified. The process is so simple that one man can attend to a dozen or so of these machines, and it takes only one hour to grow a drop large enough to be cut into a ten-carat stone. [Illustration: _PLATE XIV_ BLOWPIPE USED FOR THE MANUFACTURE OF RUBIES AND SAPPHIRES]