Itnmediately before each analytical ope ration a sufficient quantity of the oxide is ignited, and while still hot transferred to a dry tube, by plunging the mouth of the tube into the oxide in the crucible and then shaking it in piecemeal. The tube with the oxide is immediately closed with a dry cork, and al lowed to cool. Meantime the interior of the retort is completely dried by heating each por tion of it in succession in the flame of a spirit lamp, beginning at the closed end, and draw ing air through the heated tube, by means of a narrower tube passed down just beyond the heated part and exhausting by the mouth; when every part has thus been dried, the retort is corked and allowed to cool. Five or six grains of the powdered and dried sub stance are put into a perfectly dry test tube, and the whole is very accuiately weighed; its contents are then added to the oxide of copper in the mortar and the empty tube again weighed ; the difference gives the weight of the substance employed.
The best kind of mortar is one of Wedge wood ware or Berlin porcelain, capable of con taining about half a pint, with a pestle com posed of a single piece of the same material ; it should be thoroughly dried and well warmed. Much caution is requisite in charging the re tort. The warm dry mortar is placed on a sheet of glazed paper and first cleared out with a little of the dried oxide of copper, which is put aside. Oxide to the depth of an inch is poured into the combustion tube ; a small quantity of oxide is put into the mortar, then the substance to be analysed, then more oxide ; the mixture must be made quickly and care fully, adding so much oxide as shall be suf ficient to fill a little more than half the retort; the mortar is then taken in tbe palm of the left hand and the mixtuie introduced, carefully picking it up piecemeal by the retort tube it self; fresh portions of oxide are rubbed in the mortar to clear out the last traces, and the retort is then filled up with pure oxide of cop per to within two inches of the extremity.
The proportions of the mixture are repre sented in fig. 430 : the portion from the tail of the tube to the letter a consists of pure oxide of copper, from a to b of the mixture, from b to c of the rinsings of the mortar, and from c to within an inch of the cork is again pure oxide.
If the process of mixing has occupied much time, it may be advisable to subject the tube and its contents to a further operation to re move any traces of moisture that may have been absorbed. The tube is struck smartly in a horizontal position on the table, to clear the tail-like prolongation, and to make an air-way above the oxide from end to end; an exhaust ing syringe made fast to the table by a screw vice or other convenient means, is attached to a long tube filled with chloride of calcium ; and this drying apparatus is fitted by a sound cork to the retort tube. This latter is laid in a shal
low trough open at one end, which is slightly elevated ; the trough is then filled with sand heated to about 212° F., and cautious exhaus tion is performed by the syringe, taking care that none of the charge is carried out of the tube by the current of air; on gradually open ing the stop-cock air is slowly re-admitted, being dried in its passage over the chloride of calcium; it is allowed to remain in the appa ratus a few seconds, and the exhaustion re peated ; these operations are performed in suc cession ten or twelve times. It is, however, rarely necessary to resort to this process of desiccation, and it is objectionable from the ease with which many compounds rich in hy drogen decompose the oxide of copper at com paratively low temperatures.
The drying tube having been accurately weighed is next fitted to the dried perforated cork, and connected by it air-tight to the retort tube; this is now placed in the furnace, which has been disposed in a convenient place rest ing on bricks; to the drying tube the potash apparatus, also previously weighed, is attached by a connecting piece of caoutchouc, taking care that the largest bulb is on the arm con nected with the drying tube ; the potash ap paratus should be slightly inclined by placing a cork under the end of the horizontal portion nearest the open extremity. Matters being thus arranged, we proceed to ascertain if the whole be tight, and for this purpose expand the air in the large bulb by beat, so as to ex pel a few bubbles; if, on cooling, the liquid rise in the limb and maintain its elevation steadily for a few minutes, the combustion may safely be begun. Charcoal broken into pieces about the size of a walnut is ignited in a cru cible furnace or by any other convenient means, and when red-hot applied to the por tion of the tube nearest the cork where the pure oxide of copper hes ; the action of the heat is limited by a double sheet-iron screen which fits into the furnace, and has a central slit which allows it to bestride the tube; this screen can by degrees be moved further and further down the furnace until the whole tube is heated. An additional screen of single iron plate is hung over the closed end of the fur nace to protect the cork, which usually should reach to within an inch of the fire, care being taken that the heat never rises so high as to scorch it, or falls so low as to allow of the condensation of moisture in the portion of the retort which projects from the furnace.