R

a

Fig. 13.

A

a

a

C

B

B

C

Fig. 13 gives the form of this instrument. The Stamp, A, is so bored through at four parts, a, that the internal diameter of each orifice is little more than the diameter of the side: but the cross section of the outer part is nearly the double. These perforations serve for extracting the superfluous clay that has been placed in the mould for the formation of a crucible. The forming part is three-quarters of an inch in diameter at the top, and five-eighths of an inch long.

The Mould, B B, consists of two parts, accurately joined, which form at the outside a truncated cone. At the inner side, the distance of each part is one thirty-second of an inch from the Stamp ; and the edges near b are flattened, so that small depressions may be produced in the two opposite points of the mould, to prevent the crucible from being injured when the stamp is withdrawn. c c is the binding ring into which the mould is fitted, so that, when unclasped, the mould falls with it exactly into a plane.

For the formation of these crucibles, a stiff paste is prepared with fire-proof clay; when this paste acquires a certain dryness, it is worked with the fingers into small balls, each containing more clay than is required for the manufacture of one of these vessels. Before introducing one of these balls into the mould, the forming parts and the stamp are well smeared with oil, by means of a fine brush, and then the mould, with the binding ring, is placed on an anvil resting upon a folded cloth to give elasticity. When the machine is thus arranged, the stamp is struck perpendicularly upon the ball with a mallet, until the protruding part, cc, rests on the margin, d, of the mould. The stamp causes the clay in the mould to spread, and the superfluous portion escapes through the small apertures at a. After the stamp has remained in the mould for about five minutes, it is gently screwed out; by this means the clay contained in the openings, a, and which is still in conjunction with the crucible, is smoothly removed.

The binding ring

To extract the crucible requires great care. is first taken off the mould, and then the two sides B B are slightly struck with a hammer; this causes them to separate and fall open at b b, and the crucible remains perfectly detached, upon e. The crucible, when cleansed from adhering clay by a small knife, is dried and heated in the same manner as the clay basins.

9. SODA PAPER.-In heating the quantitative silver assays before the Blowpipe, HARKORT found it necessary, instead of laying the ore, mixed with reagents, upon charcoal, to enclose the mixture in a substance that will resist the first effects of the flame of the Blowpipe, and prevent any loss of the particles. After some investigations, he ascertained that fine letter-paper, saturated with a solution of soda, was the best. This paper, which is made use of advantageously, not only in silver, but also in many more quantitative analyses, is prepared in the following manner :-In a flat basin, dissolve half an ounce of crystallized carbonate of soda, perfectly free from any sulphate, in one ounce of distilled water; then steep a number of strips of fine letterpaper in the solution for a few minutes, and allow them to dry in the open air. When dry, cut them into pieces, one inch and three-eighths long, and seven-eighths of an inch broad.

10. A MIXTURE of seven parts of Charcoal with one part of Clay-This mixture is used for lining the clay basins in which Lead and Tin ores are to be calcined. It is best prepared as follows:-Weigh seven parts of fine dry charcoal powder, and one part of finely sifted clay; mix the latter with water, and then add the former; next, knead the mixture into a paste. The tenacious mass thus formed is dried in a warm stove, and reserved for use. When a basin is to be lined with this mixture, a small quantity of it must again be moistened, and strongly rubbed over the interior of the vessel employed for calcination. The thickness of the lining at the bottom of the basin must be about one sixteenth of an inch, and at the top rather less. When this operation Fig. 14. is finished, the clay dish is placed upon the triangle,

c, Fig. 3, and dried over a spirit-lamp. Fig. 14 shows the cross section of such a lined basin.

V.-INSTRUMENTS, ET CETERA, REQUISITE IN ANALYSIS WITH THE BLOWPIPE.

1. A BALANCE.-The operator can please himself as to the form and quality of the balance. It may, however, be useful to give a few rules which will be applicable in the choice of it, and then point out the methods for testing its accuracy.

It should be of sufficient strength to bear from seventy to eighty grammes on each scale, and must be enclosed in a glass case to protect it from dust. This case must not be too small, and especially its sides should not approach too near the beam; the anterior wall should be divided into three parts, the centre one being fixed, and the two ends opening as sashes.

The balance must be provided with a proper contrivance to render it immovable whilst the weights are being placed on the pans. The best arrangement is that which raises the fulcrum and end edges from their supports, keeping the scales steady at the same time. It is most convenient when the operator can perform this part of the process without opening the case. It is necessary that the balance should be provided with an index to mark its vibration, and it should have a pendulum, or two hydrostatic scales for placing the edges exactly on a horizontal level; for this purpose it is best that the case should rest upon screws.

For weighing the milligramme and its fractions, the beam should be divided into ten equal parts accurately marked; on these marks or points a centigramme crotchet or hook is suspended instead of placing the weight on the pan.

The beam should have a screw to adjust the centre of gravity, and also two screws to regulate the length of the arms.

The following experiments serve to test the accuracy and sensibility of a balance.

a. Should the scales not be perfectly equal, they are accurately adjusted by means of tinfoil, and a milligramme weight is placed in one of them. A good and useful balance must distinctly turn

with this weight; a delicate chemical balance should indicate even 0.1 milligramme.

b. When both pans are loaded with the maximum weight the construction of the balance will admit of, and a milligramme is added to the weight in one scale, the beam ought to turn to the same degree as when the pans are unloaded.

c. Both scales are equally loaded with, say fifty grammes; the weights are then interchanged so that those of the right hand pan are transferred to the left, and vice versa. A balance, the arms of which are perfectly equal, must maintain its equilibrium upon this exchange.

The balance is set in motion, then arrested, and subsequently set in motion, until it recovers its equilibrium. If its quality is good, it will invariably assume the original position.

Fig. 15 represents an OERTLING's balance, which is the one generally used.

2. A SET OF WEIGHTS.-Weights, above 100 milligrammes, are seldom required. It is best, however, to have a box containing the Gramme and its divisions.

These, to prevent their oxidation and the adherence of foreign matter, should be kept in close boxes, well lined. They should on no account be touched by the fingers, but lifted, when wanted for use, with a pair of forceps.

Fig. 16.

3. A MICROSCOPE is indispensably necessary, in various operations to distinguish the effects of the flame and the diameter of metallic globules. An instrument formed of two fine plano-convex lenses-Fig. 16-of equal magnifying power, is very suitable; they must be so arranged that, when required, one can slide over the other, by a movable arm.

4. FORCEPS. Various kinds of Forceps are needful for experiments with the Blowpipe :

a. One with platinum points, to hold a small lamina of a specimen in the flame in order to try its fusibility-Fig. 17.

Fig. 17.