cathode, are put the following ingredients: Fluoride of calcium, 234 parts by weight; double fluoride of cryolite, 421 parts by weight; fluoride of aluminum, 845 parts by weight. To these add three to four per cent of a suitable chloride—for example, calcium chloride. To this add alumina sufficient to form a very stiff mixture. Before electrolysis can begin the above are fused by means of heat, which should not exceed 1,210° F. The heat is obtained from a furnace heated by gas, coke, or charcoal, care being taken that no gases from the furnace enter the crucible. The bath fused, the electrodes are dipped into it, the current switched on, and the metal is deposited (in the best and largest of these crucibles) at the rate of one pound per five electrical horse-power hours. The current pressure required is six to eight volts, at a density of one and a half amperes per square inch. The metal from time to time is removed from the crucible by means of a siphon or a ladle, care being taken to remove as little of the haloid salts as possible. There is another method of extraction equally successful with this, but also more economical. In this other method a set of similar ingredients are placed in a crucible having one or more vertically movable carbon electrodes, which are used as one, or a collective anode, respectively. The crucible, though lined principally with carbon, has some metal exposed to act as a cathode at the beginning of the process, this to generate heat enough to fuse the bath, after which the anode is placed so that the extracted aluminum acts as a cathode. The molten metal is from time to time run out of a tap-hole into a mold, and thence cast into ingots, or granulated by being poured into cold water. The same particulars as to results apply to this crucible furnace process also, only that not nearly so much of the bath is wasted in it, and the metal needs less purifying when molten. There are, also, no loss of time and money from the use of gas, coke, or charcoal, and of an extra furnace in this method.
"A Mechanical Bootblack."—A bootblacking apparatus is one of the latest developments of the nickel-in-the slot machine, a specimen of which is undergoing trial in a French public garden. The customer drops his coin—in the present case a ten centime, or a two and-a-half-cent piece—into the receptacle, which opens the way to a compartment where a brush cleans his boots; he next puts his feet into a second compartment and has them blackened; and then into a third, where they are polished. The operation takes about a minute and a half, and during the time the customer may watch the indications
of its progress as they are shown upon the dial. The machinery working in the inside is very simple. An electric motor of small power—about eighteen kilogrammetres per second—controls the shaft on which the three rotary brushes are fixed, and the customer has only to unlock the machine, the same as all others of its kind, with his coin, and move the handle which opens the circuit and starts the motion. A representation of the machine at work is given in the accompanying illustration, for which we are indebted to La Nature.
The "Barisal Guns."—A curious phenomenon of unexplained sounds like those of explosions, occasionally heard in different places over the earth, has attracted much attention, has been made the subject of a book recording several hundred accounts of it, by M. Ernest Van den Broeck, of Brussels, and has already been mentioned in the Popular Science Monthly. The phenomenon has been most carefully observed in India, where it seems to have assumed