by going into compressed air, unless his Eustachian tubes are blocked, in which case intense pain is produced, owing to the great difference in pressure between the two sides of the ear drum. The above-described lock should be used immediately on prostrations occurring, as it seems to be of little value after some time has elapsed. A very slight increase of carbonic oxide (if it much exceeds one part in a thousand) in the compressed air chamber leads to increased sickness. The impurity never affects a man while below, but only after he comes out, and we had mules working under pressure in New York for over twelve months at a stretch, which sold at good figures after coming out. Every man should be medically examined, and hot coffee should be given to each man before he comes out of compressed air. A warm room to dress in and extra clothing for passage through the lock should be supplied. At the Blackwall Tunnel, with the experience gained and attention to the above points, we have not had a single death, notwithstanding the fact that we had men working under a pressure of thirty-seven pounds per square inch for some time. Generally sparely built men, not too full-blooded, are those who stand air pressure best. A man with weak lungs may work and improve, but one with a weak heart or any apoplectic tendency should not go in at all. Drink of all classes is bad, but such drinks as tend to thicken the blood are worse than spirits."
The Electro-metallurgy of Aluminium.—Dr. Joseph W. Richards recently delivered before the Franklin Institute a very interesting and instructive lecture on the electro-metallurgy of aluminium. Several years ago the daily press gave considerable space to descriptions of the new aluminium industry and discussions of the modifications which its cheap production would bring about in the arts. While it subsequently proved unsuited to many purposes for which it was at first thought well fitted, it has become quite an important staple, and its applications are gradually increasing. Dr. Richards thus describes the process of manufacture: Pure alumina made from ore by a chemical process is stirred into a fused solvent bath composed of the double fluorides of aluminium and sodium. This bath may be simply cryolite, but preferably cryolite to which has been added a further proportion of aluminium fluoride and a little calcium fluoride (fluorspar). The alumina is dissolved by the bath to the extent of one fifth of its weight. The electric current is then sent through this mixture, using for anodes carbon rods dipping into the bath from above. The cathode is formed by the carbon lining of the vessel, on the bottom of which the melted aluminium collects. When the dissolved aluminium has nearly all been removed, the resistance of the bath rises, and fluorine fumes, from the decomposition of the solvent, begin to appear; fresh alumina is then stirred in and the operation thus proceeds continuously. The cavity containing the fused salt has a sump in which the molten aluminium collects and from which it is removed by ladles. The action of the current, when not of too high a voltage, is to decompose only the alumina as long as it is present in the bath in sufficient amount. The oxygen simply combines with the carbon anodes and passes away as carbonic oxide. The above process was discovered independently in 1886 by Heroult in Europe and Hall in America. In 1888 Hall put aluminium thus made on the market. The plants now engaged in making aluminium on this principle are as follows: The Pittsburg Reduction Company, at New Kensington, Pa., and at Niagara Falls, having a daily capacity of 4,400 pounds; the works at the Rhine Falls in Switzerland, capacity 5,000 pounds; and works at La Praz and Saint-Michel in France, with a combined capacity of 5,500 pounds. Besides these, there are in contemplation or course of erection five other plants, which will raise the total possible daily output to 42,900 pounds.
A Convention of Dragon Flies.—Some curious movements of dragon flies were observed one September afternoon by Prof. Charles Barrois, of Lille, along a road near Morbihan, France. The insects were seen, thousands in number, seated along the telegraph wire, all in the same position, their bodies in the axis of the wire, their heads turned west toward the setting sun, and their abdomens making an angle of twenty-five degrees with the wire. New insects were coming from every side, plunging first