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POPULAR SCIENCE MONTHLY 



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 Ycrk 

 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 pas- 

 sage 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 interest- 

 ing 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 un- 

 suited to many purposes for whioh 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 pro- 

 cess is stirred into a fused solvent bath 

 composed of the double fluorides of alu- 

 minium and sodium. This bath may be 



simply cryolite, but preferably cryolite to 

 which has been added a further propor- 

 tion of aluminium fluoride and a little cal- 

 cium 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 pro- 

 ceeds 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 independ- 

 ently in 1886 by Heroult in Europe and 

 Hall in America. In 1888 Hall put alumin- 

 ium 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 ob- 

 served 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 tele- 

 graph 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 



