LIQUEFIED AIR. 



363 



the Fresnel refractors have been made to give a 

 higher coefficient of beam. Charles A. Stevenson's 

 improvements on the Fresnel lens have been intro- 

 duced in several Scottish lighthouses, and are 



Claimed to give an advantage of about 10 per cent. 



'over the older forms. 



There have been no important changes in burn- 

 ers within recent years. A mercury float carriage 

 has been introduced with some of the new rapidly 

 rotating lights erected in localities subject to seis"- 

 mic disturbances, the advantage being that the 

 float is not disturbed by earth tremors. 



The last report of the United States Lighthouse 

 Establishment shows that 1,839 light keepers are 

 employed, besides 1,226 assistants and 1,356 labor- 

 ers. There are 50 light vessels, 439 day or unlighted 

 beacons, and more than 5,000 buoys, of which 66 are 

 electric or gas buoys. 



LIQUEFIED AIR. Atmospheric air reduced 

 to liquid form by cold and pressure. Its boiling 



Soint is 191 C., its freezing point 207 and its 

 ensity 0.933. It is noteworthy as being the first 

 gas to be liquefied in large quantities. 



History. Gases were first liquefied by Michael 

 Faraday, who reduced to liquid form by pressure 

 cyanogen, carbon-dioxide, and others. As he did 

 not succeed with all, he concluded that oxygen, 

 nitrogen, hydrogen, and some other gases were per- 

 manent, and this distinction was made for some 

 time. The discovery by Andrews of the " critical 

 point " that is, the discovery that for every gas 

 there is a temperature above which pressure alone 

 can not liquefy it made it probable that the "per- 

 manence " of these gases was merely apparent, and 

 was due to the fact that their critical points are 

 very low. Efforts were therefore made to attain 

 very low temperatures, and these have been re- 

 warded by the final liquefaction of all gases, and 

 the consequent abolition of the distinction between 

 liquefiable and permanent gases. The failure to 

 liquefy certain gases is now seen to have been due 

 to the fact that their critical temperature is mostly 

 below 100 C. Oxygen was first liquefied by the 

 French physicist Cailletet, who obtained it in the 

 form of mist by compressing it in a freezing mix- 

 ture and then suddenly liberating it, thus further 

 lowering its temperature. Pictet, another French 

 experimenter, obtained the liquid gas in quantities 

 large enough to be experimented upon by using a 

 series of gases that had successively lower boiling 

 points. By first liquefying sulphurous acid and 

 then accelerating its evaporation with a vacuum 

 pump he reduced its temperature to 65 C. With 

 this he cooled carbonic acid gas, which was then 

 liquefied by pressure and connected with the 

 vacuum pump in its turn, and its temperature was 

 thus lowered so that it is solidified and reached a 

 temperature below the critical point of oxygen. 

 By using a similar series, of which oxygen itself was 

 the final member. Olszewski solidified nitrogen and 

 other gases. The first to obtain, or at any rate to 

 exhibit, liquefied atmospheric air in considerable 

 quantities was Prof. James Dewar, of the Royal 

 Institution in London. Concerning priority in the 

 liquefaction of certain gases and the use of certain 

 apparatus for this purpose, there has been a bitter 

 . controversy between him and Prof. Olszewski, re- 

 garding the merits of which scientific men are 

 divided. Prof. Dewar, by means of a series of suc- 

 cessive operations of the type just described, ob- 

 tained in 1894 about two quarts of liquefied air, 

 which he said cost him about $5,000. In 1898 

 Charles E. Tripler, of New York, made a machine 

 in which, using only the atmosphere itself as a 

 refrigerating substance, lie achieved the continuous 

 liquefaction of air, so that it can be drawn off 

 through a faucet in any desired quantity. Similar 



devices have since been operated in Europe. Mr. 

 Tripler's machine works on the principle of inten- 

 sive refrigeration. Every one is familiar with the 

 lowering of temperature produced by the sudden 

 release of a compressed gas. It is this that causes 

 the fog about the mouth of a bottle of beer or soda 

 water when it is opened. The cooling is due to the 

 conversion of heat energy into the mechanical 

 energy of expansion. In Mr. Tripler's machine con- 

 densed air is permitted to cool to the temperature 

 of the surrounding atmosphere and then allowed to 

 expand suddenly, thus lowering its temperature 

 further. This cooled air is used to cool other con- 

 densed air, by whose expansion the 

 temperature is then lowered still more. 

 By repeating the operation continu- 

 ously, the air is finally reduced to the 

 liquid state. 



Mr. Tripler's apparatus consists of a 

 triple air compressor, a cooler, and a 

 liquefier. The compressor has three 



Sumps in line on one piston shaft, the 

 rst giving 60 pounds to the square 

 inch, the second 750, and the third 

 2,000. After each compression the 

 air is cooled by passing it through 

 jacketed pipes surrounded by 

 city water, and after the third 

 compression it is cleaned. 

 It then passes to the liquefier 

 where, by means of a special 

 valve of Mr. Tripler's inven- 

 tion, whose details are not 

 made public, a portion of it 

 is allowed to expand and 

 passes into the space between 

 two concentric tubes, through 

 the inner of which the remain- 

 der of the air is flowing. The 

 latter is thus greatly cooled, 

 and by a repetition of the 

 process it is finally liquefied 

 and may be drawn off from 

 the valve at the bottom of the 

 apparatus. Tripler's original 

 apparatus, based on this prin- 

 ciple, was used in 1890, and 

 is only 12 inches long and 

 1-fk in diameter. His present 

 plant will produce 30 to 40 

 gallons of liquid air in a 

 working day of ten hours, with an expenditure of 

 40 to 50 horse power. The liquid appears in less 

 than fifteen minutes after the first pump has been 

 started. The properties of liquid air may be classi- 

 fied under three heads those due to its low tem- 

 perature ; those due to the high pressure developed 

 by its evaporation ina closed space; and those due 

 to the fact that it speedily becomes much richer in 

 oxygen than atmospheric air, since its nitrogen 

 boils away faster than its oxygen. 



Temperature. The usual temperature of liquid 

 air is that of its boiling point at ordinary atmos- 

 pheric pressure, 191 C. or 320 F. This, of course, 

 can not be raised by applying heat at atmospheric 

 pressure, which only makes the liquid boil away 

 faster. It could theoretically be raised by heating 

 the liquid under pressure, but the pressure devel- 

 oped by its own expanding vapor (ordinary air) is 

 so great that it can not safely be confined. The 

 boiling point can be lowered by lowering the atmos- 

 pheric pressure by means of a vacuum pump, and 

 thus the liquid can be reduced to a temperature of 

 210 C. Notwithstanding this low temperature the 

 hand may be dipped into the liquid with impunity, 

 being protected by the formation of a layer of vapor, 

 although severe frostbites result from continued or 



DEWAR S APPARATUS 



(1886). 



