lO ATTAINMENT OF VERY LOW TEMPERATURES. 



will be positive; below that temperature it will be negative. The in- 

 version points calculated by Witkowski are 633° and 193.7° absolute 

 for air and hydrogen respectively. Olszewski (Nature, 1902) confirms 

 the second figure, finding 192.5° absolute for the inversion point for 

 hydrogen. He points out that the inversion points are practically 

 corresponding temperatures for the two gases. 



The application of the principle of free expansion to the liquefaction 

 of air was first made practicable by Hampson in England and Linde in 

 Germany. Since Hampson and Linde's work paved the way to the 

 production of liquid hydrogen, by a very similar process, and as my 

 researches have been based upon their results, some mention must be 

 made of their work. To Dr. Hampson I am particularly indebted for 

 much advice and assistance in carrying out my investigations. 



V. The Performance of the Hampson Air Liquefier. 



The Hampson air liquefier is too well known to require a lengthy 

 description. It consists of a coil formed by winding two, or in some 

 cases four, copper tubes coaxially so as to build up a cylinder 100 cm. 

 long and 30 cm. in diameter. The coil is enclosed in a metal case. 

 Air enters the copper tube at the top under a pressure of 180 atmos- 

 pheres, and expands at a valve, in which the tubes unite at the bottom 

 of the coil. The air, cooled by expansion, presses upwards through the 

 interstices of the coil, cooling the compressed air it contains. The 

 cooling of the valve is therefore progressive, and results, after a few 

 minutes, in the partial liquefaction of the escaping air. The liquid 

 collects in a space at the bottom of the metal case containing the coil, 

 and is drawn off from time to time through a cock. 



The apparatus as installed in the Chemical Department of Uni- 

 versity College, London, is worked in connection with a Whitehead 

 " torpedo compressor " driven by an electro-motor. To deliver about 

 ten cubic meters of air per hour (measured under atmospheric condi- 

 tions), and to maintain a pressure in the liquefier of 180 atmospheres, 

 requires the expenditure of about 6.5 horse-power ; the compressor 

 runs at 360 revolutions per minute. An attempt has been made by Mr. 

 Arnold Ogden and myself to determine the efficiency of the plant. I 

 have also carried out a fairly complete investigation of the air liquefier. 



The work done in compressing the air was determined by measur- 

 ing the electrical energy absorbed by the motor under the following 

 conditions : 



Power required to run motor free 0.6 E. H. P. 



Power absorbed by motor, belt and bearings (cylinders removed) . . 1.8 " 

 Power required to run motor with cylinder in place (no com- 

 pression) 3.7 " 



