4 o 



POPULAR SCIENCE MONTHLY. 



and have produced lower temperatures. In the latest form of ap- 

 paratus used by Olszewski, liquid ethylene is used as the cooling 



agent. Its boiling point is 102 C. 

 ( 151.6 F.). By causing it to boil 

 rapidly under diminished pressure a 

 temperature below the critical tem- 

 perature of oxygen can be reached. As 

 early as 1891 Olszewski obtained as 

 much as two hundred cubic centimetres 

 of liquid air by this method. Dewar 

 has also made use of liquid ethylene. 

 This was passed through a spiral copper 

 tube surrounded by solid carbonic acid 

 and ether. It was then passed into a 

 cylinder surrounded by another cylin- 

 der containing solid carbonic acid and 

 ether. A spiral copper tube, which 

 runs through the outer cylinder and 

 also through the inner cylinder in 

 which the ethylene was boiling under 

 diminished pressure, carried the air. 

 This was liquefied and then collected in 

 a vacuum vessel below. Later he found 

 that air can be liquefied by using liquid 

 carbonic acid alone as the cooling agent. 

 A sectional drawing of his apparatus 

 described in 1896 is given herewith. 

 As he remarks: "With this simple 

 machine, one hundred cubic centi- 

 metres of liquid oxygen can readily be 

 obtained, the cooling agent being car- 

 bon dioxide, at the temperature of 

 79. If liquid air has to be made 

 by this apparatus, then the carbonic acid must be kept under exhaus- 

 tion of about one inch of mercury pressure, so as to begin with a 

 temperature of 115." 



The introduction of the vacuum vessel by Dewar has been of 

 great service in all the work on liquefied gases. A vacuum vessel 

 is a double-walled glass vessel, as shown in Fig. 1, G. The space be- 

 tween the inner and outer walls of the vessel is exhausted by means 

 of an air pump before it is closed. The vessel is therefore sur- 

 rounded by a vacuum. As heat is not conducted by a vacuum, it 

 is possible to keep specimens of liquefied gases in such vessels for 

 a surprisingly long time. Heat enough can not pass through the 



FIG. 1. LABORATORY LIQUEFACTION 

 APPARATUS OF DEWAR FOR THE 

 PRODUCTION OF LIQUID OXYGEN, 

 ETC. 



A, air or oxygen inlet ; B, carbon- 

 dioxide inlet; C, carbon-dioxide 

 valve ; D, regenerator coils ; F, 

 air or oxygen expansion valve; 

 G, vacuum vessel with liquid oxy- 

 gen ; H, carbon-dioxide and air 

 outlet; o, air coil; , carbon- 

 dioxide coil. 



