290 APPENDIX 



can perform the operation with a mass of molecules 

 a given volume of air. All we have to do is to put air 

 in a closed cylinder, compress it by means of a piston 

 and at the same time place the cylinder in a second 

 cylinder containing a material whose atoms or mole- 

 cules are quickly expanding and absorbing Ether when 

 pressure is removed, and this we can do by pressing gases 

 into a very small space and then letting them expand, 

 or by means of the exhaust pump. The material- 

 Prof. Dewar finds most suitable is carbon di-oxide. 

 This only explains the principle the apparatus used 

 by Prof. Dewar to obtain gases in the liquid condition 

 involves many and complex structural conditions. 1 



The product of these two operations is liquid air. 

 " A very faint blue " liquid. 



The molecules of the liquid are, however, in an 

 abnormal condition; they want of their own inherent 

 poii'er to again absorb Ether to become gaseous 

 which is their normal condition. 



To restrain this reaction was one of Prof. Dewar's 

 most difficult problems. He devised a double receiver 

 having an absolute vacuum (as far as gases are con- 

 cerned) between the two glass receivers. 



Fig. 4 illustrates the form of the vessel, where the 

 inner vessel is kept away from the outer by an abso- 

 lutely empty space or a vacuum. Prof. Dewar finds 

 covering the outside of the inner receiver with a thin 

 layer of silver makes the vessel more impervious to the 

 passage of Ether, Filling the space between the 

 receivers with mercury in the gaseous condition, and a 

 small excess of liquid mercury at the bottom, as illus- 



1 See Proceedings Royal Institution of Great Britain, vol. xv. 

 No. 90, 1896, p. 144. 



