Thermodynamic Properties of Air. 289 



they give fuller and more exact information than observations 

 of critical points, or other singular states. Amongst recent 

 work in this direction the very important investigations of 

 Amagat, extending to very high pressures, must be placed 

 in the first rank. They contain exceedingly valuable data, 

 relating to the compressibility and expansion of gases at 

 ordinary temperatures and at higher ones. Amagat's results 

 give a very clear and extensive idea of the behaviour of 

 several gases, chiefly of those the critical point of which is 

 not far from ordinary temperatures. 



Until quite recently the so-called permanent gases had not 

 been investigated at very low temperatures. So far as I 

 know the important paper by Wroblewski, u On the Com- 

 pressibility of Hydrogen/' published after the author's death 

 in the Sitzungsberichte of the Vienna Academy, is the only 

 one relating to this range of temperatures. This want of 

 information as regards the compressibility and expansion of 

 permanent gases in the vicinity of the critical state has 

 induced me to undertake the experimental investigation of 

 which the present forms an account. The properties of 

 atmospheric air are described here, at temperatures ranging 

 from + 100° to — 145° Cent., and for pressures from 1 to 130 

 atmospheres. 



§ 2. Outline of Method. — In order to determine the com- 

 pressibility at different temperatures, and through it also 

 the expansion of gases, two experimental arrangements were 

 chiefly used : in one of them the quantity of gas remains 

 constant, in the other its volume. We might call them the 

 manometric and the volumetric method. In the first of 

 these methods (Andrews, Amagat, &c.) a long calibrated 

 capillary glass tube, enlarged at the open end into a bulb of 

 known volume, is employed. A certain quantity of gas is 

 shut up in the tube by mercury. By means of a compressing 

 arrangement the volume of the gas may be varied at will. 

 The experiment consists in determining the volume and 

 temperature of the gas and the amount of the applied pressure. 



The method of constant volume was invented, so far as I 

 know, by Natterer. A vessel of known volume was filled 

 with gas under a known pressure. The experiment con- 

 sisted in measuring the quantity of gas contained in the 

 vessel at a certain temperature. This was done in the so- 

 called pneumatic trough, under atmospheric pressure. This 

 method has been used also by Wroblewski in his experiments 

 on the compressibility of hydrogen at low temperatures (the 

 first method being useless here on account of the freezing 

 of the mercury). 



