364 Prof. J. Dewar. The Specific Volumes of Oxygen 



2 26 '2 5 c.c. If the first two experiments are eliminated on the assump- 

 tion that the proper equilibrium of temperature had not been attained, 

 the average weight per litre would become 4'428 grammes, and the 

 specific volume 225'82. 



Taking Kegnault's density of oxygen at and 760 mm., the density 

 at 90'5 in the ordinary way would be 0'0043137, and the specific 

 volume 231 '82 c.c. Thus the volume given by the ordinary gaseous 

 laws is 1'0246 times the average observed volume ; or we may put it 

 that pv is diminished at the boiling-point of oxygen by 2 '46 per cent. 

 Again, while the ratio of the absolute temperatures is 3'017, the ratio 

 of the densities is 3'091. 



Behn* has determined by an indirect method the specific volume of 

 oxygen, and finds the value 358, which is nearly 60 per cent, greater 

 than the volume found by the direct method. The mode of proce- 

 dure he adopts is to ascertain directly three quantities out of four in 

 the ordinary thermodynamic equation correlating latent heat, tem- 

 perature, increment of pressure to temperature, and specific volume, 

 thereby deducing the unknown quantity. Now of the three experi- 

 mental values required, one, viz., increment of pressure to temperature, 

 can only be got by calculation from the vapour-pressure curve, and 

 much depends upon the accuracy of this value. Accepting Estreicher's 

 vapour pressures for liquid oxygen below its boiling-point as the most 

 reliable, a Gibbs equation gives the increment per degree near 740 mm. 

 pressure as 78'67 mm. mercury pressure or 106'93 grammes per square 

 centimetre. This value, taken along with the latent heat found by 

 Behn, and the boiling-point, gives, when inserted in the thermodynamic 

 equation, a specific volume of 223'55, which is within less than 2 per 

 cent, of the value found by the direct-density determinations.! 



Further experiments were made on oxygen vapour at 90 '5 and 

 under reduced pressures. These experiments and their results are 

 given in the following table. The same symbols are used as in the 

 preceding tables, except that d is the calculated density at 90 '5 

 absolute and pressure p, and v is l/d. The corrected volume of the 

 flask is entered under V. 



* ' Ann. der Physik,' vol. 1, 1900, " Sublimationswarme der Konlensaure und 

 die Verdampfungswarme der Luft." 



t In the same way Behn'a specific volume of carbonic acid would be contradic- 

 tory of my proof that a constant-volume gas-thermometer filled with carbonic acid 

 at about atmospheric pressure gives a very accurate value of its own boiling-point. 

 Assuming the ordinary gaseous laws, the specific volume ought to be 361'6 instead 

 of 423 given by Behn. Now my value of the increment of pressure to temperature 

 at the boiling-point is 62'84 mm. mercury pressure, and this, along with the values 

 used by Behn in the thermotlynamic equation, gives 363 as the specific volume. 

 This comes much nearer the anticipated value of the constant. 



