152 



Professor Dewar 



[June 13, 



accuracy at present, it would be useless to discuss the results with any 

 severity. All that can be inferred is that the subject is worthy of 

 farther investigation and promises important generalisation. Sarrau 

 {Comjjf. Hend. 1882) deduced the critical temperatures and pressures 

 of hydrogen, oxygen, and nitrogen by the application of Clausius' 

 formula to the experiments of Amagat ; and it is interesting to 

 compare his results with the experimental values. 



Sarrau's Calculated Values. 



Hydrogen 



Oxygen 



Nitrogen 



It will be observed that the calculated critical temperatures of 

 oxygen and nitrogen are remarkably near the truth, being respectively 

 8° and 22° too high. On the other hand, the values of the ratios of 

 the calculated critical temperatures and pressures are almost identical 

 with those obtained by direct experiment. The only peculiarity to 

 be noted is in the case of hydrogen, which has such a high critical 

 pressure, and therefore leads to a remarkably small molecular volume 

 at the critical point. If the values of the T-f-P ratio be taken as 

 proportional to the molecular volumes, then it is easy to infer the 

 densities of the fluids at their respective critical temperatures, pro- 

 vided the density of one standard substance is known by experiment. 

 The simple formula thus stated is 



S' ^ W' V 



s w w 



where S and S' are the specific gravities of two bodies, W and W 

 their molecular weights, and V and V their molecular volumes. It 

 will be convenient to take the density of carbonic acid at the critical 

 ])oint as the standard density to which the others can be referred. 

 The density of carbonic acid under such conditions may be taken as 

 0-65. Calculating with the above formula, the density of acetylene 

 would be 0-32, whereas the experimental number of Ansdell is 0'36. 

 In the same way the density of hydrochloric acid is found to be 

 • 6, the true value being 0-61. The density of oxygen would be • G3, 

 and that of nitrogen • 45. The calculated density of hydrogen at its 

 critical point would be 12, if we assume the correctness of Sarrau's 

 values for the critical temperature and pressure. A\'e may compare 

 these values with the numbers obtained by Caillctct and Hautcfeuille 

 for the densities of oxygen, nitrogen, and hydrogen from their 



