836 



W 

 point, from vvliich may he inferred tliat ^ = 0.84465 at this point. 



According tu tlie quadratic formula we should then iiave for 



W 

 Pt' — = 0.84593 corresponding to — 38°.84 C, whereas the lem- 



perature-scale of the P. T. R. gives — 38°. 89. Borrowing from 

 Hknning's paper the results of other observers: 



Steavart — 38.85 



Chappuis — 38.80 ± 0.02 



Chree — 38.86 



we see that the freezing point as determined hy Henning, when 

 reduced to ti)e lieiden scale, coincides exactly with the mean of the 

 other observers. Although this must, of course, not form the basis 

 of a final judgment on the difference of the two temperature-scales, 

 still we may see in it an indication of the cause of the deviation, 

 viz. too great a value having been attributed to Callendar's formula 

 at temperatures below — 20° C. 



Each fresh direct determination of the freezing point of mereury 

 may, moreover, lead to a decision in fa\our of one oi' the other of 

 the two temperature-scales. 



Apart from the deviation just discussed, the agreement is a very 

 good one, no greater deviations occurring than of 0.02 of a degree. 

 It remains a matter for regret, however, that for Pt^' the constants 

 of Callendar's formula were not determined, before proceeding to 

 use the thermometer at low temperatures, which would have made 

 a more direct comparison possible. With a view to the great import- 

 ance of Pt^ for low temperature-thermometry it was not deemed 

 advisable to carry out the determination in question now. ') 



§ 4. The vapour-pvi'.'<mre of oxygen. 



KamerlijSGH Onnes and Braak have determined the vapour-pressure 

 of oxygen at four different temperatures. As the resistance of P^i' was 

 measured at the same time, these vapour-pressures may be i-educed 

 to the temperature-scale as laid down in the above table. 



We find, that the temperature corresponding to a given vapour- 

 pressure may be represer\ted by the following formula which is of 

 the form as proposed by Acgust — Rankine— van der Waals. 

 ^,_ 369.83 



~ 6.98460. — log p 



Henning also gives the vapour-pressure at a few temperatures in 



1) Comp Comm. Leiden N'\ 141a 



