1182 
somewhat considerable deviation from the straight line, as a preli- 
minary plot has shown. In this manner the rectilinear character of 
the diameter is duly observed and the critical density is found in 
a more rational way than otherwise. 
The equation of the diameter being written in the form 
y=a-t bt 
the coefficients are found as follows 
a= — 0:063510. 
b = — 0:00039402. 
The 6%, 7% and 8 column of the table refer to this calculation ; 
y(C) are the calculated values of y, O — C the differences, absolute 
and in percentages of y(C) respectively. 
§ 6. Discussion. From the last column it appears that the devia- 
tions from the straight line are usually small, less than 7/100, except 
near the critical point, where they rise to almost 1/20. The experi- 
mental diameter evidently shows a small hump. A similar bump, 
although very little marked, was previously found with argon, but 
on the other hand it was very pronounced with nitrogen. The 
maximum deviation there amounted to nearly Ys. The magnitude 
of this deviation surprised us considerably at the time and made 
us suspect some irregularity in our method of observation or cal- 
culation as the cause. Since, however, we have now found a hump 
of a similar character, although four times lower, with hydrogen, 
we think we may infer that this result may be dune to some 
systematic error, not yet explained, by which in some manner 
(condensation on the wall?) a not quite correct vapour density is 
observed. 
The slope of the diameter is given by the coefficient 
b = — 000039402 
the smallest valne so far found for any substance. 
The critical density as derived from the diameter, using — 239°-91 5) 
as the critical temperature, is 
rd = 0.038102 
in complete accordance with the value previously calculated 0:0310. 
A comparison of this value with the value to be derived from 
the isothermals by means of the equation 
(3 _ (dp 
VA Jon dT coër.k. 
1) These Proceedings 20, p. 178, Leiden Comm. N°. 151c. 
