320 THE EARL OF BERKELEY, MESSRS. E. G. J. HARTLEY AND C. V. BURTON: 
and possibly fluctuations in this may help to cause the slight variations which seem 
to occur in high winds. 
Weighing Corrections .—In applying the usual buoyancy corrections there are two 
special circumstances to be considered. In the earlier part of our research we had 
been content to regard the loss of weight in the solution as a measure of the change 
in the volume of displaced air for which the buoyancy correction should be calculated. 
Having improved the manipulation and consequently the accuracy of the experiments, 
it was seen that this rough estimate could be bettered if we knew the specific volume 
of the solution remaining in the vessel at the end of the run. The data required are 
obtained approximately if we know the initial quantity of solution in each branch 
and how far crystallization has proceeded. 
The magnitude of the correction thus deduced is in no case greater than 0'0015 gr. 
and may in the calculations of osmotic pressure be neglected; it however is of 
importance when we balance the losses against the gain by the sulphuric acid in the 
end vessel. 
Similarly, but only since November 5, 1915, have we applied the corresponding 
correction to the change in the specific volume of the sulphuric acid when it absorbs 
water. In this case we have succeeded in making the correction more precise, for 
knowing that practically all the water is absorbed in the first two branches^ of the 
vessel, it is an easy matter (especially in the new quartz apparatus) to cause their 
contents to mix, so that having noted the original amount of acid present the specific 
volume of this new solution can be calculated. The correction obviously depends on 
the relative quantities of acid and water ; it has never risen to a higher figure than 
0'0045 gr. 
From a series of blank weighings with vessels treated as in an actual experiment 
it was deduced that the probable weighing error is ±'0005 gr., a quantity which is 
about the millionth part of the total weight on the pans. 
Other Sources of Error .—A few other sources of error have been investigated, but 
as, in the apparatus used, their effect is never large enough to alter the results, they 
will only be mentioned briefly. There is a small weighing error due to the fact that 
the air contained in the water vessel! is saturated, and its density is therefore less 
than that of the air in the balance case. The usual buoyancy correction might have 
to be modified were it not that the vessels are weighed before and after the 
experiment under practically the same circumstances. It is found that even two or 
three degrees difference in the temperature of the balance case, although it causes a 
change in the vapour pressure, does not affect the weighings. 
It was thought that plunging the vessels straight from the laboratory temperature 
into the bath at 30° C. might bring about some uncertainties ; these, if any, are 
* See Berkeley and Hartley, “On a New Form of Sulphuric Acid Drying Vessel,” ‘Phil. Mag.,’ 
vol. 29, 1915. 
f Care is taken that the pressure inside the vessel is the same as that in the balance case. 
