298 THE EARL OF BERKELEY, MESSRS. E. G. J. HARTLEY AND C. V. BURTON: 
apparatus, there was a difference in the final* position of the mercury level in the 
stem according as the pressure had previously been rising or falling. In all cases 
the falling pressure gave the higher reading. The cause of this was eventually traced 
to the lubricant! surrounding the tap key at the top of the piezometer. 
The piezometer shown in fig. 2 seems to 
meet both troubles. The bulb (total volume 
about 32 c.c.) is divided into two branches, A 
and B, thus giving a larger cooling surface. 
The tap is now placed at the end of a 
graduated capillary, CD, so that by careful 
manipulation the latter can be filled with 
mercury to a level which will be in the field 
of view of window C, of fig. 1. 
The remainder of the piezometer contains 
the solution, except for a centimetre or two 
of mercury at the bottom of the stem. Thus 
the solution is confined between two surfaces of 
mercury, and although the mercury meniscus 
near the tap will move slightly, if this move¬ 
ment be noted the real change in volume of 
the solution can be calculated. 
The Pressures .—The apparatus used is one 
of Schaffer and Budenberg’s dead-weight 
gauges, delivering a maximum pressure of 140 
atmospheres.J The pressure was transmitted 
by castor oil from the screw plunger to a 
mercury U-tube ; and by water from the latter 
to the compressibility apparatus. 
In the course of the research it was noticed 
that rapid changes of pressure, especially when 
on the up grade, caused some uncertainties in 
* That is when the temperature is constant again after the effects of changing the pressure have 
disappeared. Constancy of temperature is indicated by the fact that there is no further movement in the 
mercury ■ indeed, the piezometer is in effect a very sensitive thermometer. 
t For, on increasing the pressure, the lubricant is compressed, and as there is a slight difference 
in pressure due to the unbalanced height of the mercury in the stem, the key may move slightly into 
its seating; this movement obviously is not reversible. Occasionally, another source of trouble was 
encountered, namely, that caused by a small bubble of air left in the key of the tap when filling the 
piezometer; this bubble could be seen easily, but it was found to be there sometimes even in the new 
form about to be described. The presence of this trace of air would give quite fallacious results for the 
lower measurements. 
J The same apparatus as was used for the determination of direct osmotic pressure, see ‘ Phil. Trans.,’ 
A, vol. 206, 1906, p. 485. 
ft 
I 
'I 
a 
fij 
?! 
Zl 
B 
/f K 
o 
A 
graduated 
-A 
•D 
w 
graduated 
-E 
Fig. 2. 
