300 THE EARL OF BERKELEY, MESSRS. E. G. J. HARTLEY AND C. Y. BURTON: 
further cooling with the stem in mercury, caused the latter to rise to a suitable level. 
The piezometer was then quickly transferred to its position in the lower half of the 
compressibility apparatus which was already in the bath, and, with luck, the mercury 
thread carried in the stem joined the mercury already in place at the bottom of the 
apparatus. ** 
It was found that a satisfactory junction was greatly facilitated by chamfering off 
the bottom of the piezometer stem as shown at E in fig. 2. # 
A very similar procedure was followed for the experiments at 30° C. The 
piezometer being in place, the upper half of the apparatus was passed over the 
projecting bulbs and secured both to the lower half and to the pressure system. 
Supposing these operations to have been carried out successfully, the mercury 
meniscus should stand, when the bath temperature had been regained, slightly below 
the lowest window ; then a small increase of pressure will bring it opposite the 
window. 
On the other hand, should the meniscus be above this window, the apparatus has 
to be taken apart, the mercury driven out so as to replace some of it by solution, and 
after cooling, &c., replaced in the bath. 
When a steady state had been reached, the pressure on the apparatus, the level of 
the meniscus showing in the lower window and that in the topmost window 
(C of fig. l), and the temperature both of the bath and of the interior of the 
apparatus were noted. The pressure was then increased until the meniscus showed 
in the second window; this increase of pressure caused a development of heat which 
was dissipated in about three-quarters of an hour, as indicated by the constancy of 
the position of the meniscus; when steady, the five quantities were again noted, and 
so on. 
Having taken the readings for the fifth window, the pressure was reduced 
and the process just described was precisely reversed until the initial phase was 
reached. 
It is to be noted, on the assumption that there is no hysteresis in the glass of the 
piezometer,! that the same readings (corrected for small displacements of the upper 
meniscus and for any change in the temperature of the bath) should be obtained 
whether going up or going down the scale of pressures. When these readings are 
noticeably divergent we have generally found that a pellet of solution has formed 
across the mercury thread and therefore such experiments should be viewed with 
suspicion. On the other hand even if the readings are the same it is no proof of a 
* In later work this difficulty is overcome by fastening a small capsule containing mercury to the 
bottom of the stem. 
f We would draw attention to a source of error which has been observed in one of the Jena-glass 
piezometers. This error lies in the fact that some of the air bubbles usually present in capillary tubing 
(as long hair-like lines) communicate with the bore of the tube; when under pressure minute threads 
of mercury can be seen to be moving in these capillaries. 
