BETWEEN 75 AND 150 MILLIMETRES OF MERCURY. 
425 
The Observations. 
In commencing a set of observations the first step is to clear away any residue of 
gas by making a high vacuum throughout the apparatus, the mercury being lowered 
below the manometers and bulbs. The mercury having been allowed to rise into 
the pump head of the Tdpler, the gas to be experimented on is next admitted to a 
pressure of about 7 5 millims. This occupies the manometers, the bulbs, and part of 
the capacity of the intermediate chamber E. The passage through the right-hand 
manometer is then closed by bringing up the mercury to the neighbourhood of C, 
and by rise of mercury from I to H the pressure is doubled in the upper bulb. The 
next step is to cut off the communication between A and B, and to renew the 
vacuum in B. If the right amount of gas has been imprisoned, it is now possible to 
make a setting, the mercury standing at A, C, H, and in the side apparatus some¬ 
where in the upper tube below J. If, as is almost certain to be the case in view of 
the narrowness of the margin, a suitable setting cannot be made, it becomes neces¬ 
sary to alter the amount of gas. This can usually be effected, without disturbing 
the vacuum, by lowering the mercury at C and allowing gas to pass in pistons in the 
curved tube CD either from the intermediate chamber to the bulbs, or preferably in 
the reverse direction. 
When the right amount of gas has been obtained, the observations are straight¬ 
forward. On such occasion six readings were usually taken, extending over about an 
hour, during which time the temperature always rose, and the means were comljined 
into what was considered to be one observation. 
A complete set included four observations with the large bulbs at 150 millims. 
pressure and four at 75 millims. To pass to the latter the mercury must Ije lowered 
from H to I and in the left-hand manometer, and the pump worked until a vacuum 
is established in D. It was considered advisable to break up one of the sets of four; 
for example, after two observations at 150 millims. to take four at 75 millims., and 
afterwards the remaining pair at 150 millims. In this way a check could l)e 
obtained upon the cpiantity of gas, of which some might accidentally escape, and 
there were also advantages in respect of temperature changes. These eight observa¬ 
tions with the large bulbs were combined with four in which the side apparatus was 
alone in use, tire mercury standing all the while at G. Of these, two related to the 
75 millims. j^ressure and two to the 150 millims. Finally, the means were taken of 
all the corresponding observations. 
The following table shows in the notation employed the correspondence of volumes 
and temperatures : — 
3 I 
VOL. cxoviii.—A. 
