with a modified form of Sprengel-pump. 95 
bakkie and is condensed. Without this precaution I have poe 
it impossible to obtain a vacuum higher than gs-gyy-g00 
point of fact the bends should always be heated when a igh 
exhaustion is undertaken even if the pump has been standing 
well exhausted for a week; the heat should of course never be 
ne at a be stage of the exhaustion. Convers sely, I have 
often by t of heat completely and quickly removed quite 
large spiatitiek of the vapor of water that had oa purposely 
introduced. The exhaustion of the vacuum-bulb is of course 
somewhat injured by the act of using the pump mis also by 
standing for several days, so that it has been usual with me 
before undertaking a high exhaustion to incline the pump and 
reéxhaust for 20 minutes; I have however obtained very high 
vacua without using this precaution. 
During the process of exhaustion not more than one-half of 
the mercury in the reservoir is. allowed to run out, otherwise 
when it is returned bubbles of air are apt to find their way into 
the vacuum-bulb. In order to secure its quiet entrance it is 
poured into a silk bag provided with several holes. When the 
reservoir is first filled its walls for a day or two appear to 
furnish air that enters the vacuum-bulb; this action, however, 
soon sinks to a minimum om then the leakage remains quite 
constant for months togethe 
Measurement of the Gacailtik —The cylinder into which the 
gauge-tube dips is first elevated by a box sufficiently thick 
merely to close the gauge, afterwards boxes are placed under it 
_ sufficient to elevate the mercury to the base of the measuring 
tube; when the mercury has reached this point, thin boards 
and card-boards are added till a suitable pressure is obtained. 
The length of the enclosed cylinder of air is then measured 
with the cathetometer, also the height of the mercurial “ menis- 
“te ” and the difference of the heights of the mercurial columns 
in A and B, figure 4. To obtain a second measure an assistant 
removes some of the boxes and the cylinder is lowered by hand 
three or four centimeters and then replaced in its original posi- 
tion. In measuring really high vacua, it is well to begin 
with this process of lowering and raising the cylinder, and to 
repeat it five or six times before taking readings. It seems as 
though the mereury in the tube B supplies to the glass a coat- 
ing of air that allows it to move more freely; at all events it is 
certain that ordinarily the readings of B become regular, only 
after the mercury has been allowed to play up and down the 
tube a number of times. This applies particularly to vacua as 
high as Frise and to pressures of five millimeters and 
under. It is advantageous in making measurements to a ded 
