( 441 ) 
bear in mind that compressed gas must not remain behind in 
scratches, holes or other irregularities of the walls, as this could 
have the same influence as any air-bubbles which might be kept 
back by dirt gathered near the pressing valve, if we had not taken 
the necessary precautions to prevent this as described in %. 3. For 
it is not unusual for all the gas from the pump cylinder to be 
compressed to less than 1 cc. when forced through the pressing- 
valve. In addition all the parts must remain free from rust, and 
hence only perfectly dry gas can be admitted into the pump. By 
means of the precautions taken (comp. also B 1 and U 1) it is 
possible to obtain a vacuum, which in the pump cylinder is a 
primary necessity if the exhaust is to be satisfactory. 
D. 3. Into this vacuum again the gas must be so admitted 
during the exhaust, that no air can possibly enter, that the mercury 
cannot be contaminated, and that the inflow of the gas is not 
hindered, while even under the highest pressure nothing must leak 
back from the pump-cylinder towards the exhaust-side. 
In CAILLETET's original pump this is attained by a cock, which 
is opened and shut at the right moments by means of levers moved 
by blocks (comp. Pl. I) fastened to a disk on the shaft of the pump. 
Such a cock cannot work without being lubricated and a little of 
the lubricant might come on to the surface of the mercury, and 
the pump could then no longer be used (comp. U 3). We cannot 
be certain that the cock will remain properly lubricated after some 
time, and in lubricating air might come in which could contaminatie 
the gas. Besides it is difficult to keep the cock channel free from 
the lubricant and therefore large enough. 
And yet CarLLETET had good reason for using this cock. For an 
ordinary valve will generally give either an insufficient closure with 
the various pressures, at which a good closure is required, or it 
will stick at the highest pressure, so that it does not admit gas 
into the pump-cylinder during the next exhaust. 
In order therefore to replace that cock by a valve, which would 
avoid this lubrication, we had to contrive (as remarked in Comm. 
N°, 14) a special construction which would satisfy the above men- 
tioned requirements without being liable to the difficulties offered 
by ordinary valves. The valve described in § 3 enables us at 
least as far as it is concerned, to work with the pump uninterrupt- 
edly, for as long as is required. It is only rarely that the 
exhaustion fails, and then it suffices to let the pump rest a few 
moments in the exhaust position in order that the valve may again 
become loosened. 
32 
Proceedings Royal Acad. Amsterdam. Vol. IL, 
