( 453 ) 
4. The safety-cap (ks, Pl. HI) consists of a small chamber 4,,, 
on the rim of which a carefully rolled thin plate of hard brass is 
screwed by means of a nut fy. and a washer. Thickness and mani- 
pulation are so arranged that explosion will occur at a given 
pressure, while leakage which easily occurs with the usual safety- 
valves is excluded. 
fe. The cockboard. The tube k; (Pl. HI) conducts the compressed 
gas to the main tube », of the cock-board, carrying the arms n,, 
ny and #3), so that the gas can be drawn off by four ways. 
Generally the cock mj, admits the gas into the apparatus in which 
it is to be kept or used for researches, while a reservoir p is con- 
nected to the tube 4, which reservoir in many cases serves to 
maintain the pressure, when the apparatus is fastened on to the 
cock N19. 
The cock mn, serves to conduct the compressed gas to another 
apparatus or to be sampled for analysis. 
The cock 739 allows the tension of the compressed gas to be 
measured on the manometer. With cocks 49; and J, closed we can 
also measure on the manometer the tension of the apparatus connected 
to mq Or %% 3, and also with cock m2 closed and cock /, opened the 
tension of the gas in the dome of the pump. As an example: we 
may wish to test if the pump is working regularly (especially with 
the operations II and III). 
By means of the cock #33 the pump with accessories is connected 
with the sucking-apparatus (comp. § 4v) consisting, among other 
things of a tube leading to an airpump and to an open bottle with 
a safety-tube immersed in mercury. Moreover #3, is used to permit 
gas brought under pressure, to flow back to the exhaust-side of the 
pump (e.g. a gas-holder) (comp. § 4 7). 
§ 4. The accessories described above may be considered as im- 
mediately belonging, to the pump, now we have still to consider 
what is further required for the operations mentioned in § 2 and 
indicated on Pl. III by If, I, IIL. 
I. To suck gas from a space under ordinary 
or less than atmospheric pressure. 
An instance of this is the frequently occuring sucking from a 
gas-holder. To this end the sucking-tube gz, of the pump is con- 
nected to the; 
