548 Scientific Proceedings, Royal Dublin Society. 



The degree to which the exhaustion has been pushed, may be 

 accurately determined by first filling the burette with mercury* 

 and then, the stopcock being closed, lowering the reservoir suffi- 

 ciently to sink the mercury in the burette to the level of the 

 100 c.c. division line ; a torricellian vacuum will consequently be 

 obtained in the burette. Connection with the combustion tube 

 is now opened, and the tube is gently warmed. After a few 

 minutes the stopcock is again closed, and the reservoir raised ; 

 if any air has been drawn from the combustion tube, its presence 

 will be indicated as the mercury rises to fill the burette, and its 

 volume may be accurately determined. With a little care it is 

 quite possible to exhaust so completely the combustion tube, that 

 no residuum of air can be detected in it by the method here 

 described. The time required to exhaust a combustion tube, with 

 the apparatus of the size described in this paper, is about ten 

 minutes. The time, however, will vary somewhat with the dimen- 

 sions of the combustion tube. 



The combustion tube, after being exhausted, is heated to ignite 

 the water residue, the stopcock being closed during the process of 

 heating. 



During the combustion the laboratory vessel should be care- 

 fully and completely filled with mercury, and attached to the 

 apparatus in the following manner : — The laboratory vessel is held 

 in a slanting position, with its lower shoulder just resting on the 

 front edge of the shelf f, and its upper part q loosely fitting round 

 the collar at the end of the tube g. The reservoir is then ad- 

 justed so that the mercury in it, when the laboratory vessel is full, 

 shall be just above the level of the collar. The mercury slowly 

 fills the laboratory vessel, and when it has risen a little above the 

 collar, the lower shoulder of the vessel may be pushed into its 

 place. After the laboratory vessel has been properly attached, the 

 re-agent tube must be freed from air. This may be done by un- 

 screwing the bunsen clamp, and allowing a little mercury to flow 

 out of the tube. When the air has been completely driven out, 

 the re-agent tube is again closed by screwing up the clamp. If, 

 after first attaching the laboratory vessel, any traces of air are 

 detected in it (and small bubbles are especially liable to collect 

 under the indiarubber collar attached to the tube g), the operator 

 will be able to expel them by drawing the lower part of the vessel 



