( 640 ) 



tubes, but on liner Unes etched on to small glass plates E^^ — E^,, 

 provided with connections Eg^ and screws E^^ to fix them to the 

 cylindricals portions of the measuring vessel. By a proper arrange- 

 ment of these the meniscus and the lines can be sharply seen over 

 the whole length of the case. 



The measuring vessel is calibrated by temporarily bloAving on, at 

 the lower end, a small glass cock with a fine point (cf. Comm. N". 70. 

 IV. May '01). The mercury is introduced through this in the care- 

 fully exhausted apparatus and the menisci are then read in the' 

 manner described above for the measurements and with tlie same 

 precautions as to lighting and temperature. The mercury drawn olf 

 at the cock is weighed. As before the readings are made on the glass 

 plates but in order that the calibration shall not be lost if these have 

 to be removed, they are also compared Avilh the lines on the cylin- 

 drical tubes. This would be necessary if the tube had to be removed 

 cleaned and dried after the method of Comm. N". 27, but usually 

 it is sufiicient to suck iiji the various liquids and to dry by repeated 

 evacuation admission of (\vx air through >'„ and i\ of PI, II fig. 2. 



The measuring vessel can be completely shut off from the mer- 

 cury reservoir, see PL II fig. 2 or the manometer by the clamp CI^. 

 All these connections are carefully cleaned good thickwalled black 

 rubber tubes, which are strengthened by wrapping them spirall_y in 

 strong tape. The mercury meniscus (cf Comm. W. 67 Dec. '00 for 

 what is here implied) remains quite clean after a series of measure- 

 ments, if only dry gas is admitted to the volumenometer (this is 

 only true when the gas has no action on the fat of the cocks and 

 joints). The small changes in the ]:)Osition of the meniscus to bring 

 it to the required position are made by manipulating the clamp Cl^ 

 and the resevoir. During the measurements this clamp is always closed. 



The pressure of the gas in the volumenometer is given by the 

 height of the mercury in the manometer tube when CI^ is quite 

 open. This is itself connected to a barometer and a resevoir at con- 

 stant temperature by airtight connections in the manner of Comm. 

 W. 60 June '00 and the same precautions are taken to ensure 

 accuracy in the temperature determinations of these two mercury 

 columns. The volumenometer meniscus and etched lines, manometer, 

 barometer and standardscale (cf Comm. W. 60) are so placed that 

 they can be read without altering the telescopes of the cathetometer. 



The menisci of the barometer and manometer are read as described 

 before, that of the volumenometer by the help of a brass plate with 

 a 2 mm. slit in it, which is brought with a glow lamp to the same 

 level as a meniscus and gives good definition. 



