peedy Volumetric Determination of Carbonic Acid. 189 



result. It may be advisable to drive tbe air into tbe apparatus by 

 degrees as the absorption takes place, and this is my ordinary mode 

 of procedure. In order to ensure the absorption of the whole of the 

 carbonic acid, leaving behind but an insignificant fraction, the air 

 should be passed at least three times through the absorption-tube, or, 

 in other words, should be made to circulate three times through the 

 apparatus before recording the total amount of carbonic acid present. 

 The small fraction of carbonic acid left would of course become in- 

 creased after each analysis if allowed to remain in the instrument. It 

 may therefore be advisable to ran the air in the left-hand bell-jar 

 through the absorption-tube before commencing an analysis. After 

 the air has circulated three times through the absorption-tube the 

 steadiness of the reading for the volume of carbonic acid absorbed, 

 should the air be passed again and again through the absorption-tube, 

 is usually remarkable, unless there should be an alteration in the tem- 

 perature of the apparatus, when the reading is observed to follow the 

 changes of temperature. The volume of air introduced into the appa- 

 ratus to fill up the vacuum is now read off, as also the temperatures of 

 the two bell- jars and absorption- tube. All this is done without 

 drawing the shutters of the case containing the bell-jars, two little 

 windows admitting the hands to work the taps ; the absorption appa- 

 ratus is outside the case at the back (although shown in the plate 

 as inside the case). An instrument} which has necessitated such a 

 long description will be thought slow and tedious in its working, but 

 such is far from being the case, considering that the actual analysis 

 will take from 20 minutes to half-an-hour. I expect, however, an 

 increase of speed by substituting a mechanical movement for the 

 hand to turn the handle. Time can probably also be saved bypassing 

 the air to be analysed through a tube immersed in water at the 

 temperatre of the bell-jars. When the temperatures and pressures 

 become rapidly steady, the time required for the analysis is of course 

 shortened. 



I have not yet found out the shortest period required for depressing 

 or raising the bell- jars ; if the movement is too rapid a displacement 

 takes place in the level of the oil in the tanks, and a slight suction 

 follows which of course will be mistaken for so much carbonic acid, 

 a slow movement given to the bell-jars overcomes that objection com- 

 pletely. 



No additional time is taken up for running the alkaline solution 

 through the absorption-tube, as this is done by another person while 

 the observer is filling and adjusting. Of course it is only after 

 acquiring some experience that the above speed is obtained. 



Compared with Pettenkofer's method, the present instrument claims 

 much greater speed, for while by the former process the solution 

 holding the barium carbonate has to be left a day or over night for the 



VOL. xli. 



