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DR C. HUNTER STEWART ON THE 



removes this possible source of error. The Pettenkofer tube, with a capacity of about 

 130 c.c, is charged with 100 c.c. baryta solution, and the glass tube at A replaced. This 

 tube should be contracted at the end, so as to break up the entering air into small bells, 

 and thus insure the complete absorption of the carbonic acid. The air is aspirated at the 

 rate of 1 litre per 45 minutes.* If aspirated at a greater rate, then two tubes in series, 

 each charged with 100 c.c. baryta solution, are necessary. I have found by experiment 

 that by aspirating through two tubes in series, at the rate of 45 minutes per litre, there 

 was no change in the baryta solution in the tube nearest the aspirator, showing that 

 entire absorption had taken place in the one tube. After the experiment the contents of 

 the tube are emptied into a stoppered bottle, and set aside to allow the barium carbonate 



Fig. A. 



to settle. The stopper should be paraffined to prevent absorption of carbonic acid from 

 the air. Six or eight hours are necessary for the settling of the precipitate. It is some- 

 times stated that the baryta acts on the substance of the glass during these operations, 

 and thus undergoes a change quite irrespective of the carbonic acid. If this were so, a 

 serious error would exist in this method of carbonic acid estimation, especially when 

 working with small quantities, e.g., in examining outside atmospheric air. In connection 

 with this, the following experiments were made : — Two Pettenkofer tubes, A and B, and 

 two 120 c.c. white glass stoppered bottles, C and D, were taken, B and D being coated 

 inside with a layer of solid pure paraffin. Each was then rinsed out with the baryta 

 solution to absorb the carbonic acid of their contained air, drained, and charged with 



Before connecting the absorption apparatus, the air is directly aspirated from the iron tube to clear it of its 

 contained air. 



