CARBON A TES B V ABSORPTION 95 



for the purpose of absorbing any hydrochloric acid 

 vapour which may be carried over, and may obviously 

 be left out if sulphuric acid is employed. The small 

 U-tube E also contains calcium chloride. The actual 

 absorption of the carbon dioxide occurs in the potash 

 bulbs F, which are charged with 50 per cent, solution 

 of caustic potash, and in the small attached U-tube, 

 which is packed with small pieces of solid potash. The 

 use of soda-lime instead of potash is to be deprecated. 

 The U-tube G prevents the absorption of carbonic 

 acid or moisture from the atmosphere. It is filled 

 with fused calcium chloride in the limb next to the 

 absorption apparatus, the outer limb being packed with 

 soda-lime. 



Preliminary operations. — As commercial calcium 

 chloride always contains a certain amount of quicklime, 

 and consequently absorbs carbon dioxide, it is necessary 

 to pass CO2 through the calcium chloride U-tubes for 

 an hour or so, in order to transform any oxide into 

 carbonate. The excess of carbon dioxide is removed 

 by passing dry air through. 



The potash bulbs and U-tube are carefully weighed 

 and the apparatus is then fitted together, the dropping 

 funnel being filled with dilute sulphuric acid. Hydro- 

 chloric acid is not as convenient for use. 



The apparatus is tested to see whether all the 

 joints are tight, by sucking gently at h^ clipping the 

 rubber tube tightly between the fingers, and observing 

 whether any leakage causes the bubbles to move in 

 the potash bulbs. It is essential that the apparatus be 

 quite tight. 



The estimation. — According to the amount of 

 carbonate shown to be present in the soil by a pre- 

 liminary test, 5 to 20 grams of the material are 

 introduced into the flask and made into a thin paste 



