1920-21.] Solid Caustic Soda for absorbing Carbon Dioxide. 131 
mixtures were taken every five minutes and were analysed in a portable 
Haldane gas-analysis apparatus.* 
Four series of tests were carried out. For the first series the gas 
mixture was dried before it entered the caustic soda compartment. In 
the second series the gaseous mixture was saturated at room temperature, 
12° C., before coming into contact with the caustic; and the third series 
was similarly saturated at blood temperature, 37° C. ; while in the fourth 
series the mixture was allowed to saturate itself with moisture at the 
temperature at which the bath was maintained. In each of the first 
three series the weight of moisture carried by the gas was constant 
throughout the tests, while in the last it varied in accordance with the 
temperature of the test. 
I. Gaseous Mixture dried before passing over Caustic Soda. 
The 4 per cent, mixture was dried by passing through concentrated 
sulphuric acid, and through U-tubes containing calcium chloride; it then 
passed over copper sulphate. The absence of a blue tinge in the latter 
showed the gas to be dry. It must here be observed that although 
precautions were taken to keep the caustic soda dry, doubtless it 
contained a proportion of moisture. Commercial caustic soda usually 
contains water up to 10 per cent., and in exceptional cases up to 25 per 
cent. Analyses were made of the gaseous mixture entering and leaving 
the caustic compartment at frequent intervals. Their results enabled an 
average to be struck for that period of the percentage of C0 2 extracted at 
each of the temperatures of the tests. When these average extractions 
were graphed against temperature, curve A (fig. 2) was obtained. It was 
observed, at any given temperature, that the caustic soda became most 
effective after a lapse of from ten to twenty minutes, and that after that time 
the efficiency of absorption rapidly fell away. The examination of the 
material taken from the tray after an experiment showed the granules to be 
substantially unaltered in form and to have received only a thin coating of 
carbonate. The greater part of each granule was unaltered caustic soda. 
II. Gaseous Mixture saturated with Water Vapour at 12° C. 
The tests were carried out in the same manner as those just described, 
and as a result curve B (fig. 2) was obtained. The efficiency of absorption 
is evidently better in this case than in the last. The maximum efficiency 
of absorption took place in the circumstances of this test at 70°-90° C., 
where it amounted to a 72 per cent, extraction of the carbon dioxide. 
* J. S. Haldane, Methods of Air Analysis , p. 48. 
