128 Proceedings of the Royal Society of Edinburgh. [Sess. 
XIV. — Utilisation of Solid Caustic Soda in the Absorption of 
Carbon Dioxide. By Elizabeth Gilchrist, M.A., B.Sc., A.I.C. 
Communicated by Professor Henry Briggs, D.Sc., Ph.D. 
(MS. received June 17, 1921. Read July 4, 1921.) 
In the course of research on Mine Rescue Apparatus under the Department 
of Scientific and Industrial Research it was found necessary to investigate 
the conditions of utilisation of solid caustic soda for absorbing carbon 
dioxide, and particularly the effects of variations of temperature and 
water vapour upon the reaction. 
Though the experiments relate especially to the conditions prevalent 
in breathing apparatus, it is believed that they have sufficient general 
interest to warrant their description in a separate paper. The tests 
were carried out by Mr D. Penman, B.Sc., and the writer, under the 
direction of Professor Henry Briggs, and the results are published by 
permission of the Research Department. 
Preliminary Experiments. 
Preliminary comparative experiments were carried out in which air 
containing carbon dioxide was passed through U-tubes holding caustic 
soda granules. These exploratory tests showed that : — 
1. The temperature at which the caustic soda is maintained had a 
most important effect on its power of absorption, and that the absorption 
fell off almost to zero if that temperature was kept at or below 0° C. 
2. The size of particle had a great effect on the absorptive efficiency, 
small particles being more effectual than large particles. This result 
doubtless follows from the greater area of surface exposed by the small 
granules. 
3. When the soda particles act efficiently they swell considerably, 
and in so doing tend to fill up the interstices between them, thus causing 
choking unless measures are taken to prevent it. 
Apparatus and Methods. 
In view of the results of the preliminary inquiry, it was found to be neces- 
sary to carry out all subsequent experiments with granules of caustic soda 
which had been sized. The size adopted was that of from J-inch to J-inch 
diameter. Thirty grams of granules were employed in each experiment, 
