1920-21.] Solid Caustic Soda for absorbing Carbon Dioxide. 135 
of heat, as it is important to keep an equal temperature in all parts. Other- 
wise at some parts the action may become violent, with consequent rise of 
temperature and stoppage of the reaction, while at others the temperature 
may never rise sufficiently high to give the best results. 
4. It is important that the particles be of such a size as to give a fair 
surface of exposure for absorption. Excessively large particles are for this 
reason to be avoided. Provided the conditions as to moisture and tempera- 
ture are at their optima, however, there is no need to decrease the size of 
the particles unduly in order to increase the surface ; e.g., given such con- 
ditions, granules 6 or 7 mm. in diameter will, by the swelling action 
described, be entirely or almost entirely used up. 
5. The particles must be so spaced as to give sufficient room for swelling. 
There should indeed be a greater volume of interstitial space in a canister 
fitted for absorption of carbon dioxide than of actual solid caustic soda 
itself. A disregard of this requirement leads to the coalescence of the 
granules due to swelling, and to the blocking of the air passages. This, in 
turn, involves a greatly increased resistance which is highly detrimental 
in breathing apparatus, and at the same time decreases the availability of 
the caustic to the carbon dioxide. 
( Issued separately September 5 , 1921 .) 
