134 Proceedings of the Royal Society of Edinburgh. [Sess. 
the granules coalesced in a sticky state with corresponding reduction in 
surface, and therefore in the efficiency of absorption. 
In the circumstances giving the best absorption the caustic particles 
were found in all cases to have swollen sometimes to three times their 
original linear dimensions, and in many cases they were found to be hollovr 
inside. This action is of great importance to the designer of mine rescue 
apparatus or other appliances in which these granules are used for the 
abstraction of carbon dioxide from air. The action appears to be much as 
follows : — 
A granule begins its active life by taking up a considerable weight of 
moisture, and attains a plastic state. The action of carbon dioxide upon 
such a body is to coat it with a layer of spongy dry carbonate. The next 
effect is a penetration, by capillary attraction, of the soft caustic through 
its skin of spongy carbonate and a further attack by the carbon dioxide on 
the soda thus brought within its reach. The continuance of such an action 
will evidently be to cause the formation of a carbonate shell with a hollow 
interior and greatly to increase the apparent bulk of the granule. When 
absorption is at its best, this action continues until there is no caustic soda 
left behind ; in other cases until a small granule of caustic is left behind, 
apparently lying detached inside the shell of carbonate. 
Conclusions. 
The following conclusions, of value especially to the designer of rescue 
apparatus, follow from these tests : — 
1. It is evident that too much and too little moisture is detrimental to 
the action. The best results are obtained when the proportion of moisture 
carried is approximately that carried by air saturated at temperatures 
between 60° C. and 90° C. 
2. The best results for absorption are obtained when the container is 
kept at a temperature of from 60° to 100° C. Provided the moisture 
conditions are right, the average proportional absorption is then about 70 
per cent., when using the relatively small amount of caustic employed in 
the tests. Below and above those temperatures the absorption efficiency 
falls off rapidly. Below 10° C. and above 100° C. absorption is extremely 
poor. It is worthy of remark that in actual use with rescue apparatus in 
France during the war and in certain cold countries it was found by 
experience that caustic is inactive at temperatures near freezing-point, and 
the practical precaution was taken on cold days of warming the caustic 
prior to use by breathing through it. 
3. It is evident that measures must be taken to secure good conduction 
