1920-21.] Solid Caustic Soda for absorbing Carbon Dioxide. 133 
temperature was 50° C. The maximal extraction in this case being less 
than that in the last two cases would indicate that the greater amount of 
water held by the ingoing stream of gas in series 4 was detrimental to 
absorption. The curve D shows how much efficiency of absorption 
depends on the temperature of the containing vessel ; for though the 
extraction was 65 per cent, at 50° C., it was only 30 per cent, at 16° C. and 
40 per cent, at 90° C. 
Discussion of Results. 
A conceivable explanation for some part of the difference in efficiency 
of absorption is that, owing to variations of temperature and hygrometric 
state, the actual rate of flow over the caustic was different in different 
experiments. This effect, however, was found to be of negligible moment 
by carrying out special tests, in which the same mass of carbon dioxide was 
passed over the caustic per second, though contained in air currents which 
flowed at greatly different rates. A more serious criticism on the method of 
test is that the recorded temperature of the bath surrounding the caustic 
compartment was not necessarily the temperature of the caustic granules 
themselves. Undoubtedly there would be very considerable local heating, 
and the severity of this local heating may be judged when it is stated that 
in certain other experiments in which caustic granules were held on 
blotting-paper trays the blotting-paper caught fire from the heat of 
reaction. The writer has no means of judging of the temperature of 
the active surfaces at which the absorption was actually taking place. 
The general conduction of heat away from the granules, however, must 
have been moderately efficient, owing to the mass of metal with which 
they were in contact. It is advisable to state again that the whole set of 
experiments was carried out to give information which would be applicable 
for rescue apparatus in the cartridges of which the caustic soda is supported 
in much the same way as in the experiments. A slight error in the per- 
centage of carbon dioxide in the outgoing sample, as given by the results 
of the analyses, was due to the condensed moisture in the sampling-bottles 
having dissolved some C0 2 ; but on calculation it was ascertained that this 
error was negligible. 
Some remarks have already been made as to the appearance of the 
caustic soda on removal from the tray. There was a marked differ- 
ence in appearance after tests at high, low, and intermediate temperatures. 
At the lowest and highest of the temperatures in the series I., II., and III., 
the granules retained much of their original shape ; but at the high tempera- 
tures in series IV., where a great deal more moisture was present in the air, 
