1920-21.] Solid Caustic Soda for absorbing Carbon Dioxide. 129 
the caustic being weighed out in a weighing-bottle with a variation not 
exceeding ±005 gram; the soda was then transferred, immediately before 
starting the test, to the apparatus described below, and sealed in. The 
air current caused to flow over the caustic granules was made to contain 
a uniform 4 per cent, of carbon dioxide ; it thus resembled in composition 
the air expired from the lungs of the wearer of a rescue apparatus. Its 
rate of flow was 3*5 litres per minute. The supply of air to be purified 
was obtained from a Briggs gas-testing tank.* 
During any such test as those to be described, the soda becomes 
progressively less active as the test proceeds, and towards the end, though 
often a considerable percentage of hydrate remains, the caustic has virtu- 
ally ceased to extract carbon dioxide from the air. A period of test of 
forty minutes was adopted throughout. 
A special apparatus was designed for the tests. It consisted of a 
strong tin bath (fig. 1) of rectangular shape, supported on wrought-iron 
legs high enough to permit of an ordinary gas bunsen being placed under 
it. Surrounded by the water in the bath, J, were two air-tight com- 
partments, B and E. At one end of the partition separating B and E 
was an opening, C, \ inch wide and 2 inches long. The lower compart- 
ment, B, was kept either half full of water or empty, as desired, and into 
the upper compartment, E, was slipped a tray containing the caustic. 
The gas mixture entered the compartment B at the point X ; rose through 
the opening C into the upper compartment, and flowed over the surface 
of the caustic to an outlet, F, where samples could be taken. The end, 
H, of the upper compartment was detachable, and after the tray con- 
taining the caustic had been inserted the end was sealed on. 
The tray on which the caustic was spread was 11 inches long by 3 inches 
wide, and consisted of a tin base on which was fixed wire gauze crimped 
to form six Y-shaped troughs. The caustic was spread uniformly over 
the surface of the gauze along the troughs. This form of support for 
the soda has been found most effectual in rescue apparatus.]* It is to 
be noted that all temperatures given are those of the surrounding bath ; 
not necessarily of the caustic particles. The caustic, however, rested 
on metal, and, as the vessel was entirely of metal, the conduction of 
heat was fairly effective. For temperatures between 0° and 100° a bath 
of water was used. Below zero a mixture of ice and salt, and above 100° 
paraffin wax was employed. 
During the experiments samples of the ingoing and outgoing gaseous 
* Colliery Guardian, December 15, 1911. 
f Second Report , Mine Rescue Apparatus Research Committee , 1920, p. 46. 
VOL. XLI. 9 
