436 
Baker. — Quantitative Experiments on 
The variations in weight between the two check cultures are'chiefly due 
to differences in the water-supply, which is a very important factor in 
respiration in the dark. 
Physical Methods for the Absorption of Formaldehyde. 
The experiments detailed above led to the final abandonment of 
chemical methods for absorbing formaldehyde. The physical method used 
was as follows. The formaldehyde was condensed out of the air by cooling 
it to the temperature of solid carbon dioxide. Formaldehyde polymerizes 
at o° to form its polymer, the solid paraform, whose vapour pressure at this 
very low temperature would be negligible. 
Experimental Methods . 
The condensing apparatus is shown in Text-fig. 4. The air was first 
passed through an empty Walther’s condenser, which was cooled in carbon 
dioxide snow contained in a vacuum cylinder. At this temperature the 
formaldehyde should condense and the traces of carbon dioxide in the 
air pass on. The air was then warmed in a water bath at about 50°, 
and passed through a long calcium chloride tube (not shown in the dia- 
gram) to bring it to room temperature, and the humidity of air dried by 
calcium chloride, in order that no evaporation of water may take place from 
the potash bulb. It was necessary to have the glass apparatus sealed 
together with ground-glass taps because of the low temperature. The taps 
were greased with a mixture of vaseline, rubber, and paraffin-wax, and were 
air-tight. 
The carbon dioxide and water were estimated in the same way as 
in the former experiments (p. 431 et seq.), and the plants were treated 
similarly. 
Notes. — In Experiment 1 the apparatus was newly set up and was not 
dried sufficiently long before the experiment, hence the irregular wet weight 
values. 
In Experiment 4 a certain quantity of water condensed in the air 
exit-tubes, and so escaped estimation. 
The excess of water, apparent in all the formaldehyde cultures, is 
partly due to partial condensation of the formaldehyde from the air-current 
into the water on the cultures and also into the calcium chloride tube. 
After the first two experiments were completed, it was found that 
only a very small trace of formaldehyde had actually been passing over 
the formaldehyde culture, as a large proportion of the vapour was absorbed 
by the concentrated potash and the calcium chloride, which intervened 
between the culture and the source of formaldehyde. For this reason, in 
the last two experiments the positions of the paraform tube and potash 
