140 Knight and Priestley . — The Respiration of Plants 
A stream of air, free from carbon dioxide, was passed over the seeds or 
seedlings and afterwards passed through baryta solution to absorb the 
carbon dioxide which the living material had evolved. Before entering the 
respiration vessel, the air was passed through a series of wash-bottles con- 
taining a concentrated solution of potash, or, as in the later experiments, 
a long tube packed with soda-lime, in order to free it from carbon dioxide. 
To test the purity of the air-stream another wash-bottle containing clear 
baryta solution was inserted before the respiration chamber. After leaving 
the vessel containing the living material, the air-stream entered one of 
a series of Pettenkofer’s tubes, containing a known volume of a standard 
solution of barium hydrate, which absorbed the carbon dioxide, precipitating 
barium carbonate, and finally the air-current passed through a second wash- 
bottle of clear baryta to ascertain whether or not all the carbon dioxide had 
been removed (Fig. i). Various methods of keeping the air-stream in 
motion were resorted to, including aspirators, filter-pump, and a gas-holder, 
but the most satisfactory was a water-pressure filter-pump, the water-supply 
for which was maintained at constant pressure by means of an overflow 
device. The rate of the air-stream was thus rendered independent of slight 
variations of the water pressure from the main, which were found very 
troublesome at first. 
When seeds were employed, they were soaked in water for at least 
twenty-four hours before being experimented upon, and immediately before 
being introduced into the respiration chamber, the water was drained off 
and the seeds dried somewhat in a cloth. This removed the excess water 
from the outside, thus preventing the accumulation of water in the tubes of 
the apparatus. 
