For the methods employed up to the present for the determination of CO 2 in its physiological relations, Botany is indebted to all three classes of chemical analysis, to volumetric, gravimetric, and gasometric analysis. To the last belong those methods which have been of great value in giving the relation of O 2 and CO 2 exchange within a given time in either respiration or assimilation. These were the earliest used, and consist in placing a plant or part of a plant within a closed receiver containing a suitable mixture of gases and determining the exact composition of this before and after the experiment. From the reduced volumes, measured after absorption of the CO 2 by potash, and of the O by phosphorus, the quantities of either gas liberated or destroyed can be calculated. To De Saussure, Boussingault, Godlewski, and, above all, Pfeffer, we owe researches based on this method. The method has yielded valuable results in past times, but the corrections and actual analysis, according to the old standard methods of Bunsen, are lengthy and laborious, while in the modern procedure, simplification of operation is obtained somewhat at the expense of accuracy. The whole method is open to the physiological objections to the use of a closed chamber, and to the continuous, and in a lengthy experiment often considerable, variations of composition which the contained gas must undergo. Nor is it adapted to yield a series of connected results.