6 9 6 
CHEMISTRY OF KESPIRATIOX. 
water, no weighable amount of nitrogen, or of any other gaseous 
substance, is discharged or absorbed by the animal. 
The figure below represents the modification of Pettenkofer's 
apparatus, which was introduced by Voit for experiments on annuals. 
The absorption of water is effected by flasks filled with pieces of 
pumice saturated with sulphuric acid, and the carbon dioxide is in turn 
absorbed by making the air bubble through a long tube filled with a 
titrated solution of baryta, 1 
The advantages of Pettenkofer's method over those previously in use 
are these — It is possible, owing to the system of ventilation, to make 
experiments upon man ; observations can for a similar reason be much 
more prolonged without any danger of disturbance to the normal 
respiratory exchange arising from an accumulation of carbon dioxide ; 
the absorption of the carbon dioxide is more exact. Notwithstanding 
these improvements, Pettenkofer's method possesses several disadvan- 
tages and sources of error. The apparatus is complicated and costly, the 
Fig. 63. — Voit's respiration apparatus. 
determination of the moisture is liable to be inexact, owing to deposition 
on the walls of the chamber : during the process of weighing the animal 
there is an intake of oxygen, and an output of carbon dioxide and water, 
which are not determined, and can only Lie calculated approximately ; 
the absorption and estimation of carbon dioxide by the titration of the 
baryta solution has been shown by Haldane and Pembrey 2 to be less 
exact than it was thought to be. The result of these errors falls upon 
the estimation of the intake of oxygen, for since (X=C0 2 + H 2 — v, it 
is evident that the amount of oxygen may be often inexact. This has 
been pointed out and proved by C. and E. Yoit and Forster. 3 
It has already been mentioned that Voit i has constructed, upon 
Pettenkofer's principle, a smaller apparatus for the determination of 
1 Baryta was first used by Pettenkofer for this purpose, but Dalton had previously used 
titrated lime water 
•atea lime water. 
2 London, Edinburgh, and Dublin Phil. Mag., London, April 1890 
s Ztschr.f. Biol., Munehen, 1875, Bd. xi. S. 126. ' 4 Ibid., 187 
1878, Bd. xiv. S. 57. 
