WHICH SUBSISTS BETWEEN THE RESPIRATION AND IRRITABILITY. 325 
cubic inches, communicating' with a tube p q, bent at its upper part, of the 
capacity of one cubic inch, divided into tenths and hundredths, and inserted 
into a wider tube containing water, precisely in the manner of the gauge ij. 
In order to secure the exact proportion between the capacity of the pneuma- 
tometer and that of the aerometer, it is only necessary to add more or less of 
mercury to the trough. 
The whole apparatus is inclosed in a glazed frame so as entirely to obviate 
the influence of partial currents of air. It is plain that changes in external 
temperature and pressure will affect both these parts of the apparatus equally; 
and that the fluids in the gauge ij, and in the tube p q, will move pari passu. 
It is therefore only necessary to compare them, and to take the difference, for 
the real alteration in the quantity of the gas in the jar. 
Previously to noticing this difference, the fluids in the outer and inner tubes 
are to be brought accurately to the same level, by raising or depressing the 
outer tube kl, and the inner one p q. 
In order that the air within the jar and that in the aerometer may be in the 
same state of humidity, a little water is introduced into the ball o of the latter. 
When the animal is to be removed, the fluid in the inner and outer tubes of 
the gauge are to be brought to a precise level; the animal is then to be with¬ 
drawn through the mercury, by a cord attached to the little net or box in 
which it is secured ; a quantity of fluid will immediately rise in the inner tube, 
ij, equal to the bulk of the animal; the bent tube, m n, is now to be passed 
through the mercury into the jar so as to effect a communication with the 
atmospheric air; a portion of air equal to the bulk of the animal rushes into 
the jar, whilst the fluids in the gauge regain their level. 
To avoid the error which would arise from the influence of the temperature 
of the animal upon the air within the jar of the pneumatometer, the first obser¬ 
vation of the degree upon the gauge must be made the instant the experiment 
is begun, and before the temperature of the animal can have been communi¬ 
cated to it; and the last, so long after the animal has been withdrawn as to 
allow of its restoration to the temperature of the atmosphere. 
In this way all calculations for the varied temperature and pressure of the 
external air, for augmented humidity and temperature of the air of the pneu¬ 
matometer, and for the changes in the height of the fluid of the trough, are at 
once disposed of in a manner the most accurate and simple. 
2 u 
MDCCCXXXII. 
