IRRITABILITY. 
vided. into tenths and hundredths, and in- 
Serted into a wider tube containing water, 
‘precisely in the manner of the gauge (2, 7.) 
Boric. to secure the exact proportion between 
capacity of the pneumatometer and that of 
‘the aérometer, 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 
artial currents of air. It is plain that changes 
im external temperature and pressure will affect 
both these parts of the apparatus equally ; and 
that the fluids in the gauge (i,j,) and in the 
tube (p, g,) will move pari passu. It is there- 
pre Only necessary to compare them, and to 
ake 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 
Drought accurately to the same level, by raising 
‘or depressing the outer tube (A, /,) and the 
inner one (p, 9.) 
In order that the air within the jar and that 
in the aérometer may be in the same state of 
humidity, a little water is introduced into the 
glass ball (0) of the latter. 
When the animal is to be removed, the 
fuid in the inner and outer tubes of the gauge 
@ to be brought to a precise level; the animal 
is then to be withdrawn through the mercury, 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, (i, 7,) 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 
evel. 
To avoid the error which would arise from 
e influence of the temperature of the animal 
pon the air within the jar of the pneumato- 
meter, the first observation of the degree upon 
he gauge must be made the instant the ex- 
Deriment is begun, and before the tempera- 
ure of the animal can have been communi- 
tated to it; and the last, so long after the 
inimal has been withdrawn as to allow of its 
estoration to the temperature of the atmos- 
here. 
In this way all calculations for the varied 
emperature and pressure of the external air, 
br augmented humidity and temperature of 
1e air of the pneumatometer, and for the 
thanges in the Freight of the fluid of the 
rough, are at once disposed of in a manner 
he most accurate and simple. 
It now remains to determine the quantity of 
ange induced upon the air of the pneumato- 
feter, by the respiration of the animal. Two 
lews may be taken of this change; that of 
Messrs. Allen and Pepys, that the oxygen 
yhich disappears is replaced by a precisely 
qual bulk of carbonic acid; or that of M. 
uiwards, that there is generally an excess of 
ve oxygen which disappears over that of the 
arbonic acid evolved. In either case the 
uantity of respiration is ascertained by the 
VOL. If. 
33 
gauge of the pneumatometer in the following 
manner. A frame made of glass rods (r,s) 
is placed within the jar (a, 6) suspending por- 
tions of calico, imbued with a strong solution 
of pure potassa, and provided with a small 
dish of wood, so as to prevent the caustic 
liquid from dropping upon the animal beneath. 
By this means the carbonic acid is removed as 
it is evolved, or after the animal is with- 
drawn. The rise of the fluid in the gauge of 
the pneumatometer gives the quantity of oxygen 
which disappears,—whether this be entirely ex- 
changed for carbonic acid, or only partly ex- 
changed for carbonic acid, and partly absorbed, 
—and denotes the precise quantity of the respi- 
ration. 
The question itself, of thé entire or partial 
exchange of the oxygen gas which disappears, 
for carbonic acid gas evolved, is at once de- 
termined by employing the same apparatus 
without the solution of potassa: in the entire 
exchange, there is no alteration in the bulk of 
the air of the pneumatometer; in the case of a 
partial exchange, the alteration in the bulk of 
the air gives the precise excess of oxygen gas 
which disappears, over the quantity of carbonic 
acid evolved. 
But this question, and that of the absorption 
and evolution of nitrogen, with the influence 
of night and day, of season, &c. are reserved 
for a future stage of this inquiry. 
It is important that the animal should be 
left for a considerable time in the very situation 
in which it is to remain during the experiment, 
before that experiment is begun, and before the 
jar is placed over it. In this manner the effect 
of timidity or restlessness is allowed to subside, 
and prevented from mingling with that of the 
natural state of the respiration. <A bit of cork 
must also be attached to the mercurial trough, 
so as to float upon the mercury at é, and pre- 
vent the disturbing effect of the contact of this 
fluid with the animal. 
It is also well, after having placed the jar 
in the groove of the mercurial trough, to pour 
a little water over the mercury exterior to the 
jar. The apparatus is thus rendered perfectly 
air-tight, which is not always effected by the 
mercury alone. 
By means of this apparatus we readily and 
accurately determine the quantity of the re- 
spiration of any given animal, in any given 
circumstances. 
II. Of the measure of the irritability. — 
The problem to be next determined is that of 
the degree of irritability of the muscular fibre, 
and especially of the heart. The question is 
beset with scarcely fewer or less difficulties 
than that of the quantity of respiration, whilst 
it involves far greater errors and more. dis- 
crepancy of opinion on the part of physio- 
logists. 
Even Baron Cuvier has fallen into these 
errors. It will be shortly demonstrated that 
the degree of irritability is, in every instance, 
inversely as the quantity of respiration. Yet 
M. Cuvier, in a remarkable paragraph, states 
the very contrary, and even speaks of that 
which is the exhauster, as the repairer, of the 
D 
