Attached to the same mercurial trough is placed a little apparatus (o, p,) termed an ammeter, and consisting of a glass ball (o,) of the capacity of ten cubic inches, commu nicating with a tube (p, JO bent at its upper part, of the capacity of one cubic inch, di vided into tenths and hundredths, and in serted into a wider tube containing water, precisely in the manner of the gauge (i, j.) In order to secure the exact proportion between the capacity of the pneumatometer 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 (i,j,) and in the tube (p, g,) will move parr passu. It is there fore 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 (k, 1,) and the inner one (p, g.) 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 glass 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the fluid in the inner and outer tubes of the gauge are to be brought to a precise level ; the animal is then to be withdrawn through the mercury, by a cord attached to the little net or box in which it is secured ; a quantity of fluid will immediately rise in the inner tube, (i,j,) 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 pneuniato meter, the first observation of the degree upon the gauge must be made the instant the ex periment is begun, and before the tempera ture of the animal can have been communi ated to it ; and the last, so long after the mimal has been withdrawn as to allow of its 'estoration to the temperature of the atmos ffiere.
In this way all calculations for the varied emperature and pressure of the external air, or augmented humidity and temperature of he air of the pneumatometer, and for the :flanges in the height 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 hange induced upon the air of the pneumato aeter, by the respiration of the animal. Two iews may he taken of this change ; that of dessrs. Allen and Pepys, that the oxygen thich disappears is replaced by a precisely qual bulk of carbonic acid ; or that of M. :dwards, that there is generally an excess of to oxygen which disappears over that of the acid evolved. In either case the uantity of respiration is ascertained by the gauge of the pneumatometer in the following manner. A frame made of glass rods (r, s) is placed within the jar (a, b) 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 the 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.