326 
DR. MARSHALL HALL ON THE INVERSE RATIO 
It now remains to determine the quantity of change induced upon the air of 
the pneumatometer, by the respiration of the animal. Two views may be 
taken of this change ; that of Messrs. Allen and Pepys, that the oxygen which 
disappears is replaced by a precisely equal bulk of carbonic acid ; or that of 
M. Edwards, that there is generally an excess of the oxygen which disappears 
over that of the carbonic acid evolved. In either case the quantity of respi¬ 
ration 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, suspend¬ 
ing portions of calico, imbued with a strong solution of pure potassa, and pro¬ 
vided with a small dish of wood, so as to prevent the caustic liquid from drop¬ 
ping upon the animal beneath. By this means the carbonic acid is removed 
as it is evolved, or after the animal is withdrawn. The rise of the fluid in 
the gauge of the pneumatometer gives the quantity of oxygen which disap¬ 
pears,—whether this be entirely exchanged for carbonic acid, or only partly 
exchanged for carbonic acid, and partly absorbed,—and denotes the precise 
quantity of the respiration. 
The question itself, of the entire or partial exchange of the oxygen gas which 
disappears, for carbonic acid gas evolved, is at once determined 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 ex¬ 
periment 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 mer¬ 
cury at t, and prevent 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 
