IRRITABILITY. 



33 



vided into tenths and hundredths, and in- 

 serted into a wider tube containing water, 

 precisely in the manner of the gauge (i, ;'.) 

 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, q,~) will move pari 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, /,) 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 

 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 is then to be withdrawn through the mercury, 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, ;,) equal 

 to the bulk of the animal; the bent tube (?/?, ri) 

 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 pneumato- 

 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- 

 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 animal has been withdrawn as to allow of its 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 atmos- 

 phere. 



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 pneumatometer, 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. 



It now remains to determine the quantity of 

 change induced upon the air of the pneumato- 

 meter, 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 respiration is ascertained by the 

 VOL. in. 



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. 



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 t, 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 



