METHODS OF INVESTIGATION. 



those who have themselves employed it. It is altogether unsuit- 

 able for the investigation of the influence exerted by the mechanical 

 conditions of the respiration, or for the exact determination of he 

 relations of volume existing between the inspired and the _exp, red 

 air &c. For even if the apparatus is so constructed that 



animals are not compelled to take deep or frequent S* W^ 



mal be 



anm 



rations, if the air be sufficiently changed, and if the ani 

 removed from all keen draughts of air, and if the apparatus itself 

 be free from a continuous alternation in the tension of the air, &c., 

 we are only able by this method to ascertain with accuracy the 

 absolute quantities of the carbonic acid and aqueous vapour exhaled 

 in given times, for the quantity of the absorbed ^ 

 be determined approximately by weighing the animal both befo e 

 and after the experiment, and determining the oxygen contained, 

 the amount of exhaled carbonic acid and water, &c. 

 duction of perfectly dry air, which is indispensable even to th 

 moderate accuracy of the experiment, causes the animals to lose 

 more water than under ordinary circumstances in consequence 

 their breathing in an atmosphere to which they are unaccustomed 

 and hence they speedily fall into an abnormal condition. The 

 adorable investigations which Marchandhas made by thisme hod 

 on the respiration of frogs, indicate, however, that it is capable of 

 leadino- to results of the highest value to science. 



A third method was employed, first by Valentin, and subse- 

 quently by von Erlach under his guidance; animals were intro 

 Led into an inclosed space filled with atmospheric air, and they 

 were suffered to respire there for some time, when the volume ^ and 

 the composition of the expired air were compared with those 

 original atmosphere. As far as this object is concerned, the above 

 method gives highly satisfactory results, as may be seen from t 



it cannot he employed for absolute determinations, or for the imes 

 tigation of the influence exerted by mechanical and physiological 

 relations on the respiration. i l , Rpo-nanlt 



An apparatus has lately been ingeniously contrived by ' Kfgnai 

 md Reiset, by which the second and third of these methods have 

 to some extent been combined together. The animals here also 

 breathe in a circumscribed space, from which the carbonic acid and 



L portion of the expired water are constantly being removed by a 

 solution of potash, whilst a quantity of oxygen corresponding with 

 the amount absorbed is continuously being supplied from anothei 



vessel, without the pressure of the air being on that accou 



