310 RESPIRATION. 



menter is absolutely certain that the whole of the air which 

 leaves the receiver, and no more, is actually used in respiration. 



The receiver may he constructed as follows: Two glass 

 cylinders are selected, about eight inches in length, open at 

 both ends, one of which is about half an inch wider than the 

 other; the outer i about three inches in width, the inner (E 

 fig. 251) two and a half inches. Both of them are cemented 

 in the most perfect manner possible, with their axes in the 

 same vertical line into a circular horizontal plate, so that they 

 are separated from each other by a narrow space of the same 

 width everywhere. This space is to be filled with mercury. 

 Through the central part of the plate rise three vertical tubes 

 of glass, of about a quarter of an inch internal diameter. 

 Underneath the plate, which is supported on a tripod, each of 

 these tubes passes downwards for a short distance, and is 

 then bent horizontally at right angles. A third cylinder (D), 

 closed at one end and made of iron carefully protected, con- 

 stitutes the bell of the gasometer. Its diameter is the mean 

 of the diameters of the two cylinders of glass, so that it 

 descends without touching them into the space containing 

 mercury, by which they are separated from each other. It is 

 suspended by a silk cord, pulley, and counterpoise. The 

 counterpoise consists of a cup containing shot, and there is a 

 second and similar cup on the top of the cylinder. Of the 

 three tubes which enter the receiver from below, one (A) com- 

 municates with the atmosphere* (when in use), a second (B) 

 with the respiratory cavity of the animal, the third (c) is 

 usually closed. The India-rubber tubes by which these com- 

 munications are made, are guarded by the simple contrivances, 

 known as Miiller's mercurial valves. Each such valve consists 

 of a rather wide bottle containing a shallow column of mer- 

 cuiy, and closed air-tight with an India-rubber stopper. 

 Through the stopper pass two tubes, one of which is of such 

 length that its end dips just below the surface of the mercury; 

 the other is much shorter. The valve (A) is so placed that the 

 short tube is in direct communication with the receiver, the 

 long one with the atmosphere ; in (c) this arrangement is re- 

 versed. To complete the apparatus, all that is required is a 

 T tube and a third valve. The stem of the T tube communi- 

 cates with the respiratory cavity by means of a canula secured 

 air-ti jht in the trachea ; the one arm with the receiver, and the 

 other with the valve (B), through which the expired air is dis- 

 charged into the atmosphere. The quantity of air used by 

 the animal during any given period of observation must be 

 measured in the same way as before. 



The objection to which this apparatus or any other of 

 similar construction is liable, lies, as has been already hinted, 

 in the resistance offered by the mercurial valves, which is 



