BY DR. BURDON-SANDERSON. 309 



The receiver is brought into communication with the expira- 

 tory cavity of the subject of experiment by means of a face- 

 piece or mask of very perfect construction, furnished with two 

 valves, by one of which the air is expelled, while the other, 

 opening inwards, guards the orifice of a tube about an inch in 

 width, which leads from the receiver. 



By its second opening, the receiver communicates with a 

 gasometer filled with air, under a pressure somewhat greater 

 than that of the atmosphere. Between the gasometer and the 

 receiver, the tube of communication passes first through a 

 stop-cock of brass, the aperture of which can be regulated 

 verv accurately by means of a long handle, and then through 

 an 'accurately 'graduated gas meter, specially constructed for 

 the purpose. Each observation lasts ten minutes. The gas- 

 ometer is kept full of air by means of a pair of bellows, which 

 must be worked by an assistant (in default of other motor) 

 during the whole period; while the quantity of air which is 

 driven through the meter to the recipient is so regulated with 

 the aid of the stop-cock, that the receiver is kept exactly at 

 the same degree of fulness. 



The chief mechanical source of inexactitude in this appara- 

 tus is to be found in the imperfect closure of the valves, and 

 imperfect fitting of the face-piece. These defects may be 

 obviated by substituting for the face-piece a couple of tubes of 

 ivoiy, which accurately fit the anterior opening of the nostrils. 

 The wide tube with which these ivory nose-pieces are connected, 

 at once divides into two branches. Of these, one is guarded 

 by a mercurial valve leading outwrrds, the other by a similar 

 valve leading inwards for inspiration, the arrangement of these 

 valves being the same as that shown in fig. 251 to be imme- 

 diately described. 



96. In making observations of the same nature on the 

 lower animals, it is convenient to use an apparatus which not 

 only admits of accurate measurement of the quantity of air 

 breathed, but renders it possible to modify its composition by 

 the introduction of definite proportions of other gases or 

 vapors. And inasmuch as in such investigations it is, as a 

 rule, of more importance that the conditions should be 

 accurately known than that they should be identical with 

 those normally existing, the principle of completely avoiding 

 resistance, which was regarded as fundamental in the con- 

 struction of the apparatus described in the preceding para- 

 graph, must be abandoned ; for it is a mechanical impossibility 

 to construct valves which, while they close with perfect 

 accuracy, work without resistance. The apparatus to be now 

 described is so constructed that any gaseous mixture may be 

 kept in it for a length of time without change of composition 

 by diffusion, and the valves act so perfectly that the experi- 



