569 



is determined by the increase of the weight of the absorbing appara- 

 tus ; and hence it is necessary that the latter be such that it may be 

 weighed to not less than the -j^-th of a grain. Gutta percha is the 

 only known substance of which the apparatus may be constructed, 

 since it is not greatly acted upon by the caustic alkali, may be readily 

 formed by the aid of the hot iron, and does not allow the fluid to flow 

 over the floors of the chamber with the readiness observed when 

 glass or metal is used, during the act of weighing. The only prac- 

 tical absorbent is a solution of caustic potash, and its spceific gravity 

 should be 1*270. The atmospheric air which will enter the various 

 parts of the apparatus when not in use, must be expelled by blowing 

 expired air through them before they are counterpoised as a prelimi- 

 nary to the inquiry. The expired air must not be reinspired. Hence 

 boxes such as that employed by Scharling are inapplicable ; for, how- 

 ever rapid the current of air which is drawn though the box, it is 

 quite certain that some portion of the air will be again and again in- 

 spired, and moreover the dilution of the air charged with carbonic 

 acid renders the absorption of the gas much more difficult, whilst the 

 determination of the carbonic acid remaining in the box is always a 

 circumstance of great difficulty. When a mask is worn, it should not 

 have a capacity larger than necessary to contain the features, or it 

 will retain expired air, which must be reinspired. When respiring 

 through a tube placed in the mouth, it is exceedingly difficult to pre- 

 vent the escape of air, whether as it is introduced into or withdrawn 

 from the mouth, and the results cannot be relied upon. If the nose 

 be left unclosed, a variable and unknown quantity of air will enter and 

 leave by that aperture. The effect of exertion may be readily ascer- 

 tained by using a tube 15 feet in length attached to the mask and 

 the analytical apparatus. The apparatus must be placed in a central 

 position, and a space of 30 feet marked out in a right line, and this 

 must be walked over at a defined rate of speed. The potash-box 

 must be made of larger size than that required for experiments upon 

 quiet respiration, or two sets of the apparatus must be used at the same 

 time. In the latter case there will be danger of adverse pressure from 

 the air passing through so many vessels ; and if the exertion in 

 breathing be considerable, it will be impossible to measure the inspired 

 air at the same time. The tubes must be of sufficient diameter and 

 of smooth material, and be filled with expired air before the experi- 



