330 RESPIRATION. 



the facts relating to the respiratory process, which we now proceed 

 to consider. 



Before we pass to the subject itself, we are led to notice some of 

 the difficulties which attended these enquiries. It is extremely 

 difficult to procure the special materials for our investigation 

 (the expired air) in a state of purity in sufficient quantity, and 

 under perfectly normal conditions. Hence various methods were 

 adopted to obtain the products of expiration, but the only mode by 

 which the expired air could be collected pure and unmixed with 

 the products of perspiration, consisted in the direct application (to 

 the mouth) of an apparatus, by means of which the expired air was 

 conveyed to suitable receiving vessels. This was the method which 

 was usually employed in experiments on man (Dumas, Andral and 

 Gavarret, Valentin, Vierordt). The advantages of this method 

 over the one we shall presently proceed to notice, consisted not 

 merely in the exclusion of the products of perspiration, but also in 

 enabling the observer to determine the influence which certain 

 physical conditions of respiration exerted on the numerical relations 

 of its products. This method is, however, attended with some dis- 

 advantages, which although sufficiently serious, can hardly be fore- 

 seen ; thus, for instance, a person conscious of the nature of the 

 experiment which is being instituted, cannot help breathing in so 

 constrained a manner as to alter the number and depth of the 

 inspirations, so that the normal relations of the ordinarily quiet 

 and unconscious respiration are entirely changed. This inconve- 

 nience may, however, be completely remedied by long practice ; 

 and the brilliant results and investigations both of Valentin and 

 Vierordt are a sufficient proof that this method does not merit the 

 condemnation which was formerly awarded to it. The objections 

 appertaining to it may indeed be almost perfectly obviated by 

 careful attention to the construction of the apparatus. 



The second method, which Scharling and Hannover employed 

 on man, but which most other observers have used only in the 

 case of animals, consisted merely in allowing a current of air con- 

 stantly to pass through the apparatus in which the person or animal 

 was placed, on whom the experiment was to be made ; fresh air 

 was thus continuously supplied, and the products of expiration car- 

 ried off into a system of vessels, in which the constituents of the 

 expired air might be absorbed, and at the same time quantitatively 

 determined. However simple this method may at first sight 

 appear, and however conformable it may seem to nature, it pos- 

 sesses many deficiencies which can only be fully understood by 



