BY DR. BURDON-SANDE11SON. 311 



sufficient to retard the respiratory movements to a sensible 

 degree. As, however, the most important applications of the 

 method are those which relate to the influence of variable con- 

 ditions on the quantity of air breathed, this fact is of little 

 consequence; for the error arising from it may be entirely 

 eliminated by substituting, as a standard of comparison, the 

 respiration already modified by the resistance, for normal 

 respiration. For the purpose of obtaining such a standard, 

 the animal must be allowed to breathe common air through 

 the apparatus for some time before making any other observa- 

 tion. 



SECTION IV. DETERMINATION OP THE QUANTITY OP CARBONIC 



ACID GAS DISCHARGED BY AN ANIMAL PROM THE LUNGS AND SKIN 

 IN A GIVEN TIME. 



97. There are two leading methods by which this can be 

 accomplished. One of them is that of Regnault and Reiset, 

 which, with important modifications, has been used by Ludwig 

 and his pupils. The animal under observation is contained in 

 an air-tight chamber, which communicates with a second 

 chamber containing oxygen. The chamber communicates 

 with an absorbing apparatus, through which the air passes in 

 a continuous current, so that the expired carbonic acid gas is 

 removed from it as rapidly as it is formed, its place being 

 taken up by exactly the same volume of oxygen, so that the 

 constitution of the air remains unchanged. The quantity of 

 carbonic acid gas absorbed is calculated from the increase of 

 weight of the absorbing apparatus during the period of obser- 

 vation. As improved by Ludwig, the method is the best 

 suited for exact experiments. The apparatus is described in 

 Lud wig's Arbeiten for 1869. 



The second method, which is much simpler, and sufficiently 

 exact when for comparative investigations as to the influence 

 of various physiological and pathological conditions on the 

 discharge of carbonic acid gas, is that of Pettenkofer. It is 

 applicable either to large animals or small. A short account 

 of Pettenkofer's complete apparatus will now be given, as an 

 aid to the understanding of the application of the same method 

 to the small animals in common use for physiological and 

 pathological investigations. Pettenkofer's apparatus consists 

 of three parts, viz., a chamber in which a man can sit or stand 

 comfortabty; a large wet gas meter, which communicates with 

 the chamber by a tube ; a double-action air-pump, by which 

 air is continuously drawn through the meter from the cham- 

 ber ; and, lastly, clockwork, by which the pump is worked. 

 The chamber, which is of metal and glass, communicates with 

 the external air during the period of observation by the inter- 



