20 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



dane. 1 When the saturation is incomplete the pressure (in mm. Hg) 

 of the water vapour present must be determined (usually indirectly) and 

 subtracted from the total pressure. The further reduction is then 

 carried out by means of the formula 



v, - v, , p 273 



v (0 760) v (t , P) 



or corresponding tables. 



Gas volumes measured dry at 760 mm. and o are reduced to 

 weights by means of the following figures : 



litre oxygen weighs 1*429 gr. 



,, carbon dioxide ,, 1*965 ,, 

 ,, nitrogen 1*255 ,, 



,, hydrogen ,, 0*0895 > 



,, methane ,, 0*715 



The units of time most often employed in gas exchange work are, 

 24 hours, I hour, and I minute. Experiments on " normal " animals, in 

 which no special definition of conditions is attempted, are usually 

 calculated on the basis of 24-hour periods, and this is always the case 

 when the carbon balance is the object under investigation. The i-hour 

 unit is employed in most cases, while the statement of results per 

 minute is practically confined to short experiments on the pulmonary 

 gas exchange of man and mammals. Some writers calculate the re- 

 sults of experiments made under special conditions such as complete 

 muscular repose on the 24-hour basis. This appears to be irrational 

 and misleading. 



Classification of Methods. 



The methods described for the determination of the respiratory 

 exchange of animals are very divers as are indeed the problems to be 

 solved. It is obvious that the same apparatus cannot be used for an 

 ox and a mouse, and it would be strange if the same type of apparatus 

 would be suitable. Between the mouse and the egg of an insect the 

 difference in size is as great again and the methods employed must 

 obviously differ. The time factor is equally important. Certain 

 problems involve the determination of the respiratory exchange in 

 long periods up to twenty-four hours or even more, while it is essential 

 for the solution of others that determinations be made over very short 

 periods, down to a few seconds. Methods must differ accordingly. 

 There is finally the difference in respiratory medium to be considered. 



1 J. S. Haldane, " Methods of Air Analysis". London, 1912. 



