1856.] and the Means- of Determining its Amount. 239 



As then we have learnt that a fixed amount of cubic space will 

 not give us what air we want, or tell us what we have, we will turn 

 to the question, How much air we do want, and how are we to know 

 when we have got it ? According to the best experiments on 



Respiration, 



Man inhales 15,885 cubic inches of oxygen in 12 hours. 



44 cubic feet of atmospheric air contain this oxygen. 

 3*7 per hour, = 0*06 cubic foot per minute. 



Man expires . . 1 60 cubic feet in 1 2 hours, containing 4 per cent carbonic acid 

 If fresh air , .160 „ are added, there will be 2 per cent. 

 If then fresh air . 320 „ are added, there will be 1 per cent. 

 Hence, if a man has 640 „ in 12 hours, it will contam 1 per cent. 



= 53*3 „ per hour, = 0-88 per minute. 



== rather less than 1 cubic foot per minute. 



In other words, for diluting the carbonic acid we require 14^ 

 times more air than for the supply of oxygen. 



In different ages, sexes, states, and conditions, the variation in 

 these numbers is great, and no experiments have yet been made 

 with human beings in the best and most natural conditions.* The 

 following variations, obtained by single inspirations, may show how 

 easily errors may be made — 



Effect of Rapid Breathing on the amount of expired 

 Carbonic Acid. 



Effect of Holding the Breath on the amount of Carbonic 

 Acid expired. 

 Ordinary Respiration. Extraordinary deep Respiration. 



♦ Even the experiments of MM. Regnault and Reiset on the respiration of 

 animals, though perfect in their physical and chemical arrangements, are far 

 from conclusive, in consequence of the physiological conditions to which the 

 auimals were subjected. 



Vol. II. R 



