37 6 RESPIRATION. [CH. xxiv. 



carbonic acid ; No. 2 contains pumice stone moistened with 

 sulphuric acid, which frees the air from aqueous vapour. The 

 air next reaches the animal chamber, and the animal gives off to 

 it carbonic acid and aqueous vapour. It passes then through the 

 three bottles, 3, 4, and 5. No. 3 contains pumice and sulphuric 

 acid, which removes the water; No. 4 contains soda-lime, which 

 absorbs the carbonic acid ; and No. 5 contains pumice and 

 sulphuric acid, which absorbs any water carried over from bottle 4. 

 The increase of weight in bottle 3 at the end of a given time 

 (e.g., an hour) is the weight of water given off by the animal in 

 that time ; the increase of weight in bottles 4 and 5 weighed 

 together gives the amount of carbonic acid produced by the 

 animal in the same time. 



Ranke gives the following numbers from experiments made on 

 a man, who was taking a mixed diet consisting of 100 grammes 

 of proteid, 100 of fat, and 250 of carbohydrate in the twenty-four 

 hours. The amount of oxygen absorbed in the same time was 

 666 grammes; of which 560 passed off as carbonic acid, 9 in 

 urea, 1 9 as water formed from the hydrogen of the proteid, and 

 78 from that of the fat. 



Vierordt from a number of experiments on human beings gives 

 the following numbers : the amount of oxygen absorbed in the 

 twenty-four hours, 744 grammes; this leads to the formation of 

 900 grammes of carbonic acid (this contains about half a pound 

 of carbon) and 360 grammes of water. 



The respiratory interchange is lessened during sleep. It is 

 especially small in the winter sleep of hibernating animals. 



Circumstances affecting the amount of carbonic acid 'excreted, (a) Age 

 and aex. In males the quantity increases with growth till the age of 30 ; at 

 50 it begins to diminish again. In females the decrease begins when 

 menstruation ceases. In females the quantity exhaled is always less than 

 in males of the same age. 



(b) Respiratory movements. The quicker the respiration the smaller is 

 the proportionate quantity of carbonic acid in each volume of expired air. 

 The total quantity is. however, increased, not because more is formed in the 

 tissues, but more is got rid of. The last portion of the expired air which 

 comes from the more remote parts of the lungs is the richest in carbonic 

 acid. 



(c) External temperature. In cold-blooded animals, a rise in the external 

 temperature causes a rise in their body temperature, accompanied with 

 increased chemical changes, including the formation of a larger amount of 

 carbonic acid. In warm-blooded animals, it is just the reverse : in cold 

 weather the body temperature has to be kept at the normal level, and so 

 increased combustion is necessary. 



(d) Food. This produces an increase which usually comes on about an 

 hour after a meal. 



(e) Exercise. Moderate exercise causes an increase of about 30 to 40 per 

 cent, in the amount excreted. With excessive work, the increase is still 

 greater. 



