CH. XXIV.] ANALYSIS OF EXPIRED AIR 377 



which absorbs the carbonic acid ; and No. 5 contains pumice and sul- 

 phuric 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 in- 

 crease of weight in bottles 4 and 5 weighed together gives the amount 

 of carbonic acid produced by the animal in the same time. 



Kanke gives the following numbers from experiments made on a 

 man, who was taking a mixted 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, 19 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 especi- 

 ally small in the winter sleep of hibernating animals. 



Circumstances affecting the amount of carbonic acid excreted, (a) Age and sex. 

 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 pro- 

 portionate 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. 



Diffusion of Gases -within the Lungs. If two chambers con- 

 taining a mixture of gases in unequal amount are connected together, 

 a slow movement called diffusion takes place until the percentage 

 amount of each gas in each chamber is the same. Let us suppose 

 that one chamber contains a large quantity of oxygen and a small 

 quantity of carbonic acid ; and the other a small quantity of oxygen 

 and a large quantity of carbonic acid ; the oxygen moves from the 

 first to the second, and the carbonic acid from the second to the first 

 chamber. The pressure of a gas is proportional to the percentage 



