FOODS ANIMAL PRODUCTION. 243 



years of 9 and 12 tliat the difference is within the limit of experimental 

 error. The excretion anionnts to 33 to 34 gm. per honr. In the thir- 

 teenth year the carbon dioxid increases to about 42 to 45 gm. per hour, 

 and remains about the same until the nineteenth year. From the 

 twentieth year on the amount diminishes. For men between 20 and 30 

 it is 38 gin. per hour, and between 35 and GO it is from 34 to 37 gm. i^er 

 hour. With girls the carbon dioxid excretion between the ages of 8 

 and 10 is 23 to 25 gm. per hour. It then increases but does not show 

 as great an increase as in the case of boys; and from the eleventh to 

 the thirtietli year it varies only between 2G and 32 gm. As women 

 grow older the carbon dioxid excretion seems to diminish somewhat. 

 In the sixty-fifth year it is about 26 gm. per hour. 



In the case of both males and females the carbon dioxid excretion 

 per kilogram body weight is greater in young (and light) than in old 

 (and heavy) individuals, and the amount excreted per square meter of 

 body surface is greater in young than in older persons. The latter 

 fact indicates that in youth, leaving out of consideration the small 

 size of the body, metabolism is really more active than in old age. 

 In youth the excretion of carbon dioxid per kilogram body weight and 

 per square meter body surface is much greater by males than by 

 females of the same age and weight, viz, in the ratio of 140 to 100 on 

 an average. This difference between the sexes, which is so marked in 

 youth, seems gradually to diminish and finally in old age to disappear 

 entirely. 



(2) A large number of experiments of the second class were made, in 

 which the carbon dioxid, and in some cases the nitrogen, were meas- 

 ured at frequent intervals, and under varying conditions. The follow- 

 ing conclusions were drawn : 



Generally speaking, when no work is done the hourly variation of 

 carbon dioxid excretion is comparatively small. In 44 experiments 

 made with 9 individuals the mean hourly variation in 5-hour jieriods 

 was G.19 per cent. When periods of labor occurred between j^eriods of 

 rest the variations were not much greater. In G3 experiments made 

 with 7 individuals in a 5-hour period, in which rest and labor alter- 

 nated, 3 hours being devoted to rest, the mean hourly variation 

 observed during rest was 6.84 per cent of the mean value for the 3 

 hours at rest. 



In the same individuals under the same external conditions the car- 

 bon dioxid excreted varies very little from day to day; even though 

 the experiments are separated by several months the variation is 

 only 9.6 per cent of the mean daily value. During the 24 hours con- 

 siderable variations occur which in the main are caused by sleeping 

 and waking. On an average the carbon dioxid excreted in sleep is 

 to that excreted in waking as 100 to 145. The extremes are 100 to 169 

 and 100 to 132. When awake the mean variation during 2-hour periods 

 in 82 observations made witli 11 individuals was 9.32 per cent of the 

 mean value. In sleep the mean variation during 2-hour periods in 42 



