The principle 
is the same in 
20 ON THE PLACE OF FISH*IN 
to see on a frosty day that there is more water in the 
air we breathe out than that we breathe in. As we 
breathe out, then, more carbonic acid and water than 
is in the air we inhale, we know that carbon and 
hydrogen are being oxidized somewhere within the 
body. This oxidation gives rise to heat (p. 7)—and 
heat and motion go together. 
This may suffice as a rough and ready way of know- 
ing by observation what are the principles on which 
calculations as to C, H, O, and N, in foods are based. 
Experiments have, however, been carried further 
scientific work than this—not simply to find that carbonic acid and 
though the 
arrangements 
are more 
elaborate. 
water are formed by oxidation in the body, but how 
much of each is formed. To do this of course requires 
special arrangements. For example, Dr. Edward 
Smith, in ascertaining how much carbonic acid was 
given off during exertion of different degrees, wore a 
sort of mask covering his nose and mouth, and a 
flexible tube carried his breath to his apparatus for 
ascertaining the weight of water and carbonic acid 
given off in certain time. Pettenkoffer carried out 
observations on a watchmaker who consented to 
work inside a case, one day doing no harder work 
than reading, another, doing his usual light work ‘of 
watch-fitting, and another day working a treadle. The 
amount of food and of oxygen admitted to him, and 
of carbonic acid and water, &c., given off, were 
accurately weighed. 
The experiments of Fick and Wislecanus are men- 
tioned in Appendix. The two mentioned here may 
serve as examples that the statements and figures 
given by scientific chemists about the C, H, and N 
taken in as one set of compounds in food, and given 
