INFLUENCE OF FOOD. 357 



of sugar with that of fat, we see at the first glance that the carbo- 

 hydrates require far less oxygen for their perfect oxidation than the 

 fats ; in the carbo-hydrates there is no hydrogen to oxidise, since 

 the oxygen which they already contain is sufficient for the oxidation 

 of the hydrogen : hence the carbon is the only substance in them 

 requiring oxidation, and this substance is moreover contained in 

 far less quantity in the carbo-hydrates than in the fat for equal 

 weights. Certain organic acids, such as tartaric acid, citric acid, 

 and malic acid, which, as is well known, occur in many articles of 

 food, contain so large an amount of oxygen that it not only suffices 

 for the oxidation of the hydrogen, but in part also for that of the 

 carbon also. 



In reference to nitrogenous substances, we cannot, however, 

 grant the postulate that all the carbon and hydrogen is consumed 

 in the animal body, for we know that the greater part of the 

 nitrogen in these substances is not removed in a free state as 

 ammonia, but in combination with carbon, hydrogen, and a little 

 oxygen, by other means than through the lungs. Hence we are 

 led to inquire whether, and to what extent, the nitrogenous 

 nutrient substances yield materials for oxidation, and consequently 

 how much carbonic acid and water they are able to furnish to the 

 respiratory process. As we have already seen that the albuminates 

 and collagen are capable of supporting respiration, we are induced, 

 in explanation of their respiratory value, to adopt the provisional 

 hypothesis that these substances are merely decomposed into 

 carbonic acid, water, and urea in the animal body, although we 

 know that there are formed other nitrogenous products of 

 excretion besides urea. But since the quantity of urea which is 

 produced preponderates very much, and since in many organisms, 

 as, for instance, in the carnivora, urea is almost solely formed, 

 this hypothesis deserves some notice in our consideration of the 

 average value of the amount of oxygen employed in the oxidation 

 of the albuminates and the collagen. We therefore abstract from 

 the composition of the albuminates and other nitrogenous nutrient 

 substances an amount of urea equivalent to the quantity of 

 nitrogen which they contain. If, for instance, we assume that the 

 composition of the albuminates without the sulphur and salts is 

 54-36 C, 7'27H, 16'05N, and 22'32-g O, there will remain, 

 after the abstraction of the quantity of urea (= 6'88C, 2'29H, 

 and 9*18 O) equivalent to the 16*05 parts of nitrogen from 100 

 parts of an albuminate, 47*48 parts of carbon, 4*98 of hydrogen, 



