482 



SCIENCE. 



[N. S. Vol. XVIII. No. 459. 



mal— in other words, to substitute quanti- 

 tative for qualitative knowledge. 



It is well known that a fasting animal 

 consumes the tissues of its own body to 

 sustain its vital activities. Thus in one of 

 Rubner's experiments a fasting dog oxi- 

 dized per day the equivalent of 20.51 grams 

 of dry muscular tissue (lean meat) and in 

 addition 75.92 grams of his fat. He was 

 then given 740 grams per day of fresh lean 

 meat. On this ration he was found to oxi- 

 dize the equivalent of 133.89 grams of dry 

 muscular tissue, but only 30.72 grams of the 

 fat of his body. In other words, the oxida- 

 tion of 113.38 grams more of muscular tis- 

 sue derived from the lean meat eaten 

 diminished the draft upon the body-fat of 

 the dog by 45.20 grams. Plainly, these two 

 quantities were equivalent, or, after making 

 some slight corrections, 243 parts of dry 

 lean meat were equivalent to 100 parts of 

 fat A number of similar experiments 

 were made with extracted meat and with 

 various carbohydrates, while in a few 

 cases fat and carbohydrates were inter- 

 changed in the food, and a series of ratios 

 like the above were obtained, varying with 

 the material experimented on. 



All these results, however, are purely 

 empirical. They explain nothing. The 

 fact, however, that the nutrients can mu- 

 tually replace each other through so wide 

 a range, and the other fact that the animal 

 body is essentially a transformer of the po- 

 tential energy of the food into the kinetic 

 energy of heat and motion, suggest that the 

 nutrients replace each other because they all 

 serve as sources of energy to the organism. 

 Acting on this hint, Rubner proceeded to 

 determine the amounts of energy which the 

 several nutrients could liberate in the body. 

 This, by a well-known principle of thermo- 

 chemistry, is measured by the difference 

 between the potential energy of the sub- 

 stance and that of the products of its de- 



composition. The potential energy of the 

 nutrients is measured, for this purpose, by 

 their heats of combustion. In the case of 

 the carbohydrates and fats, the products 

 of their decomposition in the body are (in 

 the carnivora) substantially carbon diox- 

 ide and water, which contain no measurable 

 amount of potential energy. With the pro- 

 teids the case is different. Here we have 

 various partially oxidized compounds con- 

 tained in feces and urine, the potential en- 

 ergy of which Rubner summarily deter- 

 mined by determining the heats of combus- 

 tion of the dried excreta. In this way he 

 was able to determine how much energy a 

 given amount of lean meat or fat or starch 

 was capable of liberating in the animal or- 

 ganism, and this amount he called its phys- 

 iological heat value. 



When, now, he came to compare the 

 amounts of the several nutrients from 

 which equal quantities of energy could be 

 liberated in the body with the amounts 

 which were found to replace each other in 

 actual feeding experiments, he obtained a 

 most remarkable correspondence. The first 

 column of the table shows the amounts of 

 the several substances required to liberate 

 the same amount of energy as 100 grams of 

 fat, while the second column shows the 

 amounts which were found to replace 100 

 grams of fat in the nutrition of the animal. 



Lean meat 



Extracted meat. 



Cane sugar 



Starch 



Grape sugar . . . . 



Yielding Energy I Replacing 100 



Equal to ' Grams Fat in the 



100 Urams Fat. | Body. 



235 grams. 

 213 " 

 235 " 

 229 " 

 255 " 



243 gramK. 

 225 " 

 234 " 

 232 " 

 256 " 



The teaching of these figures seems per- 

 fectly clear. The nutrients are the fuel of 

 the body — its supply of energy— and they 

 replace each other just in proportion to 

 the energy they are capable of liberating. 



