SCIENCE. 



[N. S. Vol. XVIII. No. 4.5!). 



digest and assimilate this food. The ease 

 is analogous to that of a steam-boiler which 

 is fired by means of a mechanical stoker 

 driven by steam from the same boiler. 

 Each pound of coal fed into the fire-box is 

 capable of evolving a certain amount of 

 heat, and that heat is capable of producing 

 a certain quantity of steam. A definite 

 fraction of the latter, however, is required 

 to introduce the next pound of coal into 

 the furnace and, therefore, is not available 

 for driving the main engine. 

 1. It is plain, however, that this view of 

 the matter is inconsistent with the law of 

 isodynamic replacement as above stated. 

 If the consumption of food causes an in- 

 creased expenditure of energy by the body, 

 then we do not have simply a replacement 

 of one kind of fuel by another, such as the 

 law of isodynamic replacement predicates, 

 but in addition an actual stimulation of the 

 combustion. To recur to the previous illus- 

 tration, to prevent the oxidation of 100 

 grams of fat in the fasting animal would 

 require not only the 229 grams of starch 

 equivalent in energy to the 100 grams of 

 fat, but also enough more to supply the en- 

 ergy required to digest and assimilate the 

 starch. As a matter of fact, Rubner ac- 

 tually did find that slightly more than the 

 theoretical amount of starch was required, 

 viz., 232 grams instead of 229 grams, and 

 the same was true with the other materials 

 experimented on with the exception of cane 

 sugar. 



Such substances as meat, starch and 

 sugar, however, are comparatively easy of 

 digestion, and the latter two, at least, do 

 'not call for any large expenditure of en- 

 ergy, so that it is difficult to decide whether 

 the small differences in Rubner 's figures 

 are significant or not. A material requir- 

 ing more digestive M'ork would obviously 

 furnish a more decisive test. Such mate- 

 rials are offered in the food of her.bivora, 



particularly in the so-called 'coarse fod- 

 ders' (hay, straw, etc.), and the recent com- 

 pletion at the Pennsylvania Experiment 

 Station of a respiration-calorimeter * for 

 experiments with cattle afforded an oppor- 

 tunity to test the question with these 

 animals. 



The feed used was a rather coarse timo- 

 thy hay. The steer was fed a very light 

 ration of the hay, together with a little 

 linseed meal, the whole 'basal ration' being 

 much less than was required to support the 

 animal. On this basal ration the steer pro- 

 duced 9,229 calories of heat in 24 hours. 

 During the same time it was shown that he 

 oxidized 49.2 grams of the proteids and 259 

 grams of the fat of his body, equivalent to 

 2,578 calories of energy. 



Then 1.2 kilograms of the hay were 

 added to the basal ration. It was shown 

 that, after deducting the various unoxi- 

 dized wastes, this hay represented a fuel 

 value of 2,840 calories; that is, oxidized to 

 the fullest extent to which the organism 

 was capable of carrying it, could liberate 

 that amount of energy. If, now, the law 

 of isodynamic replacement is applicable to 

 this case, that is, if it was simply a ques- 

 tion of substituting one kind of fuel for 

 another, the digestible matter of the hay, 

 yielding 2,840 calories of energy, should 

 have prevented the oxidation of an equiva- 

 lent amount of body tissue. As a matter 

 of fact it fell far short of doing this. On 

 the increased ration, the daily loss by the 

 body of the animal M^as 7.2 grams of pro- 

 teids and 80.6 grams of fat, equivalent to 

 791 calories of energy, while the heat pro- 

 duction in 24 hours was 10,249 calories as 

 compared with 9,229 calories on the smaller 



* The apparatus is modeled after that of At- 

 water and Rosa. It was constructed and is being 

 operated in cooperation by the Bureau of Animal 

 Industry of the U. S. Department of Agriculture 

 and The Pennsylvania State College Agricultural 

 Experiment Station. 



