ISODYNAMIC VALUES OF THE FOODSTUFFS 575 



We must be careful, however, in formulating any such fundamental 

 conclusion not to go too far. We must beware of overstepping to the 

 slightest extent the sure ground of fact which our evidence affords, and 

 we must therefore candidly admit that while the evidence accumulated 

 by the remarkable series of investigations which we have briefly and 

 inadequately outlined, clearly justifies the conclusion that no source of 

 energy is contributed to or resides in the organism that is not com- 

 prised in the chemical energy of its foodstuffs, and the heat of its 

 environment, yet we cannot definitely reject the possibility that forces 

 of evanescent magnitude which are not comprised in either of the 

 above categories may influence, in the manner of a catalyzer, the rate of 

 discharge of energy from the organism. We cannot disprove this, but 

 then, on the other hand, if one should choose to assume the existence 

 of such forces the burden of proof clearly rests upon the originator of 

 the hypothesis. In the interpretation of life-phenomena, so far as we 

 have as yet been enabled to subject them to measurement, such an 

 assumption has proved to be altogether unnecessary, and hence our 

 present state of knowledge affords for it no foundation whatever. 

 No sure ground is possible in scientific discovery unless we proceed from 

 the known to the unknown. The assumption that hitherto unknown 

 forces are involved in life cannot assist but only retard its interpreta- 

 tion until and unless every previously known possibility has been 

 exhausted in a vain endeavor to reconcile the facts. But the existence 

 of unknown possibilities manifestly cannot be contradicted upon a 

 priori grounds, and a dogmatic insistence upon the sufficiency of the 

 known has only too frequently, in the history of science, served but to 

 pave the way for a subsequent recantation. 



THE ISODYNAMIC VALUES OF THE FOODSTUFFS. 



Since the products of the combustion of the Fats and Carbohydrates 

 in the diet are the same, namely carbon dioxide and water, it was sug- 

 gested at an early period in the investigation of metabolism that these 

 components of the dietary might be mutually interchangeable in 

 equicalorific quantities. This possibility was experimentally realized 

 by Rubner, who fpund that 100 grams of fat in the diet could be 

 replaced by 232 grams of starch or 234 grams of cane-sugar, the equi- 

 calorific values estimated from the heat of combustion being 229 grams 

 of starch and 235 grams of cane-sugar. The same conclusion was ulti- 

 mately reached by Atwater in a series of experiments in which the 

 subjects were made to perform external work, so that part of the energy 

 of the foodstuffs had to be expended for this purpose. The procedure 

 of the experiments was designed to test the efficacy of the fats as sub- 

 stitutes for carbohydrates in a variety of ways. Thus the diet was 

 insufficient to maintain bodily equilibrium, so that there was a loss of 

 weight throughout the duration of the experiments due to the consump- 

 tion of the subject's tissues. The loss of body-substance on the diet 



