It is with the first three of the above-named classes of food constit- 

 uents that difficulty is encountered, and for which further study of 

 methods is desirable. The object of the work which the writer has 

 undertaken, and to which he has devoted a good deal of time for 

 several months past, has been to study the means of distinguishing 

 between the first and third of these classes of constituents, the pro- 

 teids and related bodies on the one hand and the simpler amids and 

 amido-acids sometimes grouped together as <' nitrogenous extract- 

 ives" on the other. Incidentally only, some experiments have been 

 made with representatives of the gelatinoid class. 



NUTRITIVE VALUES OF THREE CLASSES. 



It is commonly assumed that proteids, gelatinoids, and the simpler 

 amids have very different nutritive values, and while all authorities 

 would agree in assigning the highest value to the first of these there 

 is probably no small difference of opinion as to the order in which the 

 second and third should be rated. In considering such a question, 

 there should be separately taken into account relative digestibility or 

 solubility, capability of undergoing osmotic absorption, and oxidiza- 

 bility in order to the production of energy. At present no definite 

 numerical statement of the relative nutritive values of nitrogenous 

 bodies of these three classes can be made. It seems much to be desired 

 that more extended experiments than have so far been recorded should 

 be made upon living animals as far as possible upon human beings 

 in regard to the utilization of both the gelatinoids and the simpler 

 amids. The latter no doubt undergo oxidation to some extent in the 

 animal body, and produce some energy in consequence. It is probably 

 true of these simpler amidic substances that much larger quantities 

 than analysis exhibits as constituents of the food consumed or than 

 analysis detects among the residua of food rejected from the body with- 

 out having undergone complete oxidation, may be constantly formed 

 among the earlier products of the metabolism of the proteids, and 

 afterwards themselves undergo further change into the simpler and 

 more stable forms of carbon dioxid, water, and urea. 



LACK OF PRECISION IN CHEMICAL CONSTITUTION. 



It must be admitted that to a chemist the .question of distinguishing 

 between the proteids and the simpler amids is not one of a scientifically 

 precise character. The proteids doubtless contain at least a part of the 

 nitrogen in the amidic relation, and where the line is to be drawn 

 between more complex and more simple amids is, of course, more or less a 

 matter to be arbitrarily decided. But, of greater importance still is the 

 doubt whether any of the so called proteids are entitled to recognition 

 as definite chemical substances. We usually understand by the term 

 "a definite chemical substance" a substance of which all the molecules 

 are exactly alike in constitution. Thus to the chemist the identity of 



