11 



FOODS INVOLVED IN THE INVESTIGATION. 



While it was especially desirable to ascertain the behavior of these 

 several substances, of both classes, in their separate condition, it was 

 borne in mind that in connection with nutrition investigations they 

 have to be dealt with as constituents of ordinary food materials, prin- 

 cipally the following: 



Meats, including poultry and fish (raw and cooked), soups and stews, meat extracts, 

 eggs, milk and cheese, bread and other farinaceous preparations such as cakes and 

 pastry, fruits and raw table vegetables, cooked fruits and table vegetables, fruit 

 conserves, undigested and uuabsorbed residues of food. 



It was assumed as probable that the same method (in detail) would 

 not be best adapted to all cases, but would be likely to need modifica- 

 tion in application under different conditions. 



Of course, the investigation undertaken has gone over ground already 

 well trodden, and the results recorded by Schulze, Barbieri, Sachsse, 

 Kormann, Boemer, Huefner, Allen, Tankard, Ohitteudeu, Osborne, 

 Wiley, and others have been carefully examined. 



The experimental work of the present investigation is reported, not 

 altogether in the order in which it was done, but rather in logical suc- 

 cession, taking account first of physical differences between the classes 

 of substances to be distinguished, then of the effects of chemical agents 

 of decomposition, and, lastly, of relations to solution and precipitation. 



METHODS OF SEPARATING NITROGEN. 



A brief notice of several methods of separating nitrogen will afford 

 means of comparison. The phospho-tungstic method which was espe- 

 cially investigated is treated more fully than the others. 



DIALYSIS. 



The attempt to separate such proteids as are soluble in water from 

 the simpler amids intentionally mixed with them by applying Graham's 

 method of dialysis was made with no great hope of success. It is, of 

 course, well known that the amorphous proteids in solution exhibit as a 

 class a very small degree of diffusive mobility, while the crystallizable, 

 simper amids diffuse through a porous septum, as of parchment paper, 

 with much greater rapidity. But separation based on this difference 

 can never be complete. Only a part of the more diffusible material 

 can be obtained in the water on the opposite side of the diaphragm 

 from the mixed solution, and that a large part shall be so obtained 

 requires that the volume of water used shall be large as compared 

 with the volume of the solution. Moreover, the absolute volume of the 

 solution itself must be large where amids of but slight solubility, 

 such as tyrosin, are present, in order that they may not be left behind 

 in the uudissolved state. But the time required for dialytic separation 



