9 



such a substance as pure common salt or cane sugar or caffeine depends 

 on the absolute identity in character of all the myriad molecules of 

 which even the smallest sensible mass is made up. But in regard to 

 such materials as these we are able to determine the relative and abso- 

 lute number of atoms of tbe elements of which each molecule is com- 

 posed, and in many cases their order of attachment to each other, or 

 the ''structure" of the molecule. On the other hand, we have reason 

 to believe that the so-called proteids are made up of molecules of such 

 extreme complexity, assemblages of such large numbers hundreds 

 of atoms of carbon, hydrogen, oxygen, and nitrogen, that we can make 

 but random guesses at their arrangement, and -can not even determine 

 with any certainty their number, relative or absolute. We talk of albu- 

 min, myosin, syntoniu, as if these terms stood for pure chemical sub- 

 stances in the same sense that attaches to ammonia, benzene, or urea. 

 But it is by no means certain that in a specimen of the most carefully 

 prepared albumin from blood or white of egg any hundred, or any ten, 

 molecules are absolutely alike. It may well be that a minute specimen 

 of such a material consists in reality of numerous more or less similar 

 but yet in some respects different molecules, which we lump together 

 under a single name merely because they have a general resemblance, 

 with certain properties in common. Furthermore, in nutrition investi- 

 gations we have to deal with articles of food representing complex 

 mixtures of substances referable to the two classes of the proteids and 

 nitrogenous extractives, with many other things besides. Hence, an 

 answer to the question to be examined must be a limited one, and such 

 only as may serve the limited purpose of practical application in con- 

 nection with nutrition investigations. Any process of separation, to be 

 of value in such application, must be reasonably simple, and capable 

 of being carried out without too great consumption of time. 



LIST OF SUBSTANCES EXAMINED. 



A good deal of work was necessary at the outset in procuring satis- 

 factory specimens of the several substances to be examined. Some of 

 these have been prepared out and out in this laboratory from natural 

 animal or vegetable sources. Some have been purchased in a more or 

 less crude state and carefully purified. Some have been purchased, 

 and their purity ascertained by testing. 



The following representatives of the simpler amidic and imidic com- 

 pounds were experimented with: 



Amido-aclds of 1he fatty series: 



Glycocin (glycocoll or amido-acetic acid). 



Alauiu (a-ainido-propionic acid). 



Leucin (amido-caproic acid). 

 Amido-acids of the succinic acid and allied series: 



Aspartic acid (amido-succinic acid). 



Asparagin. 



Glutamic acid (a-amido-oxyglutaric acid). 



Glutainin. 



