THE PROTEIN SUBSTANCES. IOQ 



cleavage products of the proteins. As this cleavage pro- 

 gresses a point is finally reached where the mixture no longer 

 solidifies on cooling; a permanent liquid solution is obtained. 

 By hydrolysis with acids this condition is much sooner reached. 

 Many bacteria also have the power of " liquefying " gelatin, 

 which depends of course on their ability to decompose the 

 complex into the more easily soluble amino acids and other 

 compounds. 



Among the final cleavage products of gelatin easily recog- 

 nizable glycocoll is probably the most abundant. Leucine, 

 alanine and various other amino acids are found in smaller 

 amount. Like other proteins gelatin yields in peptic or tryptic 

 digestion bodies which have been called gelatoses, gelatin pep- 

 tones and so on. These resemble but are not identical with 

 the true peptones, which fact has some bearing on the long- 

 discussed question of the food value of gelatin. Gelatin is not 

 converted into true protein in the animal body and for this 

 reason cannot wholly replace the albumins as food. But to 

 some extent it has the power of protecting the so-called circu- 

 lating albumin from katabolism, by undergoing destruction 

 itself. This sparing or protecting power is limited, however, 

 and the gelatin substances can not permanently replace the 

 native proteins in this way. 



EXPERIMENTS TO ILLUSTRATE PROPERTIES OF GELATIN. Dissolve enough 

 gelatin in hot water to make a solution of about one-half per cent strength. 

 Use portions of this for tests : 



To some of the solution add a solution of tannic acid; this gives a buff 

 colored precipitate. Gelatin solution is, conversely, employed as a test for 

 tannic acid. 



To some of the solution add an excess of strong alcohol; this causes 

 precipitation. This behavior is of importance in the estimation of gelatin. 



Use some of the solution with the test reagents. Apply Millon's re- 

 agent, the biuret test and the xanthoproteic test. 



Prepare a strong solution of gelatin in hot water. To some of this add 

 solution of potassium dichromate and pour the mixture out to cool in a 

 .thin layer exposed to sunlight. This treatment produces an insoluble 

 mass which is not attacked by hot water. This property finds application 

 in photo-engraving processes. 



To more of the strong gelatin solution add a trace of alkali to neu- 

 tralize any acidity and then some formaldehyde. On evaporating to dry- 



