THE ALBUMINOIDS 163 



the laOoratory : they are products of vegetable life, and form an 

 essential part of the food and of the substance of animals. 

 Bruise some fresh clover plants in a mortar, press out the sap, 

 filter into a test-tube, and warm. Note that a substance pre- 

 viously dissolved coagulates on warming. An albumin-like sub- 

 stance is thus shown to be present. Repeat the experiment 

 with fresh meat juice. Notice here agam the presence of an 

 albuminoid. 



While found in animal or vegetable substance in a soluble 

 state, albuminoids also exist in the insoluble or coagulated 

 state. Make some wheat flour into a dough with water, enclose 

 in a muslin bag, and knead in a basin of water until the starch 

 has been entirely washed out. On opening the bag, a tough 

 elastic substance, the " gluten " of wheat, will be found, this 

 being a mixture of albuminoids mostly insoluble in water. 



The albuminoids arrange themselves in several classes dis- 

 tinguished by different properties. Among these distinguishing 

 features are the different conditions under which coagulation 

 occurs. To illustrate these, the coagulation of the albumin of 

 egg, the casein of milk, and the fibrin of blood or myosin of 

 flesh, may be studied. 



Separate the white of a fresh egg. Shake it vigorously with 

 about ten times its bulk of water, and strain the liquid several 

 times through muslin until a clear solution of albumin is 

 obtained. Heat a portion in a test-tube. Observe that it 

 coagulates at a temperature below 75° C. To another portion 

 add nitric acid, and notice that this also causes coagulation. 



Allow some milk to stand for a day in a cylinder in order 

 to allow the cream to separate. Remove some of the skim 

 milk by means of a pipette, and heat it in a test-tube. Note that 

 no change occurs. Now add some nitric acid, and observe the 

 curdling of the milk due to coagulation of the casein. To 

 another portion add rennet {i.e. the solution of a soluble 

 ferment obtained from the fourth stomach of calves). Coagula- 

 tion will again take place. 



