INSOLUBLE PROTEINS. 51 



presence of peptones. They will be found to have passed 

 through the membrane, although they do not dialyze rap- 

 idly. 



121. Show that tannic acid precipitates peptones in 

 a neutral solution. 



122. Show that the peptones are not precipitated by 

 potassium ferrocyanid acidified with acetic acid, as are 

 the albumoses, if they contain none of the latter. 



FIBRIN. 



Fibrin is formed as a gelatinous mass when fresh 

 blood coagulates. If the blood is beaten during its coag- 

 ulation the fibrin collects together into strings as elastic as 

 caoutchouc, and remains so as long as it is moist. It can 

 be freed from the blood coloring matter by washing with 

 water or a salt solution. It is coagulated by heating with 

 water. 



COAGULATED ALBUMIN. 



The albumins may be converted into the coagulated 

 form by heating with water or by the continued action of 

 strong alcohol. This is insoluble in water, but can be dis- 

 solved by caustic alkalies or by heating with the strong 

 mineral acids, being thereby converted into alkali or acid 

 albumins. 



COMPOUND PROTEINS. 



This class of substances is more complex than the 

 albuminous substances. They can all be decomposed into 

 albuminous substances, on the one hand, and, on the other, 

 bodies which are not albuminous. Thus the albuminous 

 compound is, in hemoglobin, united with the hsematin 

 molecule; in the nucleins, with phosphoric acid, etc. They 



