286 PHYSIOLOGICAL CHEMISTRY 



water it may be extracted with acidified alcohol, and the extract 

 examined with the spectroscope for acid hematin. 



The identification of a stain as human blood is accomplished 

 by none of the above reactions. The final proof that a blood 

 stain has come from a human subject is obtained by agglutina- 

 tion and hemolysis tests. 



51. The globin constituent of hemoglobin may be demon- 

 strated as follows : Corpuscles are washed 2 or 3 times with iso- 

 tonic salt solution, the corpuscles being separated from the 

 liquid each time by centrifuging. A small quantity of the 

 washed corpuscles is then placed in a test tube, a small amount of 

 alcohol and of hydrochloric acid added and the mixture heated 

 on the water bath. The liquid turns a dark brown color due to 

 the formation of acid hematin, and a precipitate forms which 

 consists of the globin. This may be filtered off, and it will be 

 found to give the usual protein color tests. 



Hemoglobin, although crystallizing readily, differs from other 

 crystalloids in not diffusing through an animal membrane. 



Phosphoproteins. The phosphoproteins are compounds con- 

 sisting of a protein combined with some phosphorus-containing 

 substance other than nucleic acid or lecithin. These proteins 

 often are called nucleoalbumins ; they differ from the nucleopro- 

 teins by containing no purine bases. One member of the group 

 will be studied, casein from milk. In connection with this sub- 

 stance some other constituents of milk and egg yolk will be con- 

 sidered. 



Caseinogen. 



52. Test with litmus the reaction of the milk furnished. Fresh 

 milk is neutral or faintly alkaline. 



53. Boil a few cubic centimeters of milk. Observe that the 

 caseinogen does not coagulate. In this respect caseinogen differs 

 from most other proteins. If a skin forms over the surface of the 

 milk, it is due partly to evaporation from the surface of the 

 liquid, partly, perhaps, to coagulation of other milk proteins. 



54. To a few cubic centimeters of milk add an equal volume 



