220 



PHYSIOLOGICAL CHEMISTRY. 



plates and allow it to harden several hours at least. The prepared plates 

 are removed from the glass and soaked in a neutral solution of litmus and 

 are then allowed to dry. The test proper can now be made by putting a 

 few drops of the blood on the smooth plaster surface and allowing it to 

 remain there five minutes. It is then washed off with pure water, when 

 it will be found that the part of the plate which has been covered by the 

 blood has become blue -from the action of the alkali of the blood on the 

 neutral litmus. 



Ex. HEAT TEST. Heat the solution of blood until it is near the boiling 

 temperature and note that the red color is largely destroyed and that a 

 brownish precipitate forms which contains albumin and decomposed color- 

 ing matter. Add now a small amount of sodium hydroxide solution and 

 observe that the precipitate disappears while the blood solution becomes 

 red again by reflected light, but greenish by transmitted light. 



Ex. HEMIN CRYSTALS. When acted on by acids or strong alkalies hemo- 

 globin of blood is broken up into globin and a characteristic compound 

 called hematin. Hematin in turn is acted upon by hydrochloric acid yield- 

 ing the hydrochloride, hemin, which appears in crystalline form. From 

 the name of their discoverer, these crystals are called " Teichmann's crys- 

 tals." Their appearance constitutes one of the best tests we have for 

 blood, and can be illustrated by the following: Evaporate a drop of blood 

 on a slide, add two or three drops of glacial acetic acid, and boil. Put on 

 a cover glass and allow to cool. Minute (microscopic) plates or prisms 

 separate out. If old blood, a stain, for instance, is examined, it is neces- 

 sary to add a small crystal of sodium chloride to the acetic acid, by which 

 means sufficient hydrochloric acid is liberated for the test The crystals 

 have a dark brown color and are very characteristic. The usual forms as 

 found in human and other blood are shown below. 



2> 



t +4tl 



FIG. 9. Hemin crystals, i is from human 

 blood ; 2 from seal ; 3 from a calf ; 4 from a pig ; 

 S from a lamb ; 6 from a pike ; 7 from a rabbit. 

 (LANDOIS.) 



- * ^ \ 

 -ji > * ~~'*' 



FIG. 10. Hemin crys- 

 tals from stains of hu- 

 man blood. (LANDOIS.) 



