54 PRACTICAL HISTOLOGY. 



the air for a few minutes. It becomes thickened by 

 evaporation and dried at the edges, and crystals, 

 tetrahedral in form, may be detected in it with a 

 low power of the microscope. The preparation may 

 then be covered and examined with a higher power. 

 The crystals increase in size for a time, and new 

 ones continue to be formed. 



Haemoglobin crystals will not keep unaltered for 

 any length of time. 



Preparation 22. The name " hsemin" has been 

 given to certain very characteristic crystals which 

 are formed at the expense of the coloring matter of 

 the blood, and the production of which serves as an 

 exceedingly trustworthy test of the presence of 

 blood, although yielding no indication of the kind 

 of animal from which the blood has been obtained. 

 To see them a very small quantity of blood obtained 

 from the finger or elsewhere is smeared opon a glass 

 slide and allowed to dry. A cover-glass is then 

 placed over it, and a drop of glacial acetic acid is 

 applied from a pipette to the edge of the cover-glass 

 and allowed to run under by capillary attraction. 

 The slide is then held by one end in the fingers, and 

 the middle is gently warmed over a small flame. 

 As soon as bubbles begin to appear in the fluid the 

 warmth is discontinued, and the preparation is ex- 

 amined with a high power. If no crystals appear 

 as the slide cools a little more acid is added, to re- 

 place that lost by evaporation, and the slide is 

 warmed as before, and on cooling again examined. 

 It will be found that almost all over the preparation 

 reddish-brown short prismatic crystals, disposed 

 singly or in groups, will have made their appear- 

 ance ; most of them are very minute, but they may 

 be obtained of considerable size by re-warming the 

 preparation with glacial acetic acid once or twice. 



The presence of a chloride is necessary for the for- 

 mation of these crystals; in the case of recent blood 

 the chloride of sodium which it naturally contains 



