PRACTICAL EXERCISES 79 



glass, and again heat slowly till the liquid just begins to boil. Take 

 the slide away from the flame for a few seconds, then heat it again for 

 a moment; and repeat this process two or three times. Now let the 

 slide cool, and examine with the microscope (high power). The small 

 black, or brownish-black, crystals of haemin will be seen (Fig. 24, p. 78). 

 This is an important test where only a minute trace of blood is to be 

 examined, as in some medico-legal cases. If a blood-stain is old, a 

 minute crystal of sodium chloride should be added along with the 

 glacial acetic acid. Fresh blood contains enough sodium chloride. 



A blood-stain on a piece of cloth may first of all be soaked in a small 

 quantity of distilled water, and the liquid examined with the spectro- 

 scope or the micro-spectroscope (a microscops in which a small spectro- 

 scope is substituted for the eyepiece). Then evaporate the liquid to 

 dryness on a water-bath, and apply the haemin test. Or perform the 

 haemin test directly on the piece of cloth. In a fresh stain the blood- 

 corpuscles might be recognized under the microscope. Very few 

 liquids, however, are available for washing out the blood, as all ordinary 

 solutions, and even serum itself, cause laking of dried corpuscles 

 (Guthrie). Absolute alcohol, or 35 per cent, potassium hydroxide, 

 may be used to soak and rub up the cloth in. 



