BLOOD. 197 



dine preparations vary greatly in their sensitiveness, however. 

 Inasmuch as benzidine solutions change readily upon contact with 

 light it is essential that they be kept in a dark place. The test is 

 performed as follows : To a saturated solution of benzidine in 

 alcohol or glacial acetic acid add an equal volume of 3 per cent 

 hydrogen peroxide and one c.c. of the solution under examination. 

 If the mixture is not already acid render it so with acetic acid, 

 and note the appearance of a green or blue color. A control test 

 should be made substituting water for the solution under examina- 

 tion. The sensitiveness of the benzidine reaction is greater when 

 applied to aqueous solutions than when applied to the urine. 



14. Haemin Test. (a) Teichmann's Method. Place a very 

 small drop of blood on a microscopic slide, add a minute grain 

 of sodium chloride 1 and carefully evaporate to dryness over a low 

 flame. Put a cover glass in place, run underneath it a drop of 

 glacial acetic acid and warm gently until the formation of gas 

 bubbles is noted. Add another drop of glacial acetic acid, cool 

 the preparation, examine under the microscope and compare the 

 crystals with those shown in Figs. 58 and 59, page 198. The hsemin 

 crystals result from the decomposition of the haemoglobin of the 

 blood. What are the steps involved in this process? The haemiii 

 crystals are also called Teichmann's crystals. Is this an absolute 

 test for blood ? Is it possible to differentiate between human blood 

 and the blood of other species by means of the hsemin test? 



(b) Atkinson and Kendall's Method. Introduce a small amount 

 of the solution under examination into a tube closed at one end, 

 add sodium chloride and glacial acetic acid as in Teichmann's 

 method, 2 fuse or tightly plug the open end of the tube and heat 

 for fifteen minutes in a boiling water-bath. 3 Remove the tube and 

 permit it to cool to room temperature spontaneously. When the 

 tube has cooled, break it open, transfer the contents to a watch 

 glass or small evaporating dish and concentrate on a water-bath 

 until the volume of the fluid in the watch glass or dish has been 

 reduced to a few drops. Transfer a drop of this fluid to a slide* 

 cover with a cover slip, allow the slide to stand for a few minutes-, 

 and examine it under a microscope. Compare the crystals with 

 those shown in Figs. 58 and 59, page 198. In case crystals of 

 sodium chloride (see Fig. 60, page 200) obstruct the view of the 



1 Buckmaster considers the use of potassium chloride preferable. 



2 Care should be taken not to add too great an excess of these reagents. 



3 This process insures constancy of temperature and strength of reagents. 



