66 A MANUAL OF PHYSIOLOGY 



of the standard solution, when the troughs are placed side by side 

 on white paper. From the quantity of water added it is easy to 

 calculate the proportion of haemoglobin in the undiluted blood. 

 Greater accuracy is said to be obtamed if the haemoglobin in the 

 standard solution and that of the blood are converted into carbonic 

 oxide haemoglobin by passing a stream of coal-gas through them. 



(5) Microscopic Test for Blood-pigment. Put a drop of blood on 

 a slide. Allow the blood to dry or heat it gently over a flame, so as 

 to evaporate the water. Add a drop of glacial acetic acid ; put on a 

 cover-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 

 (Plate I., 3). 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 

 spectroscope or the micro-spectroscope (a microscope in which a 

 small spectroscope is substituted for the eye-piece). Then evapo- 

 rate 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 recognised under the 

 microscope, after the cloth had been soaked and kneaded in a little 

 glycerine. 



