PRACTICAL EXERCISES 7! 



s ame as that 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 obtained 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. 



(d) Tallquist's Method. In this method the tint produced by a 

 drop of blood on a piece of white filter-paper is compared with a 

 scale representing 10 percentages of haemoglobin (from 10 to 100 per 

 cent.). The standard filter-paper is supplied in the form of a book 

 with the scale. To make an estimation, all that is necessary is to 

 touch a drop of blood with a piece of the filter-paper, and allow the 

 blood to diffuse slowly through the paper, so as to give an even stain. 

 The position of the stain is then determined by the scale ; e.g., it 

 may be deeper than 90, but fainter than 100, in which case the per- 

 centage of haemoglobin lies between 90 and 100. The method is by 

 no means a very accurate one, but more accurate than it appears at 

 first sight. 



(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 

 (Fig. 1 6, p. 67) . 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 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. 



