174 OBTAINING THE BLOOD-SERUM 



haemoglobin, and in doubtful cases the spectroscope 

 should be used. 



The serum must be completely freed from its form- 

 elements, a point which is often neglected. A serum 

 may appear absolutely clear, and yet contain millions 

 of red blood corpuscles. The serum must be treated 

 with a good electric centrifuge until the tube shows 

 no trace of blood corpuscles, either on its sides or 

 at the bottom. The serum, after each treatment with 

 the centrifuge, is drawn off with a pipette and trans- 

 ferred to another tube, and during this operation, in 

 order to avoid any contact of the pipette with the red 

 blood corpuscles, the tube is placed upon a mirror. 

 One can then see exactly where the end of the pipette 

 is at any moment. The blood is best taken with an 

 absolutely dry needle and placed directly into a 

 sterilized centrifuge tube, or, better still, into a 

 small Erlenmeyer flask. The blood is allowed to 

 clot spontaneously, and is watched until the serum 

 separates out. Any mode of procedure which 

 accelerates the separation of the serum increases the 

 risk of haemolysis. The blood should not be placed 

 either in an ice-chest or in an incubator, but should be 

 left simply at room temperature. In the first case, the 

 risk of haemolysis is very great ; in the second, auto- 

 lysis of the form-elements generally results. Serum 

 is generally obtained in a considerable quantity 

 after five or six hours, but if enough has not 



