PRECIPITINS 145 



is desirable to dilute the suspected blood as far as 

 possible when testing, for when concentrated sera 

 are brought together reactions may occur which 

 will lead to erroneous conclusions. In medico- 

 legal work it will be well to progressively dilute a 

 suspected blood sample and to reach a conclusion 

 upon the highest (within limits) which reacts to a 

 given antiserum. In routine work one can com- 

 mence with dilutions of the suspected blood of 

 i : 100 or i : 200. We must not omit to say that it 

 is necessary to test to litmus all solutions to be 

 examined, and to neutralize any that are found 

 decidedly acid or alkaline. 



Appearance of the Reaction. When antiserum 

 is added to blood dilution it sinks to the bottom of 

 the tube, forming a milky white zone at the point of 

 contact. The milkiness gradually extends upward 

 until the whole fluid is clouded. Where the fluids 

 have been mixed by shaking this diffuse cloudiness 

 undergoes a change; after ten to twenty minutes, 

 or later, very fine granules of precipitum begin to 

 appear, and the upper layers of the fluid begin to 

 clear, due to sedimentation of the precipitum. 

 The fine particles soon become aggregated into 

 coarser ones, and these into flocculi which, gradually 

 sinking to the bottom of the tube, give rise to more 

 or less deposit of a whitish appearance. With blood 

 dilutions of, say i : 40 to i : 200 and over, the deposit 

 formed is usually sharply defined ; where more con- 



