127 



a clear serum should be kept in mind. If the precipitating value of the 

 serum is insufficient, more injections may be given before the animal 

 is finally bled. 



If the serum is not withdrawn at the proper time, its strength begins 

 to diminish and further injections no longer stimulate new antibodies. 

 It is even possible for the entire precipitin action of the serum to dis- 

 appear. 

 Titration ^^ e ^^ ow ^ n S method of titration of the strength of a serum is 



the simplest. One c.cm. of various dilutions (i : 10, i : 100, 



i : 1000, i : 10000) of the proteid under examination (precipitin- 

 ogen) is placed into different test-tubes and o.i c.cm. of the precipitating 

 serum is added to each. The tubes should not be shaken, but it is occa- 

 sionally necessary to place them in the incubator for one hour before any 

 turbidity or precipitate appears. The least amount of proteid solution 

 which still distinctly shows a precipitate is taken as the titer of the serum. 



For medico-legal purposes, Uhlenhuth advises the use of only 

 Uhlenhuth's highly valent sera. 



Method of He considers an antiserum as efficient if o.i c.cm. of it, when 

 Proteid Dif- mixed with its respective serum in the dilution of i : 1000, 

 ferentiation. p ro d uces a distinct turbidity, either at once or in one to two 



minutes at the latest; three to five minutes is the limit for an 

 indication of turbidity in the dilutions of i : 10000 and i : 20000. 



Like in all other biological reactions, control tests, here two in number, 

 are of the utmost importance. One tube must contain o.i c.cm. of the 

 precipitating serum mixed with i c.cm. of saline, another o.i c.cm. of the 

 precipitating serum mixed with a heterologous serum in the dilution of 

 i : 200 and i : 1000. Both of these tubes should show absolutely no pre- 

 cipitate after twenty minutes. In this way the specificity of the pre- 

 cipitin is determined; and it must be remembered that it is the quantitative 

 specificity which counts. 



In the process of the determination of the nature of meats, it is es- 

 pecially necessary to ascertain exactly the precipitating titer against 

 bovine and pig's serum possessed by the rabbit's precipitating serum 

 directed against horse's serum . 



When clear solutions are at hand the precipitin reaction is compara- 

 tively simple. Frequently, however, the test must be performed with old 

 and dirty blood stains, or all kinds of prepared sausage, so that the 

 first and important task is to obtain a clear solution. 



In dealing with blood, milk, or seminal stains, the parts of the clothing involved are 

 excised, divided into very minute shreds, and placed in a test-tube with a small 

 amount of 0.85 per cent, of salt solution. If the material is not too old, extraction 

 for one hour is usually sufficient, otherwise it may necessitate a period of twenty- 

 four hours or more. Stains upon solid material such as steel, wood, stone, etc., 



