THE PHENOMENON OF PRECIPITATION 257 



protein dog serum, beef serum, etc. it must be determined that 

 the precipitin in this case is strictly specific. 



The reaction can be observed with greater delicacy if it is first 

 set up by the method recommended by Fornet and Miiller, 31 which 

 we may speak of as the "ring test." The antiserum is put into the 

 tubes and the solution to be tested is allowed to flow slowly over this 

 as in Heller's nitric acid albumin test. At the line of contact be- 

 tween the two a fine white ring will rapidly appear, thickening and 

 growing heavier as the preparation is allowed to stand. After taking 

 the final readings from such a test, let us say after an hour or so, it 

 is well to shake up the tubes, set them away in the ice-chest, and again 

 read the amount of precipitates formed in the various tubes the next 

 morning. Since every test of this kind necessitates a number of 

 controls, the following example will serve as a basis for discussion : 



Forensic Blood Examination 



Material: Blood spot on trouser pocket, washed up in salt solution. Clear 

 after paper filtration. 



Antiserum: Rabbit treated with three intravenous injections, 2, 5, and 5 c. c. 

 of human serum at six-day intervals; bled on tenth day after last injection. 

 This serum has been titrated against human serum and gives precipitation 

 in dilutions up to one to ten thousand. With one to one thousand there is 

 clouding which begins in three minutes and is very distinct in eight minutes, 

 at room temperature. 32 



Test 



Tube 1. Known human serum 1 to 1,000. . 1.0 c. c. + Antiserum. . . .0.2 c. c. 



Tube 2. Unknown solution to be tested 1.0 c. c. -f Antiserum. . V 0.2 c. c. 



Tube 3. Unknown solution to be tested 1.0 c. c. + Normal rabbit 



serum 0.2 c. c. 



Tube 4. Salt solution 1.0 c. c. + Antiserum .... 0.2 c. c. 



Tube 5. Unknown solution 1.0 c. c. -f Salt solution. . .0.2 c. c. 



In this test, if the original material was human blood, tubes 1 

 and 2 should show ring formation within 5 minutes while the 

 other tubes remain clear. In addition to these controls it is well to 

 be sure that the test extract is neither strongly acid nor alkaline, and 

 that, as Uhlenhuth suggests, the material from which it is extracted 

 does not contain other substances which can give precipitates by 

 themselves when added to serum. This is especially necessary in the 

 case of cloth fabrics, and a recent instance in our own experience 

 has suggested to us the possibility that such materials may also con- 

 tain colloidal dye stuffs or other extractable substances which can 

 cause inhibition of the precipitation. In an apparently positive 

 case the reactions with a blood extract from trouser cloth were suffi- 

 ciently heavy, but regularly delayed, as in the flocculation of such 



31 Fornet and Miiller. Zeitschr. f. Hyg., Vol. 66, 1910. 



32 A mixture of too specific antisera should never be used, since such 

 sera may often precipitate each other for reasons that are discussed below. 



