128 



PRECIPITINS 



are carefully scraped off, and suspended in physiological salt solution. To obtain 

 a clear solution the extract must be passed through filter paper or eventually the lilli- 

 putian bacterial filter. 



In the examination of meats or other food stuffs, it is best to remove 

 the material for examination from the center of its thickest part, as this 

 portion has been least exposed to the methods of preservation, especially 

 the high temperatures. Three hours' extraction is usually sufficient; the 

 fresher the meat, the shorter this period. Very much salted meats are 

 best washed with distilled water, previous to extraction. Inasmuch as a 

 great deal of fat interferes with the reaction it is advisable to remove it 

 beforehand by extraction with ether and chloroform for twenty-four 

 hours (Miessner and Herbst). 



Before performing the actual test with the unknown blood stain, it is 

 best to try out the entire reaction with a similar but known blood stain in 

 order to make sure whether all the ingredients are in good working order. 

 In laboratories equipped for medico-legal examinations, stains made upon 

 linens from the blood of man, ox, pig, horse, etc., are always kept in 

 readiness for such preliminary tests. 



Uhlenhuth indicates a set of rules to be observed whenever the reaction 

 is undertaken. They are here cited in their original form, as practice has 

 shown them to be of great service. 



"In order to obtain sufficient extract for the test, a small amount of the material is 

 placed in a test-tube containing 5 c.cm. of normal salt solution. This must not be 

 shaken. After one to two hours, 2 c.cm. are poured off into another tube and gently 

 shaken. If a persisting froth appears upon the surface of the fluid, it can be taken as 

 proof that sufficient extraction has occurred, and the rest of the fluid is thereupon also 

 transferred to this tube. If no froth appears the 2 c.cm. should be returned into the 

 first test-tube and the extraction continued until repeated tests finally show the pres- 

 ence of froth. It is preferable not to disturb the sediment at the bottom of the test- 

 tube. The extract eventually obtained may have to be filtered, if not absoutely clear. 



Such an extract is, as a rule, stronger than that required for the test, i.e., i : icoo. 

 If one drop of a 25 per cent, nitric acid solution is added to i c.cm. of a i : 1000 serum 

 dilution and then heated, a faint opalescence appears. Enough saline should there- 

 fore be added to the final extract so that the nitric acid test corresponds to that 

 given by a dilution of i : 1000. 



The following mixtures are then made: 



