258 INFECTION AND RESISTANCE 



colloidal suspensions as arsenic trisulphide in the presence of a 

 protective colloid. 



In the ordinary criminal or civil case which would come under 

 consideration for precipitin tests the spots or stains are made by 

 blood as it flows from the wound and unchanged by chemical or 

 physical agencies except as these are encountered afterward, by 

 exposure. In the case of meat inspection, in which the precipitin 

 test is useful in detecting admixtures of horse flesh, dog flesh, or 

 other less desirable varieties of meat, in sausages, chopped meat, 

 etc., it often happens that such procedures as heating or smoking 

 may vitiate the results of precipitin reactions. It is of practical im- 

 portance, therefore, that we should know exactly what the effects of 

 heating (boiling) may be upon precipitinogen. Moreover, this ques- 

 tion possesses considerable theoretical interest since the coagulation 

 of proteins by heat seems to involve chiefly a physical rather than a 

 chemical change. 



Cohnheim 33 says in discussing this question : "It is still unclear 

 what the changes are that take place in coagulation. It may be that 

 there is merely an intramolecular 'Umlagerung' or there may bo 

 cleavage ; or the process may be comparable to the flocculation of col- 

 loidal clay emulsions by salts. . . . With coagulation all proteins 

 have lost the differences which they possess in the native state in 

 respect to solubility or precipitahility by salts. Physically all coagu- 

 lated proteins are alike ; they are no longer native proteins, and with- 

 out further decomposition are insoluble. Chemical differences, how- 

 ever, variations of composition, and the cleavage products which 

 they yield still distinguish them." 



The question has been experimentally approached by Obermeyer 

 and Pick 34 in connection with their general investigations upon the 

 influence of chemical and physical alterations upon precipitinogen. 

 They found that precipitin produced with unchanged (native) beef 

 serum does not react with heated beef serum, even if immunization 

 was prolonged and a very potent serum was produced. On the other 

 hand, when animals were immunized with beef serum which had 

 been boiled for a short time ("Kurz aufgekocht" 35 ) the precipitin 

 so produced reacted, not only with native beef serum, but also pre- 

 cipitated the boiled serum and a whole row of split products which 

 give no reaction to normal precipitin. The "coctoprecipitin" so 

 produced, furthermore, was found by them to be specific, acting 

 only upon beef protein or its derivatives. 



33 Otto Cohnheim. "Chemie der Eiweiss Korper Vieweg Braunschweig," 

 3900, p. 8. 



34 Obermeyer and Pick. Wien. kl Wocli., 32, 1906. 



35 Sera or other proteins used in such tests are boiled in dilutions of 1 

 to 10 or more, in order to avoid the formation of heavy flakes which cannot 

 be injected. Boiled in sufficient dilution, an opalescent suspension is formed 

 which easily passes through the syringe. 



