326 STUDIES IN IMMUNITY. 



between opposing influences. And if this is true we should be able 

 to modify the outcome of the struggle by enfeebling one or the 

 other of the influences present ; and this we have done in the follow- 

 ing manner: It is well known that heating serum to 60 to 65 degrees 

 brings it through a series of transitory stages to a dense coagulum. 

 This coagulum may be avoided by a preliminary dilution of the 

 serum, under which conditions the serum is still coagulable more 

 or less completely, according to the degree of dilution, but never 

 in a single mass. For example, if we leave horse serum, that has 

 been diluted with three times its volume of salt solution, for a quarter 

 of an hour in boiling water, we obtain a whitish fluid containing 

 albuminoids coagulated to a greater or less extent, but not in 

 a single mass; such a fluid is a true colloidal solution, but in 

 comparison with unheated serum it represents a coagulated solu- 

 tion, the particles of which have a greater tendency to clump and 

 sediment. 



We now prepare two tubes which contain the same quantity of 

 barium sulphate in the same volume of salt solution and add to 

 one a given dose of non-heated serum diluted 1-4 in salt solution, 

 and to the other the same amount of the same diluted serum that 

 has been heated for 15 minutes in boiling water. The barium 

 sulphate becomes dissociated in the first tube, but in the second 

 forms clumps that are quite different from the spontaneous sedimen- 

 tation in salt solution. After this agglutination is finished and 

 the clumps have fallen to the bottom of the tube, the supernatant 

 fluid has lost much of its milky appearance and the subsequent 

 addition of a fresh amount of barium sulphate leads to only a slightly 

 increased clarification in the fluid, the agglutinating property of 

 which has become very much diminished. The agglutination of 

 barium sulphate by heated serum is due, then, to a combination 

 between the suspension and the colloidal substances which give 

 the serum its milky appearance. Since this is true the aggluti- 

 nating property may be removed from heated serum by adding 

 sufficiently large amounts of barium sulphate. We collect at the 

 bottom of the tube by centrifugalization the sulphate contained 

 in 4 c.c. of our emulsion and remove the supernatant fluid and then 

 suspend the sediment in a mixture containing 0.6 of a cubic centi- 

 meter of horse serum plus 1.8 c.c. of salt solution (the mixture 



