284 



INFECTION AND IMMUNITY 



typhoid bacilli, which is permanent and may be kept indefinitely, 

 and may be employed for macroscopic agglutinations. 16 



Attention has been called by various workers to a source of 

 error in all these methods, known as pseudo-clumping. 17 The causes 

 for such clumping not due to agglutinins seem to lie in the presence 

 of blood cells in the serum or excessive acidity of the culture 



FIG. 35. MACROSCOPIC AGGLUTINATION. Dilutions from 1 to 10 in to 1 to 1,000. 

 The first tube contains a 1 : 20 control with the bacteria and normal serum 

 Agglutination complete in the tubes marked 10, 20, 50, 100, r 



medium. 18 In fact, agglutination of bacteria by acids in definite 

 concentration can be Carried out and seems to depend directly upon 

 the hydrogen ion concentration. 



While the microscopic methods are more suitable for clinical- 



16 Exact method of -production of ' ' Ficker 's Diagnosticum " is a proprietary 

 secret. 



37 Savage, Jour, of Path, and Bact., 1901. 



18 Biggs and Park, Amer. Jour, of Med. Sci., 1897; Block, Brit. Med. Jour., 

 1897. 



