229 



satisfactory for the preparation of Fildes agar. Since June 

 1920 I have made this in the following manner which only 

 differs from Fildes' directions in unessential details. 



To 150c.c. of 0.9o/o salt solution are added 6c.c. pure, con- 

 centrated hydrochloric acid and then 50c.c. defibrinated horse 

 or sheep blood and 2gm. pepsinum concentratum 1 Langebek 

 (Fildes uses lgm. „pepsin B. P."). The mixture which need 

 not be prepared from sterile materials or in sterile measuring 

 cylinders, is now poured into a sterile flask and is kept at 56° 

 for 5 — (1 hours (either in an air thermostat or in an oil bath). 

 NaOH is now added until a sample diluted with distilled 

 water to a light brown colour, turns red on the addition of phe- 

 nol red (phenolsulphonephthalein) without giving the alkaline 

 colour change with o-naphtholphthalein (see p. 226). Next, 

 1/4 0/0 chloroform is added, the flask is closed with a sterile 

 rubber cork and kept in the cold room. I have not observed 

 any decrease in activity during a couple of months' preser- 

 vation. (According to Fildes it lasts even for over a year at 

 room temperature). 30c.c. of this liquid is added to 600c.c. 

 agar. (It seems not to matter whether it is added while the agar 

 is hot (about 90°) or after it has been cooled to 50°— 40°). After 

 pouring it into flasks a perfectly transparent medium is ob- 

 tained but of a rather darker yellowish-brown colour than 

 ordinary agar. With less than 5 0/0 of the blood solution the 

 growth of Pfeiffer's bacillus was perceptibly weaker. 



The potent factor in the transformation of the haemoglobin 

 to a favourable nutritive medium for Pfeiffer's bacillus seems 

 to be the hydrochloric acid. The pepsin is however important 

 as a clear solution is only obtained in the presence of this 

 substance. 



The pepsin digested blood only gave good results in my 

 hands in the case of solid media but here it proved to be 

 active with extraordinary constancy, although during the period 

 it was in use many different batches of blood solution and agar 

 have been prepared. 



That Fildes agar gives good growth is evident from the 

 fact that the colonies figured in plate 2 appeared on this 

 medium. In my experience however it did not give the most 

 copious growth of Pfeiffer's bacillus it is possible to observe. 

 In cases where it is necessary to produce the greatest possible 



