Ill 



350, 600, 1000, 1600, 2500, 4000, 7000, 10000, and 15000. Al these 

 plates were inoculated with 6 strains of Pfeiffer's bacillus. On the 

 first 7 plates there was a rich growth even after 15 hours' in- 

 cubation, perhaps a little more marked on the first one or two 

 than the last. On Nos. 8—13 (1 : 350—4000) the amount of growth 

 gradually decreased. On the last three plates the growth was slight 

 after 40 hours incubation, but as it was the same on all three plates, 

 it was probably due to the haemoglobin carried over when the 

 inoculations were made. 



On 23. II. 1920 agar plates were prepared (with a moderate 

 alkaline reaction to azolitmin paper) with the following concentra- 

 tions of dissolved blood corpuscles (1) 1 : 100, (2) 1 : 300, (3) 1 : 1000, 

 (4) 1 : 3000, (5) 1 : 10000, (6) 0. All the strains mentioned on p' 

 108 were inoculated on these media from a medium identical with 

 No. 1. 



On (1) the usual good growth occurred in all cases. On (2) 

 the growth was rather less; on (3) the growth was rather poor in 

 each case, but a little better than on ordinary blood plates; on 

 (4) the growth was about the same as on ordinary blood plates 

 although considerably better in the neighbourhood of chance co- 

 lonies of other bacteria; (5) was still able to support a slight growth 

 of all the strains except H 161. The growth was not entirely produced 

 by the haemoglobin carried over at the inoculation because a con- 

 siderably greater number of strains showed practically no growth 

 on the haemoglobin-free plates (6). • 



The lowest limit of (approximately) optimum growth is therefore 

 at about lo/ dissolved blood corpuscles, and the lowest limit of 

 growth is round about 1 /i 00 °/o. This agrees quite well with the 

 experiment of 1918, in which 1/2% an d V50 / were found re- 

 spectively. It would be quite wrong however to assume these values 

 are generally applicable, since the amount of haemoglobin necessary 

 depends largely on the constitution of the (peptone-broth) agar, on 

 the nature of the haemoglobin, and on the presence of oilier kinds 

 of bacteria or other growth-promoting subslances having a similar 

 effect. The importance of the first of these three poinls is seen 

 from an experiment (1. II. 19) in which autoclaved agar was used 

 while in all other cases the agar was sterilised at 100°. This altera- 

 tion was the cause of Pfeiffer's bacillus, even on a 1 — 2"/o content 

 of dissolved blood corpuscles, only growing poorly or entirely fai- 

 ling, but it grew luxuriantly when the concentration was increased 

 to about 10<Vo. The peculiar result that the haemoglobin agar which 

 up to March 1920 had constantly been used with excellent results, 

 then began to fail, will be further discussed in the chapter on 

 nutritive media. » 



We will now revert to the experiment of 13. II. 1920. From 

 media Nos. 3, 4, 5 & 6 subcultures were made on plates with 

 the same haemoglobin concentrations. The results are tabulated below 

 where „1 : 1000" etc. denotes the concentration of dissolved blood 



