160 



to be considerably greater than Onorato did who found that 

 Pfei iter's bacillus is killed in a couple of hours at —15° 

 to —20°. 



Different strains were suspended in broth in test tubes which 

 were placed for 24 hours in the freezing mixture. The tempera- 

 ture for the first 12 hours was —18" to — 20 p , and Ihen rose 

 during the remainder of the experiment to —12°. Inoculations made 

 from them all gave growth. I thought it unprofitable to continue 

 these experiments as the process of freezing introduced the combined 

 effect of a low temperature and drying. In a greatly cooled, frozen, 

 suspension in salt solution or broth the bacteria will in fact, be 

 surrounded by a very concentrated solution of salts and other sub- 

 stances.. In addition, freezing does not set in at the same moment 

 in a series of lubes which are exposed to exactly the same cooling 

 effect, and indeed, as is well known, it may not take place at 

 all for a long time, , which I have also frequently observed. 



I have made the following experiment with reference to 

 the resistance of different strains of Pfeiffer's bacillus against 

 drying. 



In August 1918 strains I 1—17 were tested by rubbing a loopful 

 of each culture on a small area of the bottom of a Petri plate, 

 which was then" put in the incubator at 37°. After 2 hours had 

 passed each of the dried cultures was rubbed up with a little 

 broth and inoculated on haemoglobin agar plates. There was growth 

 in each case. The experiment was repeated, allowing 3, 4, 5%, 

 and 8 hours in the incubatjor. Marked individual differences were 

 found in the resistance of the cultures against drying. 2 grew 

 in all 5 tests, while 4 did not grow in any of them. In the other 

 cases the results were quite irregular as growth was just as likely 

 to occur after prolonged drying and no growth after- a short drying, 

 as the reverse. The attempt to use this test as a means of di- 

 stinguishing the individual strains was therefore abandoned. 



On testing the strains H 13—32 (Sept. 1918), the majority gave 

 growth after 8 hours' drying. 



In March 1920 the strains mentioned on p. 108 were tested. 

 After 5 hours' drying (dew-point of the air, 11°), almost all the 

 strains grew but only a few (I 1 (a), I 29, I 54, H 122, II 127 

 and Pa 5) grew after 14 hours' drying. 



In November 1920 all the strains on p. 110 were examined. 

 The Petri plates containing the dried cultures were kept 24 hours 

 in the incubator this time, the plates being placed upside down on 

 a perforated iron shelf, so that the air had free access to them. 

 The dew-point of the air was 13°, the percentage humidity there- 

 fore being about 25. None of the strains gave growth on cultivation 



