98 



TE3IPERATURE EXPP:RIMENTS. 



Thermal Death Point. 



Some difficulty was experienced in determining accurately the ther- 

 mal death point of Ps. hyacinthi owing to the slight variability in 

 sensitiveness of individual rods. Considerable trouble was also expe- 

 rienced for some time owing to the frequent unaccountable failure of 

 the germs to grow in some of the fluid cultures (see Sensitiveness to 

 acids). 



Most of these experiments were made in thin-walled test tubes 

 16 to 17 mm. in diameter, and containing exactly 10 c. c. of fluid 

 (usually beef broth) entireh' free from any trace of sediment or 

 cloudiness. These tubes were inoculated in each case with big loops 

 from fluid cultures only a few days old (1 to 11). and great care was 

 taken in making the inoculations not to wet the walls of the tube above 

 the fluid, and also to keep the tubes upright from first to last. The 

 exposures were made by plunging the inoculated tubes into a hot- 

 water bath nearly to their top. and keeping them in it at the given 

 temperature for exactly 10 minutes. They were then removed, and 

 either cooled quickly under running water or left to slowly acquire 

 the temperature of the room. Duplicate tubes were always inoccu- 

 lated and maintained at the living-room temperatures for comparison. 

 On two occasions poured plates were also made, using a large quantity 

 of the culture fluid so as to determine more precisely the proportion 

 of the germs killed by the heating. 



The hot-water bath employed was the Ostwald-Pfeffer. using a very 

 sensitive Roux metal-bar thermo-regulator. and a stream of com- 

 pressed air for the motive power. The thermometer employed was a 

 very sensitive one, belonging to a set made by ^lax Kaehler and Mar- 

 tini, of Berlin, and compared with the standard hydrogen thermome- 

 ter of the International Bureau of Weights and Measures, Washington, 

 D. C. With this apparatus, which keeps the water uniformly in 

 motion, it was easy to maintain approximately constant temperatures 

 for short periods. 



The following is a detailed account of these experiments: 



I. December 3: Oue tube of stock 204 inoculated with a large loop from tube 6 

 December 1. This tube was allowed to stand 1 hour and then plimged for 10 min- 

 utes into water at 54.30° C. Cooled at room temperature. Result: Under observa- 

 tion several weeks, but no growth. 



II. December 3: One tube of stock 204 inoculated with a large loop from tube 6 

 December 1. This tube was allowed to remain 3 hours at room temperatures and 

 then plunged for 10 minutes into water at 49.80° C. Cooled at room temperatures. 

 Eesult: Xo growth. Tube mider observation several weeks. 



III. Decembers, 1896: One tube of stock 204 (1:2 acid beef broth, i. e., no pep- 

 tone or alkali added) inoculated with a large loop from tube 6 December 1, which 



