376 The Microscope. 



In his experiments, Dr. Sternberg adopted ten minutes as the 

 standard time of exposure to a given degree of temperature. A 

 fresh culture of the organisms to be tested is introduced into capil- 

 lary glass tubes, which have an expanded extremity to serve as an 

 air-chamber, by means of which the culture fluid is drawn into or 

 forced out of the capillary tube. 



This is readily accomplished by heating the little bulb. The 

 glass tubes, hermetically sealed, are introduced into a vessel contain- 

 ing water, which is kept at a uniform temperature by personal super- 

 vision, a Bunsen burner being the source of heat. A standard 

 thermometer is placed in the vessel, and this and the capillary tubes 

 are protected from the bottom of the vessel containing them by a 

 thick plate of glass. 



A uniform temperature throughout the fluid is maintained by 

 stirring it with a glass rod. After exposure for ten minutes to a 

 given temperature, the sealed extremity of the capillary tube is 

 broken off with sterilized forceps, and the contents are forced, by 

 heating the air in the expanded extremity, into a test-tube containing 

 sterile flesh-peptone-gelatine, which has been liquefied by exposure 

 in a water bath to a temperature of 40° C. or below. The cotton 

 plug is only removed for a moment in order to introduce the contents 

 of the capillary tube, and in his extended experiments he has rarely 

 seen any accidental contamination. 



A rubber cap is next placed upon the open end of the test-tube 

 and the gelatin is spread in a uniform manner over the interior of 

 the tube by the method of Esmarch. This is accomplished by roll- 

 ing the tul^e in iced-water until the gelatin hardens. 



These tubes are then kept at a temperature a little below the 

 melting point of gelatin — 20" to 22^ C. — for at least a week. 



If the test-organism has not been killed by the temperature to 

 which it was exposed, colonies are developed in the gelatin which 

 may often be recognized by the naked eye within a day or two. 



In other cases development is retarded, and it is only at the 

 end of four or five days that evidence of growth is seen. 



The absence of growth at the end of eight or ten days is taken 

 as evidence that the vitality of the test-organism has been destroyed 

 by the temperature to which it was exposed. 



In every case a control experiment is made with material from 

 the same culture which has not been subjected to heat. 



