THERMAL DEATH-POINT. 515 



can be readily made by drawing- out a test-tube. A layer of absorbent 

 cotton should be placed on the bottom of the tube and then some 

 glass-wool which acts as a weight. The filter thus prepared is plugged 

 at both ends and sterilized. The filtered suspension should be exam- 

 ined under the microscope, and, if aggregations of bacteria are still 

 present, it should be again filtered. The filtrate may be received in a 

 large Esmarch dish, or in a sterile beaker which is covered with 

 cotton or paper. 



The suspension is drawn up into a sterile capillary 

 pipette which is then sealed below and above the liquid 

 (Fig. 62 c x, p. 459). The liquid is now contained in a capil- 

 lary tube about 10 cm. long and 1-2 mm. in diameter. A 

 sufficient number of these sealed .capillary tubes should be 

 prepared (see p. 516) to meet the requirements of the ex- 

 periment in view. They should be immersed in a test-tube 

 containing mercuric chloride in order to disinfect the 

 exterior. These tubes are used to test the action of moist 

 heat. The same suspension is used for the preparation of 

 silk threads, muslin squares and cover-glass specimens 

 {p. 517). 



MOIST HEAT. 



The ordinary Hofmann iron water-bath will answer 

 very well for these tests. It is filled with water and provided 

 with a thermometer and a thermo-regulator which should 

 be suspended so as not to touch the bottom or the sides 

 (Pig. 75). The temperature of the water can thus be kept 

 at any desired point. The water-bath of Roux, shown in 

 Mg. 64, is very useful for these experiments. The capillary 

 tubes should be placed on cross wires which are stretched 

 over a syringe holder (Fig. 41 c, p. 263), and immersed in the 

 water. At the end of stated intervals, the holder is raised 

 and a capillary tube is removed. The latter should be 

 placed at once in a tube of cold water, to prevent the fur- 

 ther action of heat. Each of the remaining capillary tubes, 

 in like manner, is removed and placed in water at the end 

 of the respective period of exposure. 



