306 METHODS OF IDENTIFICATION AND STUDY 



already been roughly determined whilst observing the 

 range. It can be fixed within narrower limits by 

 inoculating in a similar manner a series of tubes of 

 bouillon which represent smaller variations in reaction 

 than those previously employed (say, i instead of 5) 

 for five points on either side of the previously observed 

 optimum. For example, the optimum reaction ob- 

 served in the set of experiments to determine the 

 range was + 10. Now plant tubes having reactions 



of +15, +14, +13, + I2 > + IJ > + I0 +9> + 8 > +7 

 + 6, +5, and observe as before. 



IV. Resistance to Lethal Agents. 



(A) Desiccation. 



Apparatus Required: 



Mueller's desiccator. This consists of a bell glass fitted with an 

 exhaust tube and stop-cock (d), which can be secured to a plate- 

 glass base (c) by means of wax or grease. It contains a cylindrical 

 vessel of porous clay (a) into the top of which pure sulphuric acid 

 is poured whilst the material to be dried is placed within its walls 

 on a glass shelf (6) . The air is exhausted from the interior and 

 the acid rapidly converts the clay vessel into a large absorbing 

 surface (Fig. 157). 



Exhaust pump. 



Pure concentrated sulphuric acid. 



Sterile cover-slips. 



Sterile forceps. 



Culture flask containing 200 c.c. nutrient bouillon. 



Sterile ventilated Petri dish. This is prepared by bending three 

 short pieces of aluminium wire into V shape and hanging these on 

 the edge of the lower dish and resting the lid upon them (Fig. 1 58) . 



METHOD. 



1. Prepare a sunace cultivation on nutrient agar in 

 a culture bottle and incubate under optimum condi- 

 tions for forty-eight hours. 



2 . Examine preparations from the cultivation, micro- 

 scopically, to determine the absence of spores. 



3. Pipette 5 c.c. sterile normal saline solution into 

 the flask and suspend the entire growth in it. 



