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 (b). 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. 158). METHOD.-- 1. Prepare a surface cultivation on nutrient agar in a culture bottle and incubate under optimum conditions for forty-eight hours. 2. Examine preparations from the cultivation, microscopically, 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. 4. Spread the suspension in thin, even films on sterile cover-slips and deposit inside sterile "plates" to dry. 5. As soon as dry, transfer the cover-slip films to the ventilated Petri dish by means of sterile forceps. [Illustration: FIG. 157.--Mueller's desiccator.] 6. Place the Petri dish inside the Mueller's desiccator; fill the upper chamber with pure sulphuric acid, cover with the bell jar, and exhaust the air from its interior. Ten minutes later connect up the desiccator to a sulphuric acid wash-bottle interposing an air filter so that only dry sterile air enters. [Illustration: FIG. 158.--Petri dish for drying cultivations.] 7. At intervals of five hours open the apparatus, remove one of the cover-slip films from the Petri dish, and transfer it to the interior of a culture flask, with every precaution against contamination. Reseal the desiccator and again exhaust, and subsequently admit dry sterile air as before. 8. Incubate the culture flask under optimum conditions until the completion of seven days, if necessary; and determine the time exposure at which death occurs. 9. Pour plates from those culture flasks which grow, to determine the absence of contamination. 10. Repeat these observations at hourly intervals for the five hours preceding and succeeding the death time, as determined in the first set of experiments. (B) _Light._-- (a) Diffuse Daylight: 1. Prepare a tube cultivation in nutrient bouillon, and incubate under optimum conditions, for forty-eight hours. [Illustration: FIG. 159.--Plate with star for testing effect of light.] 2. Pour twenty plate cultivations, ten of nutrient gelatine and ten of nutrient agar, each containing 0.1 c.c. of the bouillon culture.