GELATIN AND POTATO MEDIA. 167 



8. — The safety valve should be set to open at about 125°. The 

 noise caused by the escape of steam when this limit is reached will 

 call attention to the apparatus in case it has been overlooked. 



The autoclave can be used, by leaving- the steam-valve open, for 

 fractional sterilization at 100°. 



The student should distinguish between steam under 

 pressure, as employed above in an autoclave, and super- 

 heated steam. A current of steam, passing' through a tube, 

 may be heated by a lamp to 13Q° or higher. Owing to the 

 expansion, since the pressure remains normal, the steam 

 will not have the destructive action of that which has the 

 same temperature, but is under pressure. Indeed, super- 

 heated steam may be said to possess but little advantage 

 over a corresponding degree of dry heat. 



Preparation of Potato Cultures. 



Select three sound potatoes and clean them thoroughly, 

 under the tap, with the aid of a brush. By means of a 

 knife remove any bad spots, or depressions that may exist, 

 since these frequently harbor bacteria which are highly re- 

 sistant to destruction. In so doing, avoid cutting ofE the 

 skin more than is necessary. Place the potatoes, thus pre- 

 pared, in a solution of mercuric chloride (1 to 1,000)' for i an 

 hour; then transfer to a steam sterilizer and steam for f of an 

 hour. The potatoes should be well cooked. Allow the 

 potatoes to remain in the pail till partially cool. 



A "moist chamber" (Fig. 28 b) is prepared by placing 

 a round filter-paper on the bottom of the lower dish, and 

 moistening it with mercuric chloride, the excess of which 

 is allowed to drain off. Three potato knives are then ster- 

 ilized. This is done by heating the blade in the flame till 



' A stock solution of mercuric chloride is first prepared by dissolv- 

 ing- 200 g. of the salt in 100 c.c. of concentrated commercial HCl. 

 5 c c. of this solution added to 1000 c.c. of water will give a 1:1000 

 solution. 



