LABORATORY STERILIZATION 273 
made by de Khotinsky is particularly to be recommended for this 
purpose.^ 
(b) Dry //eo^.— Test-tubes, flasks, Petri dishes, pipettes and other 
laboratory glassware are sterilized in the hot-air sterilizer— an oven 
heated with a gas flame. An exposure of one and a half hours at 
160° C. or one hour at 180° C. will eft'ectually kill all spores. Cotton 
browns slightly at this latter temperature, and may be used as a temper- 
ature indicator. The heat should be applied gradually and reduced 
gradually to diminish the danger of cracking. Dry heat has but 
little power of penetration. Glassware is conveniently wrapped in 
paper before sterilization to protect it from dust prior to its use. 
The cotton plugs of flasks and beakers are also covered with paper 
before sterilization, for the same reason. 
(c) Moist Heat. — I. The most satisfactory agent for the steriliza- 
tion of articles uninjured by moisture is steam under pressure. Many 
kinds of media and laboratory apparatus, and fomites as w^ell are 
quickly and completely sterilized by steam. The autoclave is com- 
monly used for laboratory purposes. It consists essentially of a 
double-walled chamber with close-fitting cover, into which steam may 
be introduced. There are many patterns, but the essential features 
are— the steam should enter the chamber from the top, and the bottom 
of the chamber should be provided with a stop-cock, through which the 
residual air and condensation water can escape. 
Operation.— A single layer of apparatus should be sterilized at one 
time. If several layers are introduced, condensation water from the 
upper layer may collect on the lower layers, permitting of subsequent 
contamination. Test-tubes containing media should not be closely 
packed in wire baskets; space between the tubes must be left to 
permit of a circulation of the steam. Large amounts of media in 
flasks— a liter or more — require considerable time for their contents 
to be raised to the desired temperature. A correspondingly longer 
exposure to steam should be allowed to insure sterilization. Steam is 
admitted to the chamber to displace the air, and the air-cock should 
remain open until live steam flows freely from it, because hot air is 
far less efficient than steam for sterilization. Also, the condensed 
steam escapes through the same orifice. When all the air is replaced 
by dry steam the pressure is gradually increased initil 15 pounds are 
recorded on the pressure gauge. This pressure is maintained from 
ten to twenty minutes, depending upon the nature of the material to 
be sterilized. In general, media in test-tubes, not too tightly packed, 
is more quickly sterilized than media in flasks. At the end of the 
allotted time, the pressure is f/radualh/ reduced until ecjuilibrium 
' It consists essentially of a tube about 12 cm. in length and 1 cm. in diameter, of 
fire clay surrounded by a resistance coil of sufficiently fine wire and numerous layers 
to heat the interior of the tube to a white heat. The charged platinum wire is placed 
in the tube, and within a few seconds it becomes white-hot. There is absolutely no 
danger from "spattering," because the extruded organisms fall upon the hot walls of 
the tube (see Fig. 22 page 235). 
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