100 BACTERIA IN RELATION TO PLANT DISEASES. 
THE CLEANING AND STERILIZATION OF GLASSWARE AND INSTRUMENTS. 
New glassware may be boiled in soap-suds, rinsed thoroughly, soaked in the 
chromic-acid cleaning mixture for some hours, rinsed in hydrant water, soaked in 
several changes of distilled water, soaked or shaken in alcohol, and finally rinsed 
in distilled water. Neglect to wash in alcohol will frequently leave behind on the 
walls of the test-tubes an invisible film which causes vexatious precipitates in beef- 
bouillon, etc. Discarded tubes, flasks, and dishes containing living organisms must 
be autoclaved or filled with the chromic-acid cleaning mixture before they are washed. 
Some responsible person should attend to this. If acid is used'it should be allowed 
to act for some hours. 
Petri dishes should fit together well, but not tightly, and should be double- 
wrapped in clean Manila paper before placing them in the hot-air oven, or else 
should be inclosed in suitable tin boxes. ‘The writer prefers to wrap them. ‘The 
paper for this purpose may be 12 by 12 inches. ‘The dish should be placed in the 
middle. ‘The sides of the paper 
are folded over it; the corners 
of the projecting ends are then 
turned in, leaving V-shaped 
flaps, which are folded down on 
to the plate. ‘The second cover- 
ing is folded at right angles to 
the first and on the other side 
of the dish. Dishes treated in 
this way and ready for steril- 
ization are shown in fig. 85. 
Pipettes should be dry-heated 
in the tin boxes already men- 
tioned (fig. 37) after having the 
upper end carefully plugged 
with cotton, which should not 
project. Knives, scalpels, scrapers, spatulas, needles, forceps, etc., may be sterilized 
in the Bunsen flame, or, if needed cold in quantity, may be wrapped in Manila paper 
or put uncovered into short tin boxes and heated in the dry oven at 140° C. for two 
hours. Petri dishes, test-tubes, and all other apparatus wrapped in paper and put 
into the oven for sterilization by dry heat should have air spaces between them, z. e., 
they should not be crowded together tightly, and the recording thermometer should 
project well down into their midst. The investigator should test the behavior of his 
oven when full and empty. Many cheap ovens give very different temperatures 
in different parts, especially if filled with apparatus, so that cotton or paper may be 
scorched in one partand not sterilized in another. ‘The best oven known to the 
writer is that made by Lautenschlager. The improved form of the Lautenschlager 
oven shown in plate 6 does not require watching and gives a uniform temperature 
Fig. 85.* 
*Fic, 85.—Petri dishes wrapped in two layers of Manila paper and ready to be dry sterilized. 
They are set on edge in the oven. 
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