IOO 



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 

 iu 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 iu, leaving V - shaped 

 flaps, which are folded down on 

 to the plate. The second cover- 



1^^ ^ ing is folded at right angles to 



\^ "S^/ ^^ 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, /. c., 

 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 part and not sterilized in another. The best oven known to the 

 writer is that made by Lautenschlager. The improved form of the I Y autenschlager 

 oven shown in plate 6 does not require watching and gives a uniform temperature 



*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. 



Fig. 85* 



