APPENDIX A 117 



in other cases it has not proved certain. Hydrogen peroxide is a 

 weak germicide but is useful in some cases. 



The volatile group of antiseptics consists of a few organic com- 

 pounds, which are more or less toxic to microorganisms. 



Formaldehyde, either as a gas or in a strong solution (known as 

 formalin), is an efficient and economical germicide. In a 4 per cent 

 solution it quickly kills all organisms with which it is in contact. It 

 is quite irritating to the skin, however, and must be employed with 

 care. The gas is readUy liberated from the solution by the use either 

 of heat or of potassium permanganate. 



Chloroform, acetone, and carbon bisulphide have more or less exten- 

 sive use in sterilizing liquid media, where it is not practical to use heat, 

 but it will not do to assume too quickly that chloroform or carbon 

 bisulphide has sterilized a solution ; the solution should be incubated 

 and observed after several days, to make sure that no growth occurs. 



Alcohol and ether have somewhat limited use as sterilizing agents. 

 Alcohol of 60 or 70 per cent strength may be used for quick treat- 

 ment of glassware when time or location forbids the use of other 

 agents. For sterilizing pipettes or tubing used for taking samples in 

 the field a brief washing with alcohol just previous to the time of 

 using serves very well. After draining off the alcohol, the tubes 

 should be rinsed with the solution to be sampled. 



STERILIZATION WITH HEAT 



Heat is the sterilizing agent most useful for general bacteriological 

 work. If properly applied, it produces no detrimental conditions in 

 the culture media, and at the same time it is efficient in killing 

 microorganisms . 



Dry or moist heat may be used, according to the nature of the 

 object to be sterilized and the pui'pose for which it is to be used. 



Hot-air sterilization. Dry sterilization is accomplished by placing 

 the objects in a hot-air sterilizer, which is a gas-heated oven con- 

 structed of sheet iron or other metal. The ovens constructed for this 

 purpose have double walls so arranged that the hot air circulates 

 through the chamber. The outer walls are covered with asbestos and 

 provided with an opening for the insertion of a thermometer. The 

 Lautenschlager oven is one of the best for dry sterilization. If a 

 simple, cheap oven is to be used, careful tests should be made, to 

 ascertain the relative temperature in its various parts. 



