CULTIVATION OF BACTERIA 55 



sticking. A tumbler or a simple cap of paper over the neck answers 

 much the same purpose. 



Preparation and Filling of Tubes. The cheaper grades of test-tubes 

 should be avoided. They are thin and break easily, and also frequently 

 frost on heating from the separation of silicic acid. The tubes of the 

 better class can be used after rinsing with hot water; cheap tubes are 

 very alkaline and must first be soaked in dilute hydrochloric acid. 



CLEANSING \XD STERILIZATION OF APPARATUS. In order to study 

 bacteria, both in culture media and in the living body, we must, as 

 already stated, separate those developed from one organism from all 

 others and study them by themselves in pure cultures. In order to do 

 this we have to take the greatest precautions to ensure that the materials 

 that we make use of for the growth of bacteria, the flasks and tubes that 

 hold these materials, and the instruments with which we transfer the 

 bacteria are sterile. In bacteriological work sterilization is practically 

 always done by means of dry and moist heat, for no antiseptic sub- 

 stances can be allowed to remain in any of the media used for the 

 growth of bacteria or on any of the apparatus which would come in 

 contact with them, as such substances w r ould inhibit the growth of the 

 bacteria which we desired to study. 



The platinum wires and loops used in transferring bacteria are ster- 

 ilized by holding them for a moment until red-hot in a gas or alcohol 

 flame. They should not be used until time enough has elapsed for them 

 to cool sufficiently not to injure the bacteria touched by them. Knives, 

 instruments, etc., are, after thorough cleansing, placed in boiling 1 per 

 cent, soda solution for three to five minutes. Hypodermic needles are 

 sterilized by boiling in soda solution, or, when this is impossible, they 

 are first frequently rinsed with boiling or with very hot water, and then 

 filled with a 5 per cent, carbolic acid solu- 

 tion for at least thirty minutes and then 

 rinsed again with sterile water. New tubes 

 and flasks sometimes require to be washed 

 in a 2 per cent, solution of nitric acid, so 

 as to remove any free alkali which may be 

 present. They are finally thoroughly rinsed 

 in pure water. Old tubes, flasks, and other 

 glassware are boiled for about thirty minutes 

 in a 5 per cent, solution of washing soda, 

 and then thoroughly rinsed off with water 

 until perfectly clean. If necessary, any dirt 

 clinging to the insides of the flasks and tubes "^Dry-heat sterilizer. 

 can be removed by bristle brushes or suit- 

 able swabs. After the tubes and flasks have been thoroughly cleaned 

 they are plugged loosely with ordinary cotton-batting, or, if that is not 

 at hand, the more expensive absorbent cotton. The tubes and flasks 

 with their cotton plugs and all other glassware are sterilized by dry 

 heat at 150 C. for one hour in the dry-heat sterilizer (Fig. 25). 



The sterile tubes and flasks are filled with the media, when small 



