• TECHNIC BACTERIOLOGICAL EXAMINATION OF SOILS. 277 



burning gas to escape. It is desirable to have the holes cut in the oven so 

 that one is not exactly above the other. By using the detachable baking 

 ovens apparatus can be sterilized some days previous to the time the samples 

 are taken and an oven with its contents can be set to one side while another 

 ovenful of glassware is sterilized. This simplifies the handling of petri plates 

 and dilution bottles as the ovens can be carried directly to the work table 

 without handling the apparatus and risking chance contamination. This 

 is very important where a large number of petri plates are used at one time. 



Autoclave. 



The American Sterilizer Company's autoclave has proven in spite of its 

 high initial cost to be very efficient and economical for within one half hour 

 from the time the gas is lit not only eighteen pounds pressure of steam can 

 be generated but one load of material can be given a ten minute sterihzation 

 under eighteen pounds pressure of live steam. The automatic valve which 

 regulates the pressure of the steam in the boiler by shutting off or turning 

 on the gas is a feature appreciated by the investigator. 



Water Bath 



A water bath, is used to cool the agar media to 45°C after it has been 

 melted in the autoclave. The media is then kept in the bath at 40°-42°C 

 until it is poured into the petri dishes. 



Absorbent Cotton 



By buying in quantity absorbent cotton is as cheap as the bat cotton at 

 retail prices. It is preferable because the fibre is cleaner and better plugs 

 can be made from it. 



Mallets. 



The mallet for driving the bacteriologist's soil sampler should be of wood 

 and not weigh (5ver three pounds. The light mallet for use in the laboratory 

 weighs about a half a pound and is made from very light wood. 



Petri Dishes 



Petri dishes 10 centimeters in diameter and 15 millimeters high have 

 been most satisfactory. The top of the dish should not have a rim deep 

 enough to come down to the table when the dish is placed right side up, on 

 a table. 



THE TECHNIC 



The technic is given as directions for handling ten samples of soil at one 

 time. In addition to the ordinary apparatus such as burners, autoclave, 

 ovens and so forth being ready for use, the following apparatus, must be ready 



