148 BACTERIOLOGY. 
How are plate cultures made? 
A test tube of nutrient gelatin or agar agar containing the organ- 
isms to be studied is melted. The contents of three sterile tubes are 
also melted and numbered 1, 2,3. The dese is first thrust into the 
original tube and then into No. 1; after mixing thoroughly with the 
dese transfer a drop of the mixture No.1 to No. 2, mix thoroughly and 
transfer a drop from No. 2to No.3. Each tube of material is now 
poured upon a separate glass plate previously sterilized and placed upon 
the leveller. When the nutrient mass has become solid, the plates are 
placed in a deep glass dish and rest upon glass bridges to keep them 
separate. They are then covered and left to develop. 
How are pure cultures prepared ? 
When the inoculated plates 1, 2 and 3 as above described have 
developed, they may be studied with a lens of medium power (one 
having a focal distance of about one-third of an inch). Colonies of 
different kinds and appearances will beseeu. The colony desired may 
be fished out with the dese and transferred to a sterilized tube, and the 
whole process repeated until it is seen that only one kind of colony 
develops upon the culture plates. 
What are Petri culture dishes ? 
They are small shallow glass dishes having a glass cover. They are 
about four inches in diameter and are extensively used instead of the 
plate method just described, as they do away with the leveller and 
tripod, the glass plates and bridges and the culture dish. 
How is the medium prepared in which to cultivate anzrobic 
bacteria. 
A plate culture may be made and covered with a sheet of sterilized 
isinglass. Still further to prevent the entrance of oxygen the edges 
may be sealed with paraffin. Another method is to introduce into 
the culture medium a three per cent. to five per cent. solution of 
formic acid. Still another method is to place the culture tube stopped 
with the cotton wool into a large tube, in the bottom of which is the 
following mixture: pyrogallic acid, 1 gram and 10 cc. of solution of 
caustic potash (5.5 grams of potash to 1 litre of water). The large 
tube is covered with arubber cap. The oxygen present is reduced by 
this mixture and air excluded by the rubber cap. Very good results 
have been secured by this method. 
What other apparatus is needed for the successful cultivation of 
bacteria ? 
An incubating oven which is a double-walled metal chamber, having 
