76 CULTURES IN SOLID MEDIA. 
without the exposure which occurs when a plate prepared by Koch’s 
method is under examination. 
In agar-agar cultures or in gelatin cultures of non-liquefying 
bacteria made in Petri’s dishes, we may examine and count colonies, 
without removing the cover, by inverting the dish. 
In pouring the liquefied gelatin from the test tubes in which the 
dilution has been made into sterilized Petri’s dishes, care must be 
taken to first sterilize the lip of the test tube by passing it through 
the flame of alamp. We may at the same time burn off the top of 
the cotton plug, then remove the remaining portion with forceps, 
when the lip has cooled, for the purpose of pouring the liquid into the 
shallow dish. 
Von Esmarch’s Roll Tubes.—Another very useful modification 
of Koch’s plate method is that of von Esmarch. Instead of pouring 
the liquefied gelatin or agar medium-upon plates or in shallow 
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dishes, it is distributed in a thin layer upon the walls of the test tube 
containing it. This is done by rotating the tube upon a block of ice 
or in iced water. Esmarch first used a tray containing iced water, 
and to prevent the wetting of the cotton filter a cap of thin rubber 
was placed over the end of the tube. It is more convenient to turn 
the tubes upon a block of ice having a horizontal flat surface, in 
which a shallow groove is first made by means of a test tube con- 
taining hot water (Fig. 45). Or, in the winter, we may turn the 
tube under a stream of cold water from the city supply—.e., from a 
faucet in the laboratory. <A little practice will enable the student to 
distribute the culture medium in a uniform layer on the walls of the 
test tube, and as soon as it is quite solidified these may be placed 
aside for the development of colonies from the bacteria which had 
been introduced. When roll tubes are made from the agar jelly it is 
best to place the tubes in a nearly horizontal position, for if placed 
upright at once the film of jelly is likely to slip from the walls of the 
