CULTURES, AND THEIR STUDY. 141

by turning the dish in all directions. The solution is
emptied out, and the dish, which is always kept closed,
is ready for use.

A levelling apparatus is required (Fig. 19). This con-
sists of a wooden tripod with adjustable screws, and a glass
dish covered by a flat plate of glass upon which a low
bell-jar stands. The glass dish is filled with broken ice
and water, covered with the glass plate, and then exactly
levelled by adjusting the screws under the legs of the
tripod. When level the cover is placed upon it, and it
is ready for use.

Method (Fig. 24).—A sterile platinum loop is dipped
into the material to be examined, a small quantity se-

FiG. 24.—Method of holding tubes during inoculation.

cured, and stirred about so as to distribute it evenly
through a tube of the melted gelatin. If the material
under examination is very rich in bacteria, one loopful
may contain a million individuals, which, if spread out
in a thin layer, would develop so many colonies that it
would be impossible to see any one clearly; hence the
necessity for a dilution. From the first tube a loopful
of gelatin is carried to a second tube of melted gelatin
and stirred well, so as to distribute the organisms evenly
through it. In this tube we may have no more than ten
thousand organisms, and if the same method of dilution
be used again, the third tube may have only a few hun-
dreds, and a fourth only a few dozen colonies.

After the tubes are prepared, one of the sterile glass
plates is caught by its edges, removed from the iron box,
and placed beneath the bell-glass upon the cold plate