8 
HOUSEHOLD BACTERIOLOGY. 
enough to see through readily and large enough 
to entirely and tightly cover the dish. After 
washing well, place both together in a pan in a cool 
oven and gradually raise the temperature until it is hot 
enough to bake bread or to yellow a piece of white 
paper in half a minute. Let them bake for an hour 
or more. Then open the oven and place the pan 
where the dish may cool slowly. When cold take 
out, without removing the covering plate, and put 
round both dish and plate a strong rubber band, or tie 
them together with a string. 
This we will call our garden-plot. But a garden is 
of little use without something growing in it, and for 
this soil is required. 
FIG. 1. PETRI DISH FOR PLATE CULTURES. 
For the soil take the following recipe: Chop finely 
one-quarter pound of lean, juicy beef. Mix this with 
one cup of warm water. Heat in double boiler, stir¬ 
ring often until water in water pan has boiled fifteen 
minutes. Remove inner dish, place directly over the 
fire and allow broth to boil ten or fifteen minutes. 
Clear by straining through two or more thicknesses of 
