How we catch Bacteria to Study Them.--To study bacteria it is first necessary to find some material in which they will grow, then kill all living matter in this food material by heating to boiling point (212 deg.) for half an hour or more (this is called _sterilization_), and finally protect the _culture medium_, as this food is called, from other living things that might grow upon it. One material in which bacteria seem to thrive is a mixture of beef extract, digested protein and gelatine or agar-agar, the latter a preparation derived from seaweed. This mixture, after sterilization, is poured into flat dishes with loose-fitting covers. These _petri_ dishes, so called after their inventor, are the traps in which we collect and study bacteria. Where Bacteria might Grow.--Expose a number of these sterilized dishes, each for the same length of time, to some of the following conditions: (_a_) exposed to the air of the schoolroom. (_b_) exposed in the halls of the school while pupils are passing. (_c_) exposed in the halls of the school when pupils are not moving. (_d_) exposed at the level of a dirty and much-used city street. (_e_) exposed at the level of a well-swept and little-used city street. (_f_) exposed in a city park. (_g_) exposed in a factory building. (_h_) dirt from hands placed in dish. (_i_) rub interior of mouth with finger and touch surface of dish. (_j_) touch surface of dish with decayed vegetable or meat. (_k_) touch surface of dish with dirty coin or bill. (_l_) place in dish two or three hairs from boy's head. This list might be prolonged indefinitely. [Illustration: Colonies of bacteria growing in a petri dish.] Now let us place all of the dishes together in a moderately warm place (a closet in the schoolroom will do) and watch for results. After a day or two little spots, brown, yellow, white, or red, will begin to appear. These spots, which grow larger day by day, are _colonies_ made up of millions of bacteria. But probably each colony arose from a single bacterium which got into the dish when it was exposed to the air.