Ordinarily, more attention must be paid to the sources of the two elements N and C than to the others, for in general the substances used to furnish these two and the water contain the other elements in sufficient amount. For very exact work on the products of bacteria, _synthetic media_ containing definite amounts of chemicals of known composition have been prepared, but for most of the work with bacteria pathogenic to animals such media are not needed. Culture media may be either _liquid_ or _solid_, or for certain purposes may be liquid at higher temperatures and solid at lower, as indicated later. Liquid media are of value for obtaining bacteria for the study of morphology and cell groupings and for ascertaining many of the physiological activities of the organisms. Solid media are useful for studying some few of the physiological activities and especially for determining characteristic appearances of the isolated growths of bacteria. These isolated growths of bacteria on solid media are technically spoken of as “_colonies_,” whether they are microscopic in size or visible to the unaided eye. It is clear that the kinds of culture media used for the study of bacteria may be unlimited but the undergraduate student will need to use a relatively small number, which will be discussed in this section. =Meat Broth (Bouillon).=[20]--This itself is used as a medium and as the basis for the preparation of other solid and liquid media. Finely ground _lean_ beef is selected because it contains the necessary food materials. Fat is not desired since it is a poor food for most bacteria and in the further processes of preparation would be melted and form an undesirable film on the surface of the medium. The meat is placed in a suitable container and mixed with about twice its weight of _cold_ water (not distilled) and allowed to soak overnight or longer. The cold water extracts from the meat water-soluble proteins, blood, carbohydrates in the form of dextrose (occasionally some glycogen), nitrogenous extractives and some of the mineral salts. The fluid is strained or pressed free from the meat. This “meat juice” should now be thoroughly boiled, which results in a coagulation of a large part of the proteins and a precipitation of some of the mineral salts, particularly phosphates of calcium and magnesium, both of which must be filtered off and the water loss restored by adding the proper amount of distilled water. The boiling is done at this point because the medium must later be heated to sterilize it and it is best to get rid of the coagulable proteins at once. The proteins thus thrown out deprive the medium of valuable nitrogenous food material which is replaced by adding about 1 per cent. by weight of commercial peptone. It is usual also (though not always necessary) to add about 0.5 per cent. by weight of common salt which helps to restore the proper concentration of mineral ingredients lost by the boiling. The chlorine is also an