Solid culture media may be either permanently solid, like potatoes and coagulated egg, or they may retain their solid properties only at certain temperatures, like gelatin or agar. The latter two, which were devised by Robert Koch, are of utmost importance in bacteriological research, for their use permits the separation of the different forms of bacteria that may happen to be in any mixture. Gelatin is advantageously used, because the majority of bacteria present wider differences, due to growth upon this medium, than upon any other. It remains solid at ordinary temperatures, becoming liquid at about 80° F. Agar, a gelatinous product derived from a Japanese seaweed, has a much higher melting point, and is used especially with those organisms whose optimum temperature for growth is above the melting point of gelatin. Besides these solid culture media, different liquid substances are extensively used, such as beef broth, milk and infusions of various vegetable and animal tissues. Skim milk is of especial value in studying the milk bacteria, and may be used in its natural condition, or a few drops of litmus solution may be added, in order to detect any change in its chemical reaction due to the bacteria. =Sterilization.= The various ingredients that are used in the preparation of culture media are not free from micro-organisms, hence the media would soon spoil if they were not destroyed, and the media subsequently protected from contamination from the air, etc. The process of rendering the media free from living micro-organisms is known as _sterilization_. It may be accomplished in a number of ways, but most often is done by the use of heat. For culture material, which is always organic in character, moist heat is employed. The various culture media, in appropriate containers, are subjected to a thorough steaming in a steam cooker. This destroys all of the vegetating cells but not the resistant spores that may be present. The media are then stored, for twenty-four hours, at temperatures favorable for the germination of the spores and are then again heated. Three such applications on successive days are usually sufficient to free the media from all living germs, since between the heating periods the spores germinate and the resulting vegetative cells are more easily destroyed. The sterile media will keep for an indefinite period in a moist place. The media are usually placed in glass containers which may be sterilized before use by heating them in an oven, it being possible to thus secure a much higher temperature than with streaming steam. All glass or metal articles may be sterilized by the use of dry heat but for organic media, to avoid burning, moist heat must be used.