Gelatin should be sterilized by discontinuous heating at 100° on three successive days, because long boiling or heating above 100° tends to hydrolyze the gelatin into gelatin proteose and peptone and it will not gelatinize on cooling. It may be heated in the autoclave for ten to fifteen minutes at 10 pounds’ pressure and sometimes not be hydrolyzed, but the procedure is uncertain and very resistant spores may not be killed. The medium should be put into the culture tubes in which it is to be used as soon as filtered, and sterilized in these, since, if put into flasks these must be sterilized, and then when transferred to tubes for use, it must be again sterilized unless great care is taken to have the tubes plugged and sterilized first, and in transferring aseptically to these tubes. These repeated heatings are very apt to decompose the gelatin, so it will not “set” on cooling. The prepared and sterilized tubes of gelatin should be kept in an ice-box or cool room, as they will melt in overheated laboratories in summer or winter. =Agar Medium.=--Agar agar, usually called agar, is a complex carbohydrate substance of unknown composition obtained from certain seaweeds along the coast of Japan and Southeastern Asia. It occurs in commerce as thin translucent strips or as a powder. It resembles gelatin only in the property its solutions have of gelatinizing when cooled. Gelatin is an albuminoid closely related to the proteins, agar a carbohydrate. Agar is much less soluble in water, 1 or 1.5 per cent. of agar giving a jelly as dense as 10 to 15 per cent. of gelatin. It dissolves only in water heated to near the boiling-point (98° to 99°) and only after much longer heating. This hot solution “jells,” “sets” or gelatinizes at about 38° and remains solid until again heated to near boiling. Hence bacteria may be grown on agar at the body temperature (37°) and above, and the agar will remain solid, while gelatin media are fluid above about 25°. No pathogenic bacteria and none of the saprophytes liable to be met with in the laboratory are able to “liquefy” agar.