All kinds of materials may be sterilized by treatment with steam under pressure. An exposure for a few moments at 250° F., a temperature attained with 15 pounds steam pressure, will destroy all kinds of bacteria and their spores. This method of sterilization is used in the canning of meats and vegetables and in the preparation of evaporated milk. To avoid contamination of the media after sterilization, the flasks and tubes are, after being filled, stoppered with plugs of cotton-wool, which effectually filter out all bacteria and mold spores from the air, and yet allow the air to pass freely in and out of the containers. =Methods of determining the number of bacteria.= The method of determining the number and kinds of bacteria in any substance can be illustrated by the process as applied to milk. For this purpose the method of procedure is as follows: Sterile gelatin in glass tubes is melted and then cooled until it is barely warm. To this melted gelatin a definite quantity of milk is added. The medium is gently shaken, so as to thoroughly mix the milk and gelatine, and the mixture then poured into a sterile, flat, glass dish, and quickly covered, where it is allowed to cool until the gelatin hardens. After the culture plate has been left for twenty-four to thirty-six hours at the proper temperature, tiny spots will begin to appear on the surface, or in the depth of the culture-medium. These spots are called _colonies_, and are composed of an almost infinite number of individual cells, the result of the continued growth of a single organism that was in the drop of milk and which was firmly held in place when the gelatin solidified. The number of these colonies represents approximately the number of living bacteria that were present in the amount of milk added to the tube of gelatin. If the plate is not too thickly sown with the bacteria, the colonies will continue to grow and increase in size, and as they do, minute differences will begin to appear. These differences may be in the color, the contour, and the texture of the colony, or the manner in which it acts toward gelatin. [Illustration: Fig. 4.--Plate Culture. Each of the dots is a colony that has been formed by the growth of an organism embedded in the solid culture-medium. By counting the colonies, the number of living bacteria in the amount of milk added to the culture is determined.]