Potatoes are sometimes used in “potato plate cultures.” The term “plate culture” is a relic of the time when flat glass plates were used for this and other “plate cultures.” Now glass dishes of the general form shown in Fig. 115, called “Petri dishes,” or plates are used for practically all plate culture work. For “potato plates” slices from potatoes are cut as large and as thick as the relative sizes of potato and dish permit (Fig. 116). The slices should be thin enough not to touch the lid and thick enough to be firm. [Illustration: FIG. 115.--Petri dish with the lid partly raised. × 1/2] [Illustration: FIG. 116.--A potato plate. × 1/2] It is a good plan to wrap each dish separately in paper to retain the lid securely, then sterilize as for potato tubes, and leave plates wrapped until wanted. It sometimes happens that the natural acidity of potatoes is too great for the growth of many organisms. The acidity is sufficiently corrected by soaking the pieces of potato in a 1 per cent. solution of sodium carbonate for an hour before they are put into the tubes or plates. _Glycerinized potato tubes_ are conveniently prepared by covering the potato in the tube with glycerin broth, sterilizing and pouring off the excess broth immediately after sterilizing, taking care that the tubes do not become contaminated which is not very probable if the work is quickly done while the tubes are still hot. =Blood Serum Media.=--Blood serum, usually from the larger, domestic animals on account of convenience in securing it in quantity, is used in the study of the bacteria causing disease in man and animals. Most commonly the serum is collected from the clotted blood after it has well separated (usually about forty-eight hours is required for this). It is then run into tubes which are plugged with cotton and placed in an apparatus for coagulating the serum by heat. A copper water bath with a tightly closed air compartment or the horizontal autoclave (Fig. 81) is sufficient for this purpose, though special forms of apparatus are to be had. It is important that the temperature be raised slowly so that the blood gases escape gradually. Three to five hours or longer should be allowed for the temperature to reach the boiling-point. If the tubes are heated too rapidly, the serum is filled with bubbles and badly torn since the gases are driven off suddenly. _Löffler’s serum_ is made by adding one part of dextrose broth to three parts of serum and then coagulating as above. The solidified serum in either case is best sterilized discontinuously, though with care the autoclave at 15 pounds pressure may be used for a single sterilization. This is very apt to cause a greater darkening of the serum and frequently also a laceration of the solid mass by escaping gases.