Blood serum is also used in the liquid state. For this purpose it is best to collect it aseptically; or it may be sterilized discontinuously at a temperature of 55° or 56° on seven to ten consecutive days. Novy has recently suggested dialyzing the serum to free it from salts and thus prevent its coagulation when heated. Whether the removal of the various “extractives” which diffuse out with the salts deprives the serum of any of its advantageous properties remains to be ascertained. From the discussion of the physiological activities of bacteria in Chapters IX-XII it is apparent that a very great variety of culture media other than those described is necessary for the study of special types of bacteria, but such media are beyond the scope of the present work. The ideal culture media are without a doubt the _synthetic media_, that is media of definite known chemical composition, so that the various changes due to the growth of bacteria can be accurately determined and thus a means of sharply differentiating closely related organisms be secured. Such media have been prepared and every bacteriologist believes strongly in their future usefulness when media of wider application shall have been devised. An example of this type of culture media is Uschinsky’s synthetic medium, of which the following is one of the modifications: Distilled-water 1000 parts Asparagin 4 „ Ammonium lactate 6 „ Disodium phosphate 2 „ Sodium chloride 5 „ A criticism of this medium is that the elements K, Ca, Mg, Fe, Mn, and S which have been shown to be essential are not present if chemically pure salts are used in the preparation. CHAPTER XVII. METHODS OF USING CULTURE MEDIA. The way in which culture media shall be used depends on the purpose in view. By far the larger part of bacteriological work is done with cultures in “bacteriological culture tubes.” Various laboratories have their own special types but all are more or less after the “Board of Health” form. They differ from ordinary chemical test-tubes in that they are usually longer, have no “lip” and have much thicker walls to prevent breakage and consequent loss of the culture as well as danger from pathogenic organisms. The author finds two sets of tubes most serviceable for student use--one size 15 cm. long by 19 mm. outside diameter (No. 9, Fig. 119), the other 15 cm. long by 13 mm. (Nos. 1 to 7, Fig. 119). Culture tubes are conveniently used in “wire baskets” circular or square in section and of a size to correspond with the length and number of tubes used. These baskets are light, do not break, and if made of good galvanized wire netting do not readily rust (Figs. 117 and 118).