4. Take a piece of thin glass rod, stout platinum wire or best of all a piece of aluminium wire (say 2 mm. diameter) about 28 cm. long. Bend the terminal 4 cm. at right angles to the remainder, making an L-shaped rod (Fig. 126). Sterilise the short arm and adjacent portion of the long arm, in the Bunsen flame, and allow it to cool. 5. Now raise the bottom of the Petri dish in the left hand, leaving the cover on the laboratory bench, and holding it vertically, smear the drop of inoculum all over the surface of the gelatine with the short arm of the spreader by a rotatory motion, (Fig. 127). Replace the dish in its cover. 6. Raise the bottom of plate 2 and rub the infected spreader all over the surface of the gelatine--then go on in like manner to the third plate in the series. 7. Sterilise the spreader. 8. Label and incubate the plates. [Illustration: FIG. 127.--Spreading surface plate.] After incubation, plate No. 1 will probably yield an enormous number of colonies; plate 2 will show fewer colonies, since only those bacteria adhering to the rod after rubbing over plate 1 would be deposited on its surface, and by the time the rod reached plate 3 but very few organisms should remain upon it. So that the third plate as a rule will only show a very few scattered colonies, eminently suitable for detailed study. (b) ~Agar Surface Plates.~-- 1. Liquefy three tubes of nutrient agar--nutrose agar or the like. 2. Pour each tube into a separate Petri dish and allow it to solidify. 3. When quite solid invert each dish, raise the bottom half and rest it obliquely on its inverted cover (Fig. 128) and place it in this position in an incubator at 60° C. for forty-five minutes (or in an incubator at 42° C. for two hours). This evaporates the water of condensation and gives the medium a firm, dry surface. 4. On removing the plates from the incubator close each dish and place it--still upside down--on the laboratory bench. [Illustration: FIG. 128.--Drying surface plate of agar.] 5. Inoculate the plates in series of three, as described for gelatine surface plates 3-8. Hanging-drop Cultivation. ~Apparatus Required.~-- Hanging-drop slides. Cover-slips. Section rack (Fig. 75). Blotting paper. Bell glass to cover slides. Original culture. Tubes of broth, or liquefied gelatine or agar. Forceps. Platinum loop. Bunsen burner. Grease pencil. Sterile vaseline. Lysol. (a) ~Fluid Media.~-- 1. Prepare first and second dilutions of the inoculum as directed for plate cultivations (_vide_ pages 228-229, sections 4 to 6), substituting tubes of nutrient broth for the liquefied gelatine. 2. Sterilise a hanging-drop slide by washing thoroughly in water and drying, then plunging it into a beaker of absolute alcohol, draining off the greater part of the spirit, grasping the slide in a pair of forceps, and burning off the remainder of the alcohol in the flame.