390 BACTERIA. 



manner : — Thin plate glass is cut into squares of four and a half inches, the 

 sharp edges are removed with a file, and the glass is carefully cleaned. It may 

 then be sterilized in one of the following ways : Wrap each plate separately 

 in a sheet of hard tissue paper, and place in the hot air chamber, leaving it 

 for about an hour subject to a temperature of 150° C. It may then be taken 

 out and kept in a dry place until required for use. If the plate is required 

 at once, leave it slightly damp after cleaning, then, laying hold of it with a 

 pair of strong forceps, heat it carefully over a Bunsen burner or spirit lamp 

 until the whole moisture disappears, great care being taken that one side at 

 any rate is sterilized by the action of the flame, wrap up in a piece of steri- 

 lized paper and leave it until the other materials are ready. Or the plate 

 may be tilted against a clean wooden block or iron upright with the more 

 carefully heated surface downwards. Bell jars and glass benches have pre- 

 viously been prepared by being thoroughly washed with soap and water, 

 and then with a l per l ,000 solution of bichloride of mercury ; a piece 

 of absorbent filter paper thoroughly saturated with the bichloride solution 

 is placed in the bottom of one of the jars ; on to this all germs that are 

 suspended in the air within the jar gradually fall and are destroyed. On 

 a surface made as level as possible by means of a tripod levelling apparatus 

 and a small round spirit level, (if these can be obtained), a plate of metal 

 resting on three metal feet is placed in a mixture of ice, salt, and water; on 

 this the sterilized plate of glass, with the more carefully prepared side upper- 

 most, is laid, and the whole is covered with a bell jar that has been previously 

 sterilized by means of heat or of bichloride of mercury. A test tube, with 

 a large overhanging cotton wadding stopper, containing gelatine or a 

 mixture of agar and gelatine is taken, the stopper is removed for a second 

 or two, and the lip of the glass tube is carefully heated in a flame, the 

 cotton wadding stopper is also thoroughly singed and then replaced. The 

 gelatine is melted by placing the tube in water at a temperature of about 

 3S°~39° C. for gelatine, and a much higher one for agar gelatine. As soon 

 as the medium is thoroughly melted, the quantity of water that is to be used 

 is dropped from the sterilized pipette into the tube, then with a rolling 

 motion the water is thoroughly incorporated with the nutrient medium, 

 great care being taken that as few air bubbles as possible shall find their 

 way into the viscid fluid. The plate by this time being thoroughly cooled, 

 the gelatine is poured out so as to form an equal and regular layer ; it is 

 spread gradually from the centre of the plate to near the margins, over 

 which, however, it is not allowed to run ; it is then allowed to solidify, 

 after which the plate is transferred to the bell jar prepared for its recep- 

 tion. By means of slips of glass, or of glass, porcelain, or zinc benches, 

 carefully sterilized, three plates may be placed in the same bell jar ; they 

 are then allowed to incubate at the temperature of the room, and at the 

 end of two or three days colonies of bacteria make their appearance 

 (each one from a single seed if the mixing has been perfect), and may 

 be isolated and described. Where the number of organisms is unknown 

 the method described for the isolation of cholera organisms (see under 

 Cholera, p. 155) should be utilized. It is sometimes necessary to make 

 a whole series of plate cultures to obtain a single pure growth, especially 

 where zooglcea masses are formed. This method is also used for the 

 separation of different species of micro-organisms, and it can be easily 

 understood how micro-organisms may be more or less isolated by being 

 shaken in a fluid medium, and how when once they are isolated they 



