96 TEXT-BOOK OF BACTERIOLOGY. 



Now, the liquid gelatin in the test-tubes must be suitably spread 

 out and solidified. The larger the surface on which it is spread 

 the further the germs are separated from each other and the 

 easier it is to keep the different species apart. 



We therefore pour the contents of the test-tubes upon plates of 

 glass to consolidate. These plates, from which the widely-cele- 

 brated Koch's " plate process " takes its name, are cut out of mod- 

 erately thin glass, and must, of course, be thoroughly sterilized 

 before being used. This is best done in a box of sheet-iron, ar- 

 ranged to receive about twenty plates at once. Packed in this 

 manner, the plates are heated for half an hour in the hot-air oven, 

 after which the lid of the plate-box is removed, and with the 

 thumb and finger of the right hand a plate carefully drawn out 

 without touching its surface. If the attempt be made to pour the 

 gelatin over it at once, it would be found impracticable. Gelatin 

 hardens but slowly at the ordinary temperature of rooms, and 

 unless it can be managed to have the plate perfectly horizontal, 

 the gelatin will soon flow over the lowest end. 



Koch has, therefore, invented a special " plate apparatus." A 

 vessel is filled with pounded ice and water to the edge, and covered 

 over with a sheet of opaque glass. The whole is placed on a level- 

 ling stand with adjusting screws and provided with a spirit-level, 

 so that a horizontal position is easily obtained. 



When the plate is placed on this instrument the gelatin may 

 be poured out without difficult}', and on the horizontal, strongly- 

 cooled surface it spreads evenly and hardens quickly. In spite 

 of this aid, it requires a certain degree of practice to spread the 

 gelatin properly over the plate. The process may be made much 

 easier by quickly sterilizing the mouth of the test-tube in the flame 

 of the Bunsen burner immediately after the inoculation, and after- 

 ward directing the flow of the gelatin by using the mouth of the 

 test-tube as a " lip." Remember, however, to push the cotton plug 

 a short distance into the tube, that it may not be scorched or 

 burned, and take care that the mouth of the tube is fulty cooled 

 again before proceeding to pour out the gelatin. 



The whole operation of the " plate process " may be summarized 

 as follows: 



Three test-tubes with gelatin are melted in the water bath at 30 

 to 40 C.; inoculation and dilution are next performed, the original 

 being first prepared, and the first and second dilution by the trans- 

 fer of three drops with the loop. Next sterilize the mouths of the 

 tubes; while they are cooling, fix the levelling apparatus to an 

 exact horizontal by means of the spirit-level; open the plate-box, 



