74 APPLIED BACTERIOLOGY 



The tubes containing the nutrient agar are stood in a 

 beaker of boiling water until the contents are completely 

 melted. After this the water in the beaker is cooled 

 somewhat, and then allowed to stand, with a thermometer 

 immersed in the water until a temperature of 40° C. is 

 reached. The tubes are then immediately inoculated and 

 the contents poured into a plate, as previously directed 

 for the preparation of gelatine plates. It is a good plan 

 to warm the dishes or plates to 40° C. before pouring the 

 agar, and, above all, to work quickly, as the agar solidifies 

 at 40° C, and after solidification has begun to take place 

 an even distribution of the medium is no longer possible. 



The gelatine plate cultures are kept at room -temperature 

 until the individual colonies show themselves. The different 

 bacteria develop at very different rates at the ordinary 

 room-temperature. It is possible that on the following day 

 colonies of bacteria will be apparent to the naked eye ; but 

 often one has to wait two or three days, or even longer, 

 according to the special kinds of organisms that are present. 

 Of course, growth in the summer is very much more rapid 

 than in the winter. At fir§t it, is. difficult to distinguish 

 the colonies from small air-bubbles. As they grow, 

 however, the different colonies may be distinguished one 

 from the other by a great number of different character- 

 istics. Some are spherical, and these may be transparent 

 or opaque, or they may have an opaque nucleus surrounded 

 by a transparent zone. Again, the outlines may be irregular, 

 giving rise to amcsba-like, rosette or star-like forms, with 

 fringed or bushy-like margins. In the case of those 

 which liquefy the gelatine, they will be seen to sink some- 

 what, and to liquefy the gelatine in more or less wide 

 circles, while in others the liquefaction is a much slower 

 proqess, and is only visible after some time. Most of the 

 liquefying organisms liquefy gelatine in a very characteristic 



