PLATE-CULTIVATIONS. 73 



also to examine under a low power of the 

 microscope the individual colonies of bacteria, 

 and to distinguish, by their characteristic appear- 

 ances, micro-organisms which, in their individual 

 form, closely resemble one another, or are even 

 identical. The same process, with slight modifica- 

 tion, is also employed in the examination of air, 

 soil, and water, as will be referred to later. 



The preparation of plate-cultivations, therefore, 

 must be described in every detail, and to take 

 an example, we will suppose that a series of plates 

 are to be prepared from a test-tube-cultivation. 



Arrangement of Levelling Apparatus. In 

 order to spread out the liquid jelly evenly on the 

 surface of a glass plate, and hasten its solidifica- 

 tion, it is necessary to place the glass plate upon a 

 level and cool surface. This is obtained in the 

 following manner : Place a large shallow glass 

 dish upon a tripod stand, and fill it to the brim 

 with cold water; carefully cover the dish with a 

 slab of plate glass, or a pane of window glass, and 

 level it by placing the spirit-level in the centre and 

 adjusting the screws of the tripod. Replace the 

 level by a piece of filter paper, the size of the glass 

 plates to be employed, and cover it with a shallow 

 bell glass (Fig. 8). 



Sterilisation of Glass Plates. The glass 

 plates are sterilised by filling the iron box (p. 27), 

 and placing it in the hot-air steriliser, at 150 C., 

 from one to two hours. As these plates are used 



