NUTRIENT MEDIA AND METHODS OF CULTIVATION. 



107 



vessel, and held in position by means of a clip. The vessel can be 

 fixed on the inclined stage of the microscope, and the contents of the 

 tube conveniently examined with low-power objectives. 



Plate-cultivations. — By this method, as already mentioned, a 

 mixture of bacteria, whether in fluids, excreta, or in cultivations on 

 solid media, can be so treated that the different species are isolated 

 one from the other, and perfectly pure cultivations of each of the 

 cultivable bacteria in the original mixture established in various 

 nutrient media. We are enabled also to examine under a low 

 power of the microscope the individual colonies of bacteria, and to 

 distinguish by their characteristic appearances, micro-organisms 

 which, in their individual form, closely resemble one another, or 

 are even identical. The same process, with slight modification, is 

 also employed in the examination of air, soil, and water, which will 

 be referred to later. 



The preparation 

 of plate-cultivations, 

 therefore, must be 

 described in every de- 

 tail ; and to take an 

 example, we will sup- 

 pose that a series of 

 plates is to be pre- 

 pared from a test- 

 cube-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. Substitute for 

 the spirit-level a piece of filter-paper the size of the glass plates to 

 be employed, and cover it with a shallow bell-glass. 



Sterilisation of Glass Plates. — The glass plates are sterilised in 

 an iron box placed in the hot-air sterihser, at 150° C, from one to 

 two hours. As these plates are used also for other purposes, a 

 quantity ready sterilised should always be kept in the box. 



Preparation of Damp Chambers. — The damp chambers for the 

 reception of the inoculated plates are prepared thus : Thoroughly 



Kg. 36.— Levelling Appabatds. 



