PETRPS CAPSULES. 



61 



first as to their purity and then with a view to their 

 identification. 



2. Glass Capsules. While in certain circumstances, as 

 when the number of colonies has to be counted, it is best 

 to use plates, in the usual laboratory routine Petri's cap- 

 sules are to be preferred. The technique of the inocula- 

 tion of the tubes is, of course, the same. The contents of 

 the tubes are then poured out into three capsules, and these 

 are labelled. Not only is it not necessary to use the 

 leveller, but the risk of atmospheric contamination is less 

 than with plates, as the cover of the capsule is lifted only 

 sufficiently to allow the gelatine to be poured in, and the 

 latter is thus practically not exposed to the air at all. In 

 examining a capsule when growth has taken place, it can be 

 held up to the light with cover in situ, and the colonies 

 observed ; and, further, it can be turned upside down on 

 the stage of the microscope, and the colonies 



examined with a low power through the 

 bottom. 



As the naked -eye and microscopic 

 appearances of colonies are often very 

 characteristic, plate cultures, besides use in 

 separation, are often taken advantage of 

 in the description of individual organisms. 

 The plate-culture method can also be used 

 to test whether a tube culture is or is not 

 pure. The suspected culture is plated (three 

 plates being prepared as usual). If all the 

 colonies are the same, then the cultures may 

 be held to be pure. 



3. Esmarctts Roll Tubes. Here the 

 principle is that of dilution as before. In 

 each of three test-tubes \\ or ij inch in 

 diameter, gelatine to the depth of f of an 

 inch is placed. These are sterilised. The 

 gelatine is melted and inoculated with the culture, 

 bacterial mixture as in making plate cultures, but instead 

 of being poured out it is rolled in a nearly horizontal 



FIG. 20. 

 Tube 

 Esmarch's 



for 

 roll 



