44 BACTERIA 



at the very outset certain facts may be obtained, viz., the size, 

 elevation, configuration, margin, colour, grouping, number, 

 and kinds of colonies, all of which facts are of importance, 

 and assist in final diagnosis. Moreover, in the case of gel- 

 atine plates (it is otherwise in agar) one is able to observe 

 whether or not there is present what is termed liquefaction 

 of the gelatine. Some organisms produce in their develop- 

 ment a peptonizing ferment which breaks down gelatine into 

 a fluid condition. Many have not this power, and hence 

 the characteristic is used as a diagnostic feature. 



2. Microscopic Examination of Colonies, which confirms or 

 corrects that which has been observed by the naked eye. 

 Fortunately some micro-organisms when growing in colon- 

 ies produce cultivation features which are peculiar to them- 

 selves (especially is this so when growing in test-tube 

 cultures), and in the early stages of such growths a low 

 power of the microscope or magnifying glass facilitates 

 observation. 



3. Make cover-glass preparations: (a) unstained-^-" the 

 hanging drop " ; (b) stained single stains, like gentian-violet, 

 methyl blue, fuchsin, carbol fuchsin, etc. ; double stains 

 Gram's method, Ziehl-Neelsen's method, etc. 



This third part of the investigation is obviously to prepare 



\ 



THE HANGING DROP 



specimens for the microscope. " The hanging drop " is a 

 simple plan for securing the organisms for microscopic 

 examination in a more or less natural condition. A hollow 

 ground slide, which is a slide with a shallow depression in it, 

 is taken, and a small ring of vaseline placed round the edge 



