66 A MANUAL OF BACTERIOLOGY 



and after neutralisation the whole is heated over a water- 

 bath for half an hour before filtration. 



The Cultivation and Isolation of Micro- 

 organisms 



It should be clearly understood that micro-organisms 

 cannot usually be identified by their microscopical char- 

 acters alone. We can state from a microscopical examina- 

 tion the form of an organism, that it is a bacillus or a 

 micrococcus, or a sarcina, its size, that it is motile or non- 

 motile, sporing or non-sporing, but we cannot as a rule 

 go beyond this. It is necessary in most cases to ascertain 

 the characters of the growths of organisms on the various 

 culture media before species can be identified, and this is 

 the principal reason for having a varied assortment of 

 nutrient soils. It is likewise necessary for the successful 

 cultivation of pathogenic organisms, i.e. those connected 

 with disease processes and developing in or upon the 

 bodies of man and of animals, to maintain the cultures 

 at a temperature approximating to that of the host. For 

 this purpose some form of incubator is required. This 

 consists of a box or chamber of copper or iron with 

 double walls (Fig. 10), the space between which is filled 

 with water, the outside being covered with wood or felt, 

 or some other non-conductor. The water between the 

 walls is heated by means of a small burner, the gas supply 

 for which passes through some form of regulator inserted 

 in the water, so that the temperature, indicated by a 

 thermometer inserted through a hole in the top, can be 

 kept constant. The regulator is usually a mercurial one, 

 such as Page's or Reichert's, the principle of its action 

 being that as the temperature rises the mercury expands 

 and at a certain point cuts off the greater part of the gas 

 supply, only sufficient gas then passing to keep the flame 



