66 PRACTICAL BACTERIOLOGY. 



stocking filled with liquid. Often the bacteria fill 

 this liquid gelatine with a uniformly thick grey mass, 

 but sometimes the colonies sink to the lower part of 

 the fluid, leaving the upper part quite clear. The 

 fluid often sinks down from the surface, so that an 

 air-bubble is drawn into the mouth of the funnel ; 

 whilst, with other species, no such bubbles are formed. 

 Sometimes also the liquefaction first spreads itself 

 over the surface of the gelatine until it reaches the 

 edge of the nutrient medium, when it sinks down 

 gradually and evenly without a funnel being formed. 

 With some species — with those that liquefy the gela- 

 tine, as well as with those that do not — bubbles of 

 gas may be formed, sometimes in considerable numbers, 

 in the interior of the medium. 



All these characteristics must be carefully observed 

 in order to distinguish the diSereut species from one 

 another, as they aSbrd us far more reliable data than 

 the morphological peculiarities of the bacteria. There 

 are a great many bacteria which, if their individual 

 forms alone were to be considered, could not be dis- 

 tinguished from one another, but they show such 

 marked and constant differences in their manners of 

 growth in the nutrient medium, and in the form and 

 peculiarities of their colonies, that we are bound to 

 consider them as different species. If we have suc- 

 ceeded in distinguishing by means of their culture 

 characteristics two kinds of bacteria, morphologically 



