APPENDi 



OF THE ^ 



UNIVERSITY 



457 



of any of the contained 



doubt as to the disease-producing action 

 organisms. 



By working through some such scheme as the above, it is possible to 

 detect what quantity and species of organisms, saprophytic or parasitic, 

 a water or similar fluid contains. For, observe what information has 

 been gained by following out these five steps in procedure. We have 

 learned the form (whether bacillus, micrococcus, or spirillum), size, 

 consistence, motility, method of grouping, and staining reactions of each 

 micro-organism ; the characters of its culture, colour, composition, 

 presence or absence of liquefaction or gas formation, its rate of growth, 

 odour, or reaction ; and, lastly, its effect upon living tissues. Here, then, 



FIG. 42. Types of Liquefaction of Gelatine. 



are ample data for arriving at a satisfactory conclusion respecting the 

 qualitative estimation of the drop of water under examination. 



As to the quantitative examination, that is fulfilled by counting the 

 number of colonies which appear, say by the third or fourth day, upon 

 the gelatine plates. Each colony has arisen, it is assumed, from one 

 individual, so that if the colonies be counted, though we do not thereby 

 know how many organisms there are upon the plate, it is known 

 approximately how many organisms there were when the plate was 

 first poured out, which are the figures we require, and which can at once 

 be multiplied up and returned as so many organisms per drop, or if the 

 quantity of water were measured, per c.c. 



When counting colonies in a Petri's dish, it is sufficient to divide the 

 circle into eight equal divisions, and counting the colonies in the 

 average divisions, multiply up and reduce to the common denominator of 

 1 c.c. (or a Pakes' or Jeffer's Counting Disc may be used). For example, 



