BACTERIOLOGICAL STUDY OF WATER 625 



A simpler method than by the use of Esmarch's apparatus 

 may be employed for counting the colonies in rolled tubes. 

 It consists in dividing the tube by lines into four or six 

 longitudinal areas, which are subdivided by transverse 

 lines about 1 or 2 cm. apart. The lines may be drawn with 

 pen and ink. They need not be exactly the same distance 

 apart nor exactly straight. Beginning with one of these 

 squares at one end of the tube, which may be marked with 

 a cross, the tube is twisted with the fingers, always in one 

 direction, and the exact number of colonies in each square 

 as it appears in rotation is counted, care being taken not 

 to count a square more than once; the sums are then added 

 together, and the result gives the number of colonies in 

 the tube. This method may be facilitated by the use of a 

 hand-lens. 



In all these methods there is one error difficult to eliminate : 

 it is assumed that each colony has grown from a single 

 organism. This is probably not always the case, as there 

 may exist clumps of bacteria which represent hundreds or 

 even thousands of individuals, but which still give rise to 

 but a single colony — obviously this is of necessity estimated 

 as a single organism in the water under analysis. 



Where grounds exist for suspecting the presence of these 

 clumps they may in part be broken up by shaking the 

 original water with sterilized sand. 



What has been said for the bacteriological examination 

 of water holds good for all fluids which are to be subjected 

 to this form of analysis. 



The Sewage Streptococcus. — Houston' reached the con- 

 clusion that there is constantly present in sewage a particular 

 form of streptococcus which is really more positively indica- 



' Ann. Report, Local Gov. Board, xxviii. 

 40 



