BACTERIOLOGICAL STUDY OF WATER. 457 



face, as when in rolled tubes. The principles and 

 methods of estimation are practically the same as those 

 given for Wolff hiigePs apparatus. If the number of 

 colonies in an Esmarch tube is to be determined, a sim- 

 pler method than the use of his apparatus may be em- 

 ployed. It consists in dividing the tube by lines into 

 four or six longitudinal areas which are subdivided by 

 transverse lines drawn about 1 or 2 cm. apart. The 

 lines may be drawn with pen and ink, They need not 



FIG. 94. 



Esmarch's apparatus lor counting colonies in roll tubes. 



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 



