150 BACTERIOLOGY. 



original plate or tube, as a rule, will be of no use be- 

 cause of the great number of colonies in it ; plate or 

 tube No. 2 may be of service ; but plate or tube No. 3 

 will usually contain the organisms in such small num- 

 bers that there will be nothing to prevent the charac- 

 teristic development of the colonies originating from 

 them. 



For reasons of economy the " original," tube No. 1, is 

 sometimes substituted by a tube containing normal salt- 

 solution (0.6 to 0.7 per cent, of sodium chloride in 

 water), which is thrown aside as soon as the dilutions 

 are completed, and only plates or tubes Nos. 2 and 3 

 are made. 



THE SERIAL TUBE METHOD OF SEPARATION. 

 Another method for the separation of bacteria and 

 their isolation as single colonies consists in the making 

 of dilutions upon the surface of solid media, such as 

 potato, coagulated blood-serum, agar-agar, and gelatin. 

 In pursuance of this method one selects a number of 

 tubes containing the medium set in a slanting position. 

 With a platinum needle a bit of the substance to be 

 studied is smeared upon tube No. 1 ; without sterilizing 

 the needle it is passed in succession over the surface 

 of the medium in tubes Nos. 2, 3, 4, etc. When de- 

 velopment has occurred essentially the same conditions 

 as regards separation of the colonies will be found as 

 when plates are poured. If a slanted medium be em- 

 ployed, about the most dependent angle of which water 

 of condensation has accumulated, as blood-serum, agar- 

 agar, and potato, the dilutions may be made in this 

 fluid, and this is then to be carefully smeared over the 

 solid surface of the medium. The tubes thus treated 



