THE RECOGNITION OF BACTERIA. 185 



having exhausted the pabulum necessary for their 

 development, may have settled to the bottom, 

 where they form a white pulverulent precipitate, 

 while the fluid above is transparent. In the lat- 

 ter case, a milky appearance is produced by shak- 

 ing the tube so as to distribute the organisms 

 throughout the fluid. 



There is usually no difficulty in recognizing, by 

 means of the microscope, the minute unicellular 

 plants to which this change in our culture-fluid is 

 due. But for this purpose it will often be neces- 

 sary to use comparatively high powers, e. g., a 

 good one- tenth inch objective, and to resort 

 to the use of staining reagents. For information 

 relating to the optical and chemical tests by which 

 bacteria are to be distinguished from inorganic 

 substances, and from albuminous or fatty granules, 

 etc., the reader is referred to Part First of the 

 present volume, which treats of their morphology, 

 and especially to the remarks in the second chap- 

 ter, pages 49-53. 



Motile bacteria are at once recognized as living 

 organisms ; but care must be taken not to mistake 

 movement due to currents in the fluid, or the 

 molecular motion brownien which minute par- 

 ticles undergo when suspended in a fluid, for a 

 vital movement. This is to be distinguished by 

 the fact that the movements are vibratory, and do 

 not result in a change in the location or relative 

 position of the moving particles. Bacteria which 

 have exactly the same refractive index as the 



