The Bacterial Suspension 



271 



rate than those in old cultures. Such a culture may be taken up 

 in a platinum loop, transferred to a test-tube containing some 

 0.85 per cent, sodium chloride solution, and gently rubbed upon the 

 glass just above the fluid, allowing the moistened and mixed bacterial 

 mass to enter the fluid little by little. 

 If the culture be older or of a nature that will not separate in 



Fig. g2. — Grinding bacteria (Miller). 



this manner (tubercle bacillus), it may be necessary to rub it between 

 two glass plates, or in a small agate mortar with a drop or two of 

 salt solution, other drops being added one at a time, until a homo- 

 geneous suspension is secured. Such clumps of bacteria as may 

 remain in the suspension are easily removed by whirling for a few 

 seconds in a centrifuge. 



The next step is the standardization of the suspension. Wright 

 recommends for this purpose and for 

 the standardization of the bacterio- 

 vaccines that the number of bac- 

 teria shall actually be counted. This 

 he does by mixing one part of the 

 bacterial suspension with an equal 

 volume of normal blood and three 

 volumes of physiological salt solu- 

 tion. After thorough mixing a smear 

 is made upon a slide, the smear 

 stained, and the number of bacteria 

 and corpuscles in successive fields of 

 the microscope counted until at least 200 red blood-corpuscles have 

 been enumerated. As the number of red corpuscles per cubic milli- 

 meter of blood is 5,000,000, the number of bacteria per cubic centi- 

 meter can be determined from the results of the counting by a 

 simple arithmetical process. To facilitate the counting the eye- 

 piece of the microscope is prepared by the introduction of a dia- 

 phragm. The prepared suspension must usually be greatly diluted 

 before using, but the reduction of bacteria is, of course, easily cal- 



Fig. 93. — Diaphragm of eye- 

 piece showing hairs in position 

 (Miller). 



