32 A MANUAL OF BACTEEIOLOGY 



there is air mixed with the hydrogen, and the gas must be kept 

 flowing. When the escaping hydrogen is pure, close the Novy 

 jar by turning the stopper. Shut off the flow of gas from the 

 generator. Disconnect the apparatus and put the Novy jar in 

 the culture room. 



Exercise 57. Preparation of Sterile Water Blanks 



Water blanks are used for diluting cultures containing large 

 numbers of bacteria. Physiological salt solution (0.6 per cent 

 NaCl) or distilled water may be used. 



Water blanks usually contain 10 cc, 100 cc, 200 cc, or 500 ec. 

 The liquid should be measured with a burette or accurate pi- 

 pette into vessels of approximately twice the volume of the solu- 

 tion to be used. Large test tubes or bottles are generally used. 

 The mouths of the vessels should be rather tightly plugged with 

 cotton and should be sterilized in the autoclave. During steri- 

 lization there is slight evaporation, which, however, is usually 

 corrected when 1 cc. of the test solution is added. 



SECTION VII 



FUNDAMENTAL METHODS USED IN THE STAINING AND 

 EXAMINATION OF BACTERIA 



The general forms of bacteria may be studied in a living con- 

 dition, as has already been done ui Section I. However, if the 

 exact shape and structure of the bacteria are to be studied, they 

 should be stained. In some instances the results of staining are^ 

 valuable as diagnostic characters. Most of the stains commonly 

 employed in bacteriological laboratories are solutions of aniline 

 dyes. 



Solutions of stains should not usually be kept long before 

 being used, although in a few cases their staining powers im- 

 prove with age. 



By the use of certain chemical agents called mordants the 

 staining power of the aniline dyes is increased. Various metallic 

 salts and organic acids are used as mordants — for example, 



