144 



A MANUAL OF BACTERIOLOGY 



sodium carbonate to ordinary distilled water until slightly alkaline, and 

 boiling until about one fourth has evaporated. The residual water is 

 ready for use when cool. Am- 

 monia-free water is used in 

 the preparation of the follow- 

 ing reagents and wherever the 

 contamination w r ith ammonia 

 would influence the result. 



2. Sodium carbonate solution. A boiled 

 saturated solution. 



3. Nessler's reagent. Prepare a potassium 

 iodide solution by dissolving 35 g. in 100 cc. 

 of water, and a mercuric chloride solution by 

 dissolving 17 g. in 300 cc. of water. Heat 

 may be applied, to hasten the solution of the 

 mercuric chloride, but the liquid must be 

 cooled before it is used. The mercuric 

 chloride solution is added to the potassium 

 iodide solution until the precipitate of red 

 mercuric iodide ceases to redissolve. The 

 solution is then diluted to 1 liter with a 

 20 per cent solution of sodium hydroxide. 

 Then add more of the mercuric chloride 

 solution until a slight permanent precipitate 

 again forms. Allow this to settle, keeping- 

 it in a well-stoppered bottle and drawing 

 off small quantities into an- 

 other bottle from time to 



time as required for the tests. 

 The reagent should have a 

 light yellow color ; if color- 

 less, more mercuric chloride 

 must be added. Its sensi- 

 tiveness should be tested 

 from time to time with a 

 very dilute solution of am- 

 monium chloride. 



FIG. 46. The Schreiner Colorimeter 



A, A, immersion tubes ; J3, graduated tubes ; 

 C, clamps ; D, reflector ; E, mirror 



4. Standard ammonium chloride solution. Dissolve 0.7405 g. of 

 pure ammonium chloride in ammonia-free water and dilute to 

 1 liter. Dilute 10 cc. of this stronger solution to 500 cc. This 



