148 NITRIFICATION 



placing the dish cover downwards so that the exuded water will 

 not remain on the surface of jelly. The deep colonies of the 

 nitrate organisms are very small and appear as refractive points 

 with dark rims, and as a rule gradually become surrounded by a 

 small clear space, from which the magnesium carbonate is dis- 

 solved as growth and nitrification proceeds : examined with a 

 low-power microscope they appear granular. Remove colonies 

 or small portions with a fine capillary tube, obtained by melting 

 a piece of ordinary thin glass tube and drawing it out rapidly 

 when soft ; transfer to flasks containing the ammonium sulphate 

 solution (Ex. 74). The tip of the tube conveying the nitrite 

 organism can be easily broken off and left in the flask by pushing 

 against the bottom of the latter. Incubate the inoculated solution 

 at 30 C. and 



(1) test for nitrate at intervals; 



(2) examine it with the microscope and note the form, size, and 



motility of the organisms present ; stain some ; 



(3) test for the presence of foreign, non-nitrifying species by 



introducing some of the solution into bouillon : note if 

 the latter remains clear and without signs of growth for 

 twelve to fourteen days at least. 



The nitrite organisms stain with carbol-fuchsin and are 

 roundish or oval, 1.25 to 1.8 /A long and .9 to i p thick, some- 

 what irregular in size. 



Ex. 79. Stevens- Temple Silica Jelly. The following method 

 of preparing a " silica jelly " is due to Stevens and Temple, and 

 has the advantage of avoiding the process of dialysis, which in 

 the hands of beginners is troublesome and uncertain in its 

 results. The method leaves a certain amount of sodium chloride 

 in the "jelly," but this has not been found to interfere with the 

 cultivation of nitrifying organisms. 



(a) Take 3 to 4 c.c. of a solution of sodium silicate (water 

 glass). Place in a porcelain dish and add 5 to 10 c.c. of con- 

 centrated hydrochloric acid to precipitate the silicic acid. 

 Evaporate to dryness : moisten again with hydrochloric acid 



