REAGENTS AND PROCESSES 551 



Methyl -violet. Starch-grains may be stained by treatment with a dark aqueous 

 solution of methyl-violet. If the starch-grains after staining are treated with a 

 very dilute solution of calcium nitrate, the stain becomes deposited particularly 

 in the less dense layers of the grains. Useful as a stain for elaioplasts. See under 

 this head in the next chapter, page 567. See also in this chapter under Staining 

 Intra Vitam. 



Millon's Reagent. This should be prepared fresh, as needed, by dissolving 

 mercury in an equal weight of nitric acid and then diluting this solution with an 

 equal weight of distilled water. Proteids are colored a brick-red with this reagent. 

 Sections to be tested are to be mounted in a drop of the reagent on a glass slip. 

 Warming the slip hastens the reaction. 



a-Naphthol. When sections which are rather thick are treated on a slide with 

 a drop of a 20 per cent, aqueous solution of a-naphthol and then two or three drops 

 of concentrated sulphuric acid are added, the sections will be colored violet in a few 

 minutes if cane-sugar, milk-sugar, glucose, laevulose, maltose, or inulin is present. 



Nessler's Reagent. Used as a test for the presence of ammonium. Prepared 

 by dissolving 2 Gm. of potassium iodide in 5 mils of water, and then adding mer- 

 curic iodide to the solution while warm until a part remains undissolved. After 

 cooling, 20 mils of water are added to the solution and then, after standing for a 

 time, the solution is filtered, and to each 20 mils of it are added 30 mils of a concen- 

 trated solution of caustic potash. The solution must be filtered as often as it 

 becomes turbid. The solution is changed to a yellow color in the presence of 

 ammonia. However, other organic compounds may give the same reaction. 



Nutrient Media. Nutrient media must be sterilized by heat to keep them from 

 spoiling and to make it possible to grow in them pure cultures that is, cultures of 

 organisms of any desired species. Sterilization may be accomplished by steaming 

 the medium for about twenty minutes each day on three days in succession, after 

 having poured it into test-tubes or flasks which have previously been tightly plugged 

 with cotton rolled into the form of a stopper of the proper size and baked in an 

 oven until the cotton is slightly scorched. The tubes and cotton plugs should be 

 baked together. Or, if an autoclave is available in which steam can be generated 

 under pressure, and accordingly at a higher temperature than that of boiling water 

 at ordinary atmospheric pressure, the cotton plugs and tubes, or flasks, will not 

 need to be baked but may be sterilized, together with the nutrient medium al- 

 ready poured into them, by subjecting them for fifteen minutes to a temperature 

 of H5C. in the autoclave. At this temperature a single sterilization suffices. 



A good artificial nutrient medium for years is made by adding 0.05 per cent, of 

 tartaric acid to a 10 per cent, solution of cane-sugar. A filtered aqueous extract 

 of malted barley also gives good results. To prepare this, barley is germinated 

 until the plumule just begins to protrude; the barley is then dried and ground up, 

 and water is poured over it until there is about twice as much water by volume as 

 of the powdered malt. The water should stand over the malt, with, occasional 

 stirring, for about an hour, when it may be filtered off and sterilized. Sterilized 

 grape juice is also an excellent nutrient medium for yeasts. Cultures of yeasts 

 grown in the above media may be made to produce spores in about twenty -four 

 hours if some of the culture is transferred to the surface of sterilized bits of flower- 

 pots which are half submerged in water and kept covered by a bell-jar. 



Cohn's normal solution for the culture of bacteria is prepared as follows: Dis- 

 solve in 200 Gm. of distilled water I Gm. of acid potassium phosphate, I Gm. of 

 magnesium sulphate, 2 Gm. of neutral ammonium tartrate, and o.i Gm. of calcium 

 chloride. 



An infusion of meat for the culture of bacteria is prepared by covering finely 



