72 LABORATORY EXERCISES IN BACTERIOLOGY. 



distilled water) ; (c) a decinormal solution of hydrochloric acid (3.65 grams to 1000 cubic 

 centimeters distilled water) ; (d) a normal solution of hydrochloric acid (36.5 grams to 

 1000 cubic centimeters distilled water) ; (e) a 0.5 per cent, solution of phenolphthalein 

 in fifty per cent, alcohol. In the reaction-correction of a given medium (supposing it 

 to be excessively acid), the volume is first to be corrected to a definite amount. This is 

 most readily done by correction of weight, the weight of a given volume of the medium 

 having been previously determined by practice or calculation for the temperature of the 

 room (20 C.). If the weight be less than it should be, water is to be added to the re- 

 quired amount ; if excessive, the excess is to be evaporated by boiling. This attended 

 to, five cubic centimeters (or the equivalent weight, preferably) are withdrawn, forty- 

 five cubic centimeters of distilled water added, and this boiled for several minutes 

 in a porcelain evaporating dish to expel any carbon dioxide present. One cubic centi- 

 meter of the phenolphthalein solution is now added, and the mixture titered while hot 

 with the decinormal sodium hydroxide solution from a burette provided with a glass 

 stop-cock. The alkaline solution is added slowly, drop by drop, with constant stirring, 

 until a distinct pink color results. The number of cubic centimeters used of the deci- 

 normal sodium hydroxide solution is now read from the burette markings, indicating 

 the quantity of solution of this strength necessary for the complete neutralization of five 

 cubic centimeters of the medium. If there were originally 1000 cubic centimeters of the 

 medium, 995 cubic centimeters would remain; and for the complete neutralization of 

 this amount the quantity of decinormal sodium solution can readily be calculated. 

 Addition of so large an amount of fluid as this would entail would, however, be compli- 

 cating in its effect ; and to avoid this, one-tenth the same quantity of a normal solution 

 is substituted. However, the adopted standard is not the absolute neutral point, but 

 a reaction of such acidity as to require for the complete neutralization 1.5 cubic centi- 

 meters of normal sodium hydroxide .solution for each one hundred cubic centi- 

 meters of the remaining medium. Therefore, from the total quantity of the normal 

 alkaline solution calculated to be required for the complete neutralization of the remain- 

 ing 995 cubic centimeters of the medium are to be deducted 14.9 (9.95 by 1.5) cubic 

 centimeters; the remaining number of cubic centimeters of the normal alkaline 

 solution are then to be added and diffused to render the medium of the standard (+1.5) 

 reaction. If the medium was originally alkaline, on the addition of the phenolphthalein 

 it should have been titered in the same way with a decinormal solution of hydrochloric 

 acid until the discharge of its pink color. The number of cubic centimeters thus 

 used is employed as basis for calculation of the quantity of the same strength acid solu- 

 tion necessary for the neutralization of the remaining medium. One-tenth this amount, 

 plus 1.5 cubic centimeters additional for each one hundred cubic centimeters of the 

 remaining medium, is added in the form of the normal solution of the acid in order to 

 produce the standard reaction. 



I. CARBOHYDRATE MEDIA. 



i . Potatoes. These vegetables are most frequently employed as a culture medium 

 for the chromogenic bacteria and for a few other organisms, as those of typhoid fever and 

 of glanders, which produce rather characteristic growths upon them. Their preparation 

 is simple, consisting merely of proper cleansing, cooking, and sterilization, inoculations 

 being made upon the cut surface of the vegetable. It is not customary to modify the 

 natural (acid) reaction of the potato in such use. They may be employed either in dish 

 or tube cultures, preferably the latter. 



