EVAPORATION AT LOW TEMPERATURES. 



iii fig. 58 is very convenient. The side tube is attached to the suction-pipe of an 

 air-pump and into the neck is thrust a rubber stopper carrying a thermometer and 

 a U-shaped glass tube of small bore, the outer arm (36 inches long) ending in a 

 beaker of mercury. Heat may be applied by means of a water-bath. By substitut- 

 ing a funnel for the thermometer the same device may be used to hasten the filtration 

 of thick liquids, hard-pointed filter papers being employed. 



SENSITIVENESS TO PLANT ACIDS. 



The tests should be made with malic, citric, lactic, oxalic, and tartaric acids 

 added to neutral beef-broth, peptone-water, or plant-broths, or to synthetic media 



(see Am. Nat., 1899, p. 208). It is best to titrate with or solutions, to acidify 



2N 4.N 

 with - or - solutions, and to reckon the acidity in cubic centimeters of normal 



N 



solution ( ) required per liter of medium. If pre- 

 ferred, it may be calculated on 100 cc. portions and 

 expressed in per cents, but there is no advantage in this, 

 and it has the disadvantage of introducing fractions. 



SENSITIVENESS TO ALKALIES (POTASSIUM OR SODIUM 

 HYDRATE). 



Determine in each case the optimum reaction of the 

 medium for growth. For the majority of bacteria this 

 is said to lie between +10 and + 15 of Fuller's scale.f 

 The best neutral litmus paper should be used freely, but 

 acid and alkaline media should be titrated with phenol- 



N N 



phthalein and or solutions. In some media 

 10 20 



e.g., gelatin, juices of various plants the end-reaction 

 with phenolphthalein and caustic soda is not very sharp. 

 In these cases the titration should be stopped at the first 

 trace of change of color. If one adds alkali until the 

 fluid is decidedly red, then a distinct statement to that 

 effect should be made, since otherwise no comparisons of 

 any value can be made. All of the writer's + and refer- 

 ences to media are based on a reaction stopped at the 

 first distinct trace of pink color. As much again alkali 

 must sometimes be added to obtain a deep-red color. 



*Fic. 60. Stock bottle of sodium hydrate solution. The small bottle at the right holds con- 

 centrated potash liquor to remove the carbon dioxide from the air which enters the bottle. About 

 one-fourth actual size. 



fThe plus and minus on Fuller's scale denotes, respectively, acid and alkaline media. The + ID, 

 for example, means that exactly 10 cubic centimeters of normal alkali must be added to a liter of 

 the culture medium to render it exactly neutral to phenolphthalein, and, correspondingly, 10 means 

 that the fluid is alkaline to phenolphthalein and that 10 cc. of normal acid would need to be added to 

 bring I liter back to the neutral point. The student should not confuse the litmus neutral point and 

 the phenolphthalein neutral point, as they are about 23 apart, e. g., + 10 of Fuller's scale (acid side) 

 is distinctly alkaline to litmus. (Consult '95, Fuller, Bibliog., XVI.) 



