EVAPORATION AT LOW TEMPERATURES. 69 
in fig. 58 is very convenient. The side tube is attached to the suction-pipe of an 
ait-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 x solutions, to acidify 
with 2N or = solutions, and to reckon the acidity in cubic centimeters of normal 
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.+ 
The best neutral litmus paper should be used freely, but 
acid and alkaline media should be titrated with phenol- 
phthalein and = or x solutions. In some media— 
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. 
+The plus and minus on Fuller’s scale denotes, respectively, acid and alkaline media. The + 10, 
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 1 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.) 
