46 CULTURE MEDIA. 
According to Abbott the filtration of agar-agar does not require 
the use of a hot-water funnel or any other device for maintaining the 
temperature of the mass. He gives the following directions for its 
préparation: 
‘‘Prepare the bouillon in the usual way. Agar-agar reacts neutral or 
very slightly alkaline, so that the bouillon may be neutralized before the 
agar-agar is added. Then add finely chopped or powdered agar-agar in the 
proportion of one'to 1.5 per cent. Place the mixture in a porcelain-lined iron 
vessel, and on one side of the vessel make a mark at the height at which the 
level of the fluid stands. If a litre of medium is being made, add about two 
hundred and fifty to three hundred cubic centimetres more water, and allow 
the mass to boil slowly, occasionally stirring, over a free flame, from one and 
a half to two hours; or until the excess of water—t.e., the two hundred and 
fifty or three hundred cubic centimetres that were added—has evaporated. 
Care must be taken that the liquid does not boil over the sides of the vessel. 
From time to time observe if the fluid has fallen below its original level ; if 
it has, add water until its volume of one litre is restored. At the end of the 
time given remove the flame and place the vessel containing the mixture in a 
large dish of cold water ; stir the agar-agar continuously until it has cooled 
to about 68° to 70’ C., and then add the white of one egg which has been 
beaten up in about fifty cubic centimetres of water; or the ordinary dried al- 
bumen of commerce may be disso] ved in cold water in the proportion of about 
ten per cent and used—the results are equally as good as when eggs are em- 
ployed. Mix this carefully throughout the agar-agar and allow the mass to 
boil slowly for about another half-hour, observing all the while the level of the 
fluid, which should not fall below the litre mark. It is necessary to reduce the 
temperature of the mass to the point given, 68° to 70° C. ; otherwise the co- 
agulation of the albumen will occur suddenly in lumps and masses as soon 
as it is added, and its clearing action will not be uniform. The process of 
clarification with the egg is purely mechanical; the fine particles, which 
would otherwise pass through the pores of the filter, being taken up by the 
albumen as it coagulates and being retained in the coagula. At the end of 
one-half hour the boiling mass may be easily and quickly filtered through a 
heavy-folded paper filter at the room temperature.” * 
For special purposes various substances are added to the above- 
described solid and liquid media. A favorable addition for the 
growth of a considerable number of bacteria is from one to three per 
cent of glucose. The phosphorescent bacteria grow best in amedium 
containing two to three per cent of sodium chloride. The addition 
of three to four per cent of potassium nitrate is made in conducting 
experiments designed to test the reducing power of certain bacteria, 
by which this salt is decomposed with the production of nitrites. 
Acids are also added in various proportion to test the ability of 
bacteria under investigation to grow in an acid medium. From 
1: 2,000 to 1:500 of hydrochloric acid may be used for this purpose. 
The addition of [itinus to milk or other culture media is fre- 
quently resorted to for the purpose of ascertaining whether acids or 
alkalies are developed during the growth of bacteria under investi- 
gation. The addition of aniline colors which are variously changed 
by the products of growth of certain species has also been resorted 
to in the differentiation of species. Various disinfecting agents, such 
' Abbott’s “Principles of Bacteriology ” Fifth edition. pp. 100 and 101, 
