4t> Ct'LTl'KE 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 

 preparation : 



" 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 i.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 tlameand 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- 

 bumeiiof commerce maybe dissolved 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 a 11 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 

 cant of glucose. The phosphorescent bacteria grow best in a minlmm 

 containing two to three per cent of sodium chloride. The addition 

 of three to four per cent of itotitxxinin- n it rate 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 litmus 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 an if in c culorx which are variously changed 

 by the products of growth of certain spiv'u's has also been resorttnl 

 to in the differentiation of species. Various disinfecting agents, such 

 1 Abbott's "Principles of Bacteriology " Fifth edition, pp. 100 and 101. 



