4; CULTURE MEDIA. 



sphere of copper, supported upon a tripod, is so constructed that an 

 upper hemispherical segment can be removed to give access to the 

 interior. An opening at the bottom contains a perforated rubber 

 cork, through which the stem of an enamelled iron funnel passes. 

 A simple filter of filtering paper is used in this funnel, and this is 

 filled to a depth of two centimetres with well-burned kieselgur (dia- 

 tomaceous earth in which the organic matter has been destroyed by 

 heat). The hot solution of agar is poured into the funnel, and hot 

 water into the space between it and the copper vessel ; this must not 

 come too near the top of the funnel not nearer than three centi- 

 metres. The hemispherical cover is then secured in its place by 

 means of a clamp screw shown in the figure. By placing a Bunsen 

 burner under the projecting arm the water is made to boil and a 

 sufficient steam pressure secured. A small stopcock attached to the 

 cover of the copper vessel permits the escape of steam if the pressure 

 is too great. According to Unna, solutions containing as much as 

 three per cent of agar can be filtered by means of this apparatus, and 

 a litre of two-per-cent agar will pass through it in about two hours. 



Schultz' Rapid Method of Preparing Nutrient Agar- Agar. 

 Place one thousand five hundred cubic centimetres of water in an en- 

 amelled iron pot; add eighteen grammes of agar-agar, broken in small 

 pieces, and place upon a gas stove ; boil for half an hour ; add while 

 boiling two grammes of Liebig's extract of beef ; remove from fire and 

 cool to 60 C. ; then add ten grammes of dry peptone, five grammes 

 of sodium chloride, and the contents of one egg beaten up in a 

 sufficient quantity of water to supply that lost by evaporation ; neut- 

 ralize the mixture by the addition of dilute hydrochloric acid ; boil 

 again for five or ten minutes; filter through white filter paper. If 

 the filtrate is not entirely clear add to it the albumen of a second 

 egg and boil until this is coagulated ; then filter again. Ahvays mois- 

 ten the filter with water before filtering solutions containing 

 gelatin or agar-agar. When the process is completed the amount 

 of filtered culture medium should be about one thousand cubic centi- 

 metres. 



For serial purposes various substances are added to the above- 

 described solid and liquid media. A favorable addition for the 

 .u r n >wth of a considerable number of bacteria is from one to three per 

 cent of glucose. The phosphorescent bacteria grow best in a medium 

 < ontaining 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 



