46 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-ceiit agar will pass through it in about two hours. 



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 a medium 

 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 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 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 

 as carbolic acid, etc. , have also been used for the same purpose, and 

 it has been shown by experiment that some bacteria will grow in a 

 medium containing such agents in a proportion which would entirely 

 restrain the development of others. 



Quite recently the soluble silicates which form a jelly-like mass 

 have been proposed as a culture medium for certain bacteria which 

 do not grow in the usual media. Kiihne (1890), Winogradsky 

 (1891), and Sleskin (1891) have made experiments which indicate 

 that this medium has considerable value. 



