130 METHODS OF PUEE CULTURE. 



is used. Moreover, these agar-agar media do not lose their power 

 of solidification if stored for a long time at 100 to 120 C. They 

 are liquefied only at temperatures exceeding 40 C., and since this 

 last-named temperature is for many organisms the highest support- 

 able maximum, the agar-agar is used in the following way when 

 designed for the separation of a bacterial mixture : The recipient 

 tubes are immersed in boiling water to induce liquefaction of the 

 contents, which are then cooled down to 40 C. (at which tempera- 

 ture they are still just fluid), inoculated quickly, shaken up and 

 mixed thoroughly, and poured out on to the aforesaid glass plates, 

 which rest on a support warmed to 40 C. 



For pure cultures at temperatures above 50 C. agar-agar 

 cannot be used, since it then begins to soften. For such (rare) 

 cases, Miquel, when experimenting with Bacillus thermophihis, 

 replaced agar-agar in the nutrient solution by 2.5-3.0 per cent, 

 of Caragheen moss (Irish moss), from Chondrus crispus. For 

 special purposes suitable indicators are also added to the nutrient 

 media. For example, if it is desired to separate merely the acid- 

 forming species from a bacterial mixture, then a little litmus is 

 added to the medium before sterilising; the colonies of acid- 

 forming bacteria in the subsequent plate culture will then become 

 surrounded by a red halo standing out conspicuously against the 

 blue background. For the same purpose BEYERINCK (V. ) recom- 

 mended an addition of fine levigated chalk, which forms an opaque 

 chalk nutrient medium, becoming, however, clear at the parts of 

 the plate culture occupied by acid-forming bacteria, in consequence 

 of their solvent action on the calcium carbonate. 



Certain organisms, such, for example, as the nitrifying bacteria, 

 do not thrive in the solidified nutrient media hitherto described. 

 Therefore, in order to prepare pure cultures of the same by the 

 aid of the plate method, recourse is had to the medium prepared 

 from precipitated silica, proposed by W. KUHNE (I.). Silica pre- 

 cipitated from water-glass (alkali silicate) and carefully purified 

 will, when used as a 3.4 per cent, aqueous solution, set within an 

 hour to a firm mass if mixed with 0.25 per cent, of Nad. The 

 salt is added to a sterilised solution which also contains the other 

 requisite nutrient substances. In this solution is distributed a 

 small portion of the bacterial sample to be separated, the cells of 

 which will, when the medium has set, be fixed and develop into 

 colonies. Further particulars concerning the preparation of this 

 silica medium will be found in the above-mentioned treatise, as 

 also in one by Winogradsky which will be referred to later. 



The number of nutrient media employed in practical mycology 

 is very large, and are more fully described in the handbooks of 

 Hueppe, Eisenberg, Tiemann-Gartner, and Bernheim, but only 

 one need be briefly noticed, viz., the potatoes employed for the 

 so-called potato cultures. The potatoes the better sorts used 

 (in Germany) for salad-making after being carefully cleaned 



