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on the said medium. They gTOw on tlie a.sar as tliiii. sno\v-white 

 or rosy-tinted, very dry, flatly extended layers, which strongly remind 

 of the pellicle floating on the liquid. 



Also on silica plates, prepared in glass dishes, whicii, after extraction 

 of the chlorides are soaked with a nutrient solution, ^. ö/iV/ocrtröoy>A//?/,y 

 can produce very fine cultures, appearing after some weeks, as 

 snowwhite colonies with indented margin, and which by a right 

 selection of the salts, can finally spread over tlie whole plate. 

 Tlien tlie remarkable phenomenon is observed, that the silica liquefies 

 a little in the centre of the colonies and sinks in by evaporation. 



The silica plates are made as follows. A commercial solution of 

 potassium silicate, diluted with a known quantity of water, is titrated 

 with normal hydrochloric acid. As the solidification is much favoured 

 by an alkaline reaction, a complete neutralisation at the preparation 

 of the plate should not occur, and as a plate, with a high jiercent- 

 age of silica, contracts strongly after coagulation, and expresses much 

 water, the dilution must be sufficient for this contraction to be delayed. 

 Into a small beaker-glass was introduced, in a certain case, 5 l-W 

 of }»otassium silicate diluted with 25 cM'^ of water, and into a 

 second glass the required quantity of hydrochloric acid, amounting 

 to 10 cM'' of normal acid. The acid is mixed with the diluted 

 silicate and the mixture poured into a glass dish. The solidification 

 delays the longer as the mass is more diluted, but it is easy, after 

 some i)ractice, to make very solid plates. Tlie plate is first freed 

 from the chlorides l)y streaming tap-water, then washed out with 

 boiled water, and afterwards treated ^^ith the solution of nutrient 

 salts. When these have sufficiently diffused into the plate, the glass 

 dish is gently Avarmed at the underside, until the adhering water 

 has evaporated and the plate shows a "dry", glossy surface. The 

 surface is flamed in the BuNSEN-burner, by which only a partly but 

 sufficient sterilisation is to be attained. 



Not only B. oligocarhophilm-, but also the ferments of nitrification 

 grow on this medium very w^ell. By mixing of the diluted solution 

 of the silicate with chalk, magnesium carbonate, or ammonium- 

 magnesium phosphate, snow-white jilates may be obtained, whicli 

 are particularly fit for the culture as well of all these microbes as 

 of several lower algae. Even earth-diatoms, of the genus Nitzschia 

 will grow thereon. 



Once more it must be observed, that in the silica plates organic 

 substances must be absent, even fragments of cork, fallen into the 

 silicate solution, may disturb the experiment. 



The pure cultures, obtained on agar or silica plates, are as well fit 



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