42 CULTURE MEDIA. 



ing and sterilizing in as short a time as is practicable. The French 

 gelatin used comes in thin sheets ; this is broken up and added to 

 the flesh-peptone solution. 



Usually we prepare a litre of nutrient gelatin at one time, and for 

 this quantity one hundred grammes of gelatin will be required for the 

 standard preparation (ten per cent). It is well to allow it to soak for 

 a time in the liquid before applying heat for the purpose of dissolving 

 it. Then apply gentle heat until it is completely dissolved. The gela- 

 tin of commerce usually has an acid reaction, and it will be necessary 

 to carefully neutralize the medium after it has been added. A slightly 

 alkaline reaction is usually no disadvantage, but certain pathogenic 

 bacteria will not grow when there is a trace of acid present. The 



FIG. 17. 



next step consists in clarifying the nutrient medium. It is allowed 

 to cool to about 50 C., and an egg, previously broken into one 

 hundred grammes of water, is gradually added while stirring the 

 liquid with a glass rod. A whole egg is used for a litre of the solu- 

 tion. Heat is again applied and the solution is kept at the boiling 

 point for about ten minutes, during which time the egg albumen is 

 precipitated apd carries down with it all insoluble particles, which 

 without this clarifying process would have interfered with the trans- 

 parency of the medium, even when carefully filtered. The hot 

 solution is then filtered, A hot- water funnel (Fig. 17) is usually 

 employed, as the gelatin solution does not pass through filtering 

 paper very rapidly, and when cooled to near the point of solidifying 

 ceases to pass. 



