NUTRIENT GELATIN. 103 



cally all free acid has been carefully washed. 1 The 

 gelatin is added in the proportion of 10 to 12 per cent. 

 Its complete solution may be accomplished either over 

 a water-bath, in the steam sterilizer, or over a free flame. 

 If the latter method be practised, care must be taken 

 that the mixture is constantly stirred to prevent burn- 

 ing at the bottom. 



It is now almost the universal practice to use enam- 

 elled iron saucepans, instead of glass vessels for the 

 purpose of making both gelatin and agar-agar ; by this 

 means the free flame may be employed without danger 

 of breaking the vessel, and, with a little care, without 

 burning the media. Under any conditions it is better 

 to protect the bottom of the vessel from the direct 

 action of the flame by the interposition of several layers 

 of wire gauze, a thin sheet of asbestos-board, or an ordi- 

 nary cast-iron stove-plate. 



When the gelatin is completely melted it may be 

 filtered through a folded paper filter supported on an 

 ordinary funnel ; if solution is complete, this should be 

 very quickly accomplished. 



For the filtration of such substances as gelatin and 

 agar-agar it is of importance to have a properly folded 

 filter. Inability to fold a filter properly is so common 

 with beginners that a detailed description of the steps 

 may not be out of place. To fold a filter correctly, 

 proceed as follows : a circular piece of filter paper is 

 folded exactly through its centre, forming the fold 1, 

 1' (Fig. 16) ; the end 1 is then folded over to 1', form- 

 ing the fold 5; 1 and 1' are each then brought to 5, 

 thus forming the folds 3 and 7 ; 1 is then carried to 



i Hesteberg's acid-free, gold label gelatin has given us entire satis- 

 faction in this respect. It is an imported article. 



