110 BACTERIOLOGY. 



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 and consequent breaking of the flask, 

 if a flask is employed. 



It is now almost the universal practice to use enam- 

 elleled iron saucepans, instead of glass flasks, 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 much 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 cor- 

 rectly, 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', 

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

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

 the point 7, and the fold 4 is formed, and by carrying 

 V to 3 the fold 6 is produced ; and by bringing 1 to 3 

 and 1' to 7 the folds 2 and 8 result. 



