112 BACTERIOLOGY 



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 burning at the bottom. 



It is now almost the universal practice to use enamelled 

 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 inter- 

 position of several layers of wire gauze, a thin sheet of asbes- 

 tos-board, or an ordinary 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. 



To Fold a Filter. — For the filtration of such substances 

 as gelatin and agar-agar it is of mportance 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 1' to 3 the fold 6 is pro- 

 duced; and by bringing 1 to 3 and 1' to 7 the folds 2 and 8 

 result. 



Thus far the ridges of all folds are on the side of the paper 



