PREPARATION OF GELATIN 125 



Liebig's. It is used in the strength of from two to four 

 grams to the liter of water. Peptone and sodium chloride 

 are added as in the bouillon made from meat-infusion. 

 The advantages of meat extract are: it takes less time; 

 affords a solution of more uniform composition if used in 

 fixed proportions'; and in general use gives results that are 

 equally as satisfactory as those obtained from the employ- 

 ment of infusion of meat. The disadvantage is the possible 

 presence of antiseptics or preservatives. 



Nutrient Gelatin. For the preparation of gelatin the 

 bouillon is first made in the way given, except that its 

 reaction is corrected after the gelatin has been completely 

 dissolved, which occurs very rapidly in hot bouillon. The 

 reaction of the gelatin of commerce is frequently more or 

 less acid, so that a much larger amount of alkali is needed 

 for its neutralization than for other media. It is possible, 

 however, to obtain from the makers an excellent grade 

 of gelatin from which practically all free acid has been 

 carefully washed. The gelatin is added in the proportion 

 of 10 to 12 per cent. Its complete solution may be accom- 

 plished 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. 



