and Laboratory Methods. 



2523 



changed at all or will be turned only to a slightly deeper blue. The process of 

 neutralization should be carefully performed. Care must be taken to stir 

 thoroughly after each addition of soda. Only a small strip of litmus paper need 

 be used, since the portion wetted each time can be easily torn off and the 

 remainder used again. 



After neutalization the gelatine is boiled for a few moments and then cooled 

 down to 60° C. (140° F.) and an egg is added. If beef was used it will not be 

 necessary to add the egg, but if the extract was used it is best at this point to 

 beat an egg with a little water, add it to the gelatine and, after thoroughly mix- 

 ing, heat as hot as possible in the double dish and continue until the egg is 

 thoroughly coagulated, i. e., cooked into a solid mass. It is best not to stir 

 during the cooking. 



When the egg is thoroughly coagulated the gelatine is to be filtered through 

 a little absorbent cotton (ordinary cotton will not do) in the neck of a funnel as 

 shown in Fig. 18. The filtration will be quite rapid 

 if the egg was thoroughly cooked, otherwise it will 

 be slow, sometimes distressingly so. In the latter 

 case it must be kept in a warm place, as in a steam- 

 bath, until the filtration is complete. When the 

 gelatine is filtered it should be quite transparent. 

 The clearing and filtration require patience. They 

 may be omitted, but in that case the gelatine will 

 be more or less opaque and unsatisfactory. 



One of the most important things to remember 

 in the preparation of gelatine is that the continued 

 application of heat injures its solidifying powers, fig 18.-Apparatus for filtering 

 and that if the heat is applied too long it will not media through absorbent cot 

 harden on cooling. The greatest care, then, must 

 be exercised to heat hot while necessary and then 

 stop as soon as the purpose of the heating has been 

 accomplished. 



After the gelatine has been filtered it should be distributed into test-tubes or 

 bottles, if the laboratory is supplied with test-tubes and Petri dishes (see Fig. 

 6). In other cases the gelatine is best put in iiat bottles, such as those shown 

 in Fig. 5. The amount to be put in each tube or bottle is such that it will form 

 a layer about an eighth of an inch deep in the Petri dish or on the side of the 

 bottle. The glassware cannot, however, be used for this purpose until it has 

 first been sterilized. And before sterilization it must be plugged with cotton. 

 For this purpose ordinary cotton wool can be used, but absorbent cotton is 

 better. The cotton plugs should be rolled so that they will be compact, and so 

 made that when in place they will leave no creases for the entrance of dust. 

 They should not be put in too tight, only sufficiently so to prevent their being 

 readily pulled out. The general rule is tight enough to support the weight of 

 the vessel and its contents. Glassware is sterilized in the hot-air sterilizer at 

 150° C. for one hour, or where the thermometer is not available the heat is 

 applied until the cotton begins to turn brown, i. e., to scorch. At this point 



ton : d, layer of cotton ; /i, 

 tubes for making connection 

 with air pump ; c, Bunsen valve 

 to prevent entrance of water 

 into flasks. 



