84 LABORATORY EXERCISES IN BACTERIOLOGY, 



tion and filtration the bouillon will be rendered suitable for the preparation of these 

 special media. 



Exercise 26. Of the bouillon made by the student in the preceding 

 exercise, after testing with Fehling's solution to determine that no sugar 

 is present, withdraw three portions of one hundred cubic centimeters 

 each, the amount retained for the preparation of the sugar bouillons. 

 (If these amounts be withdrawn from a general stock, the remaining por- 

 tion is to be resterilized.) Each of these portions is to be placed in a 

 clean beaker or flask, and one gram of glucose added to the first and a 

 like amount respectively of lactose and saccharose to the second and third. 

 Aided by gentle warming and stirring with a clean glass rod, these sub- 

 stances are dissolved, and the product placed in fermentation tubes or 

 retained in the flasks, and sterilized. 



DISTRIBUTION OF LIQUID MEDIA TO TUBES, ETC. 



The distribution of fresh blood-serum, bouillon, peptone solution, and other 

 nutrients liquid at ordinary temperatures, and of gelatine and agar preparations 

 after liquefaction by heat, is performed in the same way whether the destination of 

 the material be to tubes, flasks, or dishes. In case of blood-serum intended for the 

 preparation of a clear jelly, the utmost care must be taken to avoid contamination 

 by microorganisms, since in the further steps of its preparation it will not be possible 

 to depend certainly upon the low degree of heat alone permissible for sterilization. 

 In case of gelatine, too, inasmuch as this substance, if overheated or heated too fre- 

 quently or too long, is liable to alterations and loss of its power of setting, caution 

 should be exercised to prevent infection of the finished preparation. With the other 

 forms of nutrient media, including blood-serum intended for the opaque white solid 

 serum, there need not be particular care in the process. When small quantities are 

 to be distributed, probably the most convenient method will be found in the withdrawal 

 of the liquid or liquefied medium from the stock-flask by means of a pipette of fifty 

 to one hundred cubic centimeters capacity (if convenient the pipette should be sterile, 

 but this is not essential). It is convenient here that an assistant should hold the 

 tubes, withdraw the cotton stoppers for introduction of the medium, and thereafter 

 replace the same in the tubes. However, a stand can easily be made by boring holes 

 of suitable diameter and depth into a block of wood, to serve as a stand for the tubes 

 in the process, which can then be readily conducted by one person. In introducing 

 the medium into the tubes it should not be permitted to come in contact with the 

 upper part of the inner surface, lest after the application of the cotton stopper the 

 latter may be contaminated and perhaps glued to the glass. 



A convenient method is to distribute from a separation funnel (or an ordinary 

 funnel covered by a plate to prevent contamination, as shown in figure 24), a rubber 

 tube fitted with a pinch-cock connecting a glass pipette to the lower end of the funnel. 



If care is taken there is no objection to pouring the medium from the flask into 

 the tubes, first having flamed the lip of the flask to destroy bacteria which are apt 

 to be lodged upon it. 



Where great caution is to be observed, as in the distribution of fresh blood-serum, 

 siphonage is to be recommended, the siphon arranged as shown in the accompanying 



