74 TEXT-BOOK OF BACTERIOLOGY. 



flame or in Koch's steam generator. Then follows the neutraliza- 

 tion of the liquid (which is as a rule strongly acid) by the cautious 

 addition of a saturated solution of carbonate of soda, changing- its 

 reaction till a drop taken out with a glass rod no longer reddens 

 blue litmus paper, but gives a slightly blue tinge to red test paper. 



After this is done the liquid is boiled for about an hour longer. 

 The coagulable albuminous substances are now curdled, one part 

 floating on the surface of the clear broth as an opaque conglobated 

 scum, the rest lying as a solid mass at the bottom. When the 

 liquid is cool pour it through a filter which has been moistened 

 with distilled water, and let the clear, almost colorless bouillon run 

 out below. After this filtration it must still be alkaline, or at least 

 neutral, and must not become in the least turbid when repeatedly 

 boiled. 



If it fails to answer these requirements, the faults must be rec- 

 tified and the filtration repeated. 



If, in spite of all efforts, the clearing remains unsatisfactory, the 

 white of a hen's egg should be added to the filtered solution. If we 

 boil it again for half an hour, the white of egg coagulates and car- 

 ries off with it the slight turbidity that was present. 



When the bouillon is thus satisfactorily perfected, pour into 

 well-sterilized test-tubes, Erlenmeyer's small flasks, etc., about 10 

 cm. each, and provide each with a good plug of ordinary cotton- 

 wool. But before proceeding further the food solution must be 

 made sterile, and with the bouillon this requires special care, since 

 it possesses, in common with all liquid foods, the disadvantage of 

 presenting but little opposition to the invasion and unlimited 

 growth of bacterial strangers. It is, therefore, best to expose the 

 vessels, with their contents, for about an hour to the sterilizing 

 action of the current of steam in the generator, and to make quite 

 sure it is best to repeat the operation once more on the following day. 



The bouillon is then ready for use and forms a very valuable, 

 often useful, form of nourishment, originally intended for the breed- 

 ing of parasitical bacteria. It is also very suitable for the sapro- 

 phytic species, and as yet we know but few micro-organisms which 

 are able to thrive on other culture media and not in bouillon. 



It is necessary in some cases to increase its nourishing power by 

 certain additions. Thus the supplement of 1 or 2$ of grape sugar 

 or from 3 to 5$ of glycerin has sometimes proved advantageous. 



The object of all our bouillons and artificial foods is to imitate 

 the natural nourishing media. If these in particular cases "have 

 their peculiar nature, we must note the fact and proceed accord- 

 ingly in our experiments. 



