86 TEXT-BOOK OF BACTERIOLOGY. 



This precaution should not be neglected, for to obtain a good 

 medium, perfectly germ-free, it must be carried out to the letter. 

 This interrupted heating, which reminds us of the discontinuous 

 sterilization, is also the best method of killing the spores which 

 possess the most tenacious vitality, since they germinate in the 

 intervals and are then sure to perish at the next heating. 



When the tubes have been sterilized three times they may be 

 used. 



The kind of gelatin whose preparation has just been described 

 in detail, and which is chiefly employed on account of its composi- 

 tion, is called " ten-per-cent meat- water peptone gelatin," and when 

 in future nutrient gelatin is spoken of, it is always this mixture that 

 is meant. But of course it is possible to employ a variety of com- 

 pounds differing from the above 



On the one hand, the quantity of gelatin put into the different 

 food solutions may be largely varied, and some, indeed, prefer to 

 work with a 7-J$ or 5$ mixture. Still further diminution of the 

 quantity of gelatin is not advisable, since we then have something 

 approaching too nearly the liquid foods, which softens with a slight 

 degree of heat and does not offer sufficient resistance to the pep- 

 tonizing influence of many kinds of bacteria. 



On the other hand, gelatin may be mixed with some other nutri- 

 ent solution instead of with the usual meat-bouillon. Some, for in- 

 stance, do not employ fresh meat for their bouillon, but take beef 

 extract. The following is a recipe : 1,000 grams water, 30 grams 

 peptone, 5 grams extract of meat, 100 grams gelatin. The sterili- 

 zation here requires particular care, since the extract of meat is 

 extremely rich in very tenacious germs. 



Besides the different varieties of meat-bouillon, other kinds of nu- 

 trient solution as, for instance, blood-serum, milk, spices, and urine 

 have been consolidated by the addition of gelatin. It would lead 

 too far to go into all the particulars concerning them, yet a few 

 words must be written about a number of special admixtures em- 

 ployed for particular purposes, and which are usually added to the 

 bouillon gelatin when it is fully prepared and ready to be poured 

 into the test-tubes. 



Thus, for instance, 4 to 6$ of glycerin is added to the gelatin, 

 because it has been found that certain bacteria thrive better with 

 this addition. An increase of nourishing power may also be ob- 

 tained by adding from | to 2# of grape sugar (dextrose), which is, 

 indeed, frequently done. 



But the grape sugar has another valuable property : it is a re- 

 ducing substance which partly absorbs the oxygen, and thus ren- 



