3 oo CLEAN 'MILK 



characters of the cultures which have been named, Bad. lactis acidi 

 and Bad. aerogenes, see No. 5. 



10. Fermentation Tubes. — Inoculate a series of fermen- 

 tation tubes (dextrose, lactose and saccharose) with different bac- 

 teria. Among those chosen should be one culture of Bad. lactis 

 acidi, and one of Bad. aerogenes. In two days, note whether gas 

 has been produced in the closed arms, and, by means of litmus paper, 

 whether the bouillon has become acid. If gas shows in the fer- 

 mentation tubes, determine the gas ratio as follows : 



Without disturbing the gas, fill up the bulb to the top with 

 a 2% NaOH. (By a mark, note the level of the gas in the glass 

 arm.) Place the thumb over the opening in such a way that there 

 will be no air bubble between the thumb and the surface of the 

 liquid. Now invert the tube, allowing the gas to flow out into the 

 bulb, and, by turning back and forth, mix the gas with the NaOH 

 solution in the bulb, keeping the thumb very tightly closed over the 

 opening. After thoroughly mixing, turn the tube once more so that 

 all of the gas will be in the closed arm. Remove the thumb ; it will 

 usually be found that the level of the gas in the closed arm rises. 

 If so, it will be due to the fact that C0 2 has been dissolved by the 

 NaOH solution. By determining the amount of the gas before and 

 after the test, the proportion of C0 2 to the gases not thus dissolved 

 may be obtained. This is called the gas ratio. Some gas-producing 

 species of bacteria produce large quantities of carbon dioxide, and 

 small quantities of other gases, while other species produce no carbon 

 dioxide. 



From one or two of the test tubes inoculated above, that show 

 an acidity, determine the amount of acid produced. For this pur- 

 pose remove from the tube either 5 or 10 c.c. of the liquid, dilute 

 it with 10 times its bulk of water, heat and titrate. 



11. To Determine Motility. Inoculate a little of the bac- 

 teria growth of No. 21 into a test tube of bouillon. Allow it to 

 grow for about 24 hours. By this time the liquid will become some- 



