296 PRACTICAL DAIRY BACTERIOLOGY 



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 CO 2 has been dissolved by the NaOH solution. By 

 determining the amount of the gas before and after the test, the 

 proportion of CO 2 to the gases not thus dissolved may be ob- 

 tained. 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 

 purpose 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, as 

 described in experiment No. 4, b. 



No. 31. To Determine Motility. Inoculate a little of the bac- 

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



I 23456 



FIG. 86 TYPES OF SURFACE ELEVATION 



1. Flat. 2. Raised. 3. Convex. 4. Pulvinate. 5. Capitate. 6. Urabilicite. 

 7. Umbonate 



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

 somewhat cloudy. Transfer a loopful of this material to a glass 

 slide and place upon it a cover-glass. Place the whole under 

 microscope, preferably using a i/6-inch objective, and narrow 

 the diaphragm below the stage until most of the light has 

 been cut off. Now focus very carefully upon the bacteria 

 under the cover-glass, and, if they are motile, it will be seen 

 that they are moving around through the microscope field with 

 more or less rapidity. If they are stationary, it may be assumed 

 that they have no flagilla. This test requires great care, for 



