GENERAL BACTERIOLOGY 81 



stopper. Incubate the gelatin cultures at room tem- 

 perature, all other cultures at 25 or 37 C. in the incu- 

 bator. Make descriptions after 24 hours, 48 hours, 

 and 6 days, as outlined in Section 9. 



EXERCISE 5 



Select two species of yeasts and inoculate three 

 fermentation tubes with each species, using the looped 

 needle, so as to have each species act on the three differ- 

 ent sugars. Measure the gas evolved after 24 hours 

 and after 48 hours by means of Frost's fermentation 

 chart or gasometer (back cover). The chart is to be 

 placed between the open arm and the bulb and moved 

 until the extreme upper end of the closed arm is level 

 with the top of the chart and parallel with the vertical 

 lines on the chart. Express the results in percentages 

 as read from the gasometer. 



Gas production is not a constant accompaniment 

 of fermentations. Carbohydrates are fermented by 

 many organisms without gas formation, the usual 

 product being an acid, often lactic acid. Such fer- 

 mentations produce turbidity, but no gas. Growth 

 therefore must be described and can be observed by 

 turbidity, forming either in the closed arm, the bulb, 

 or both. Observations are to be noted in the space 

 for this purpose on the first page of the description 

 charts. 



Analysis of gas produced in the closed arm 



The gas consists chiefly of carbon dioxid and hydro- 

 gen, as may be proved by the following method: Fill 

 the bulb with a 2 per cent solution of NaOH, and close 

 the mouth with the thumb, taking care not to leave any 



