THE STUDY OF YEASTS 119 



Sodium carbonate and water are formed, leaving the other 

 gases free. 



Collect in the long arm of the tube all the gases remain- 

 ing. Remove the thumb. The difference in the per cent 

 of gas before and after treating with the NaOH equals the 

 per cent of CCb which was present. 



8. Place the thumb over the mouth of the tube and 

 collect all remaining gas in the short arm. Light a match, 

 remove thumb and immediately touch off the remaining gas. 

 If Eb is present the typical reaction occurs. Other gases 

 are often present, but in too small amounts to allow of 

 testing. 



10. Record the relative proportions of C02 and H2 

 formed. 



B. Durham's Fermentation Tube 



Durham's fermentation tube is simply an ordinary test 

 tube containing a sugar broth, in which a smaller test tube, 

 inverted, has been placed before sterilization. 



This apparatus possesses some advantages over the 

 Smith fermentation tube if only the presence of gas produc- 

 tion is to be noted, as the tubes are more easily cleaned, 

 sterilized and handled. 



The amount of gas may be roughly estimated, but the 

 kind of gas may not be determined by the use of this 

 apparatus, 



EXERCISE 42. THE STUDY OF YEASTS 



The object of this exercise is to demonstrate how 

 to differentiate yeasts by microscopical and cultural 

 methods. 



Apparatus. Clean cover-glasses; three clean concave 

 slides; five clean fermentation tubes; one tube sterile 

 2% dextrose broth;* one tube sterile 2% lactose broth; 



* The sugar and glycerin broths are furnished by the laboratory. 



