DETERMINATION OF GAS PRODUCTION. 28l 



are studied. The longer the plates grow the better the 

 differentiation which is obtained. 



If it is desired to study the different bacteria further, sam- 

 ples of the different types of colonies are to be picked out 

 of the plates with a platinum needle and inoculated into agar 

 or gelatin to be subsequently studied by bacteriological 

 methods. The description of these methods does not lie 

 within the scope of this work. 



To Determine the Fermentative Power (Gas Production) 

 of Bacteria. Fill a considerable number of fermentation 

 tubes (Fig. 42) with common bouillon used for bacteriolog- 

 ical work, but containing 3% milk sugar. The closed arm 

 of the tube is to be filled full as shown at Fig. 42, A, and the 

 tubes are then to be sterilized as usual by steaming. To 

 determine whether a bacterium produces a fermentation of 

 sugar and develops gas the bacterium from a pure culture or 

 from a colony growing upon a petri dish, is inoculated into 

 the bouillon in the fermentation tube. If it produces gas by 

 fermenting the sugar gas will be found to collect at the top 

 of the closed arm in a day or two as shown at Fig. 42, B. 

 If no gas appears in the closed arm it indicates that the bac- 

 terium is not a gas-producer. It is always to be remem- 

 bered that gas-producing bact^ia are troublesome in the 

 dairy, and this test of gas production is, therefore, sometimes 

 of great importance. The useful lactic bacteria of the dairy 

 should not produce gas when subjected to this test. 



PRACTICAL VALUE OF A BACTERIOLOGICAL ANALYSIS. 



A bacteriological examination of the milk promises to be 

 of practical value, at least in two directions. It has already 

 proved itself to be "a useful test in some of the modern high- 

 class dairies where the dairyman is interested in obtaining the 

 very best product. These high-class dairies make a frequent 

 bacteriological analysis of their milk, and thus keep a general 

 oversight of the bacterial contamination. By carefully watch- 



