82 EXPERIMENTAL DAIRY BACTERIOLOGY 



In butter almost no sugar is present. Without it the 

 growth of the butyric-acid bacteria cannot occur. 



The butyric-acid bacteria form very resistant spores. Milk 

 that has been heated will frequently undergo an acid fermen- 

 tation, especially when incubated at 37C. Gas is usually 

 abundant. This acid fermentation of heated milk is due to 

 the spores of the butyric-acid bacteria that have not been 

 killed by the heating. Such milk usually has a very offen- 

 sive odor. 



In order to isolate the organisms in pure culture some 

 one of the many ways suggested for the removal of the 

 free oxygen from the culture vessel must be employed. The 

 oxygen can be absorbed by an alkaline solution of pyrogallic 

 acid, or the air may be replaced by an inert gas such as hy- 

 drogen. A combination of the two methods is preferable. 

 By exhausting the air from the vessel by means of a water 

 pump, and then filling the vessel with hydrogen and again 

 exhausting, the time required to remove the oxygen may be 

 shortened. 



The apparatus illustrated in Fig. 15 will be found very con- 

 venient. The Kipp hydrogen generator and two wash bottles, 

 one containing a 10 per cent solution of lead nitrate, the 

 other a solution of silver nitrate, are fastened to a base which 

 is placed on a shelf near a sink. Some form of a Bunsen 

 pump is placed on the wall above the sink. Brass tubing 

 connects the water pump with a mercury manometer and 

 with the gas generator by means of a Y-tube. The third limb 

 of the Y-tube is connected to the vessel in which the cultures 

 are to be placed. 



In the bottom of a wide-mouthed bottle is placed a layer of 

 sand. A solution of pyrogallic acid is poured upon the sand. 

 The bottle is closed by a rubber stopper provided with one 



