86 



LABORATORY EXERCISES IN BACTERIOLOGY. 



diagram (Fig. 25). The principle of the ordinary chemical wash-bottle, too, may be 

 easily utilized for distribution from stock-flasks, the tube carrying the compressed 

 air into the flask being provided with a cotton plug to prevent the entrance of con- 

 taminating organisms, and force being applied from any convenient source, as a reversed 

 siphon, compressed air tank, carbonic oxide tank, or from the mouth (Fig. 26). Of 

 course, in all cases in which it is essential to prevent contamination the tube of the siphon 

 or other appliance introduced into the medium should have been properly sterilized. 



The distribution flasks devised by various workers, several of which are shown 

 in figure 17, are convenient, but by no means essential. 



In tubing it is customary to put about five cubic centimeters of medium into 



FIG. 24. DISTRIBUTION OF LIQUID MEDIUM 

 FROM COVERED FUNNEL TO CULTURE 

 TUBE. 



FIG. 25. 



each tube, approximation of this amount being sufficient for ordinary work. It is 

 well, however, to provide a number of tubes containing exact amounts, for special 

 purposes. For dilution experiments four or nine cubic centimeters will be found 

 convenient quantities. Tubes of the same size having been selected, the desired 

 number of centimeters of water are placed in one tube, and from the level of this 

 the other tubes are marked and the medium subsequently filled in to the mark upon 

 each. 



2. Peptone Gelatine ("Gelatine"). This medium consists of the ordinary 

 bouillon to which has been added from ten to fifteen per cent, of the best sheet gelatine 



