214 BACTERIOLOGY 



bubbles. Remove the slide from the lower surface of the 

 block, and invert the cover-slip so that the agar-block is 

 uppermost. With a platinum loop run a drop or two of 

 melted agar along each side of the agar block where it is 

 in contact with the cover-slip. This seal hardens at once, 

 preventing slipping of the block. Place the preparation in 

 the incubator again for five or ten minutes to dry the agar 

 seal. Invert this preparation over a moist chamber and seal 

 the cover-slip in place with white wax or paraffin. Vaseline 

 softens too readily at 37 C., allowing shifting of the cover- 

 slip. The preparation may then be examined at leisure." 



Aerobic bacteria receive sufficient oxygen by diffusion, 

 and for anaerobic bacteria it will suffice to hang the block 

 in a chamber containing a little alkaline pyrogallic acid solu- 

 tion. This absorbs all oxygen. 



Study of <Jelatin Cultures. As has been previously stated, 

 the behavior of bacteria toward gelatin differs some of 

 them producing apparently no alteration in the medium, 

 while the growth of others is accompanied by an enzymotic 

 action that results in liquefaction of the gelatin at and 

 around the place at which the colonies are growing. In 

 some instances this liquefaction spreads laterally and down- 

 ward, causing a saucer-shaped excavation; while in others 

 the colony sinks almost vertically into the gelatin and may 

 be seen lying at the bottom of a funnel-shaped depression. 

 These differences are constantly employed as one of the 

 means of differentiating otherwise closely allied species and 

 varieties. (See Fig. 32.) Studies upon the spirillum of 

 Asiatic cholera and a number of kindred species, for 

 example, reveal decided differences in the form of lique- 

 faction produced by these various organisms. The minutest 

 detail in this respect must be noted, and its frequency or 

 constancy under varying conditions determined. 



