212 BACTERIOLOGY. 



the covering of collodion on the exterior of the capsule 

 is of sufficient thickness /. e., about that of writing 

 paper it is allowed to dry for a short time, and is then 

 filled with water and placed in a pan of hot water. This 

 will dissolve out the gelatin frame-work and leave the 

 collodion envelope. The water is now removed from 

 the capsule by means of a capillary pipette. The nutri- 

 tive fluid is introduced by the same means, and the filled 

 capsule is dropped into a tube of bouillon and sterilized, 

 after which the fluid is inoculated and the glass tube is 

 sealed off in the gas-flame. Before using the inoculated 

 capsule its tightness must be tested. This is done by 

 dropping it into a tube of sterile bouillon and placing in 

 the incubator for twenty-four hours. If growth appears 

 in the bouillon of the tube, then either the wall of the cap- 

 sule is permeable to bacteria, or the bouillon has been con- 

 taminated during the manipulation. As the wall of the 

 capsule is to possess the physical qualities of an osmotic 

 membrane, through which only dialyzable substances can 

 pass, it must, naturally, be free from cracks or air-holes, 

 and in making the capsules the greatest care must be 

 taken to eliminate these defects. 



STUDY OF GELATIN CULTURES. As has been pre- 

 viously stated, the behavior of bacteria tow r ard gelatin 

 differs some of them producing apparently no altera- 

 tion 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 downward, caus- 

 ing 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 depres- 



