MEDIA MAKING AND APPARATUS 275 



Serum — apart from characteristics of growth and morphology, 

 serum becomes translucent and later liquified under the action 

 of certain bacteria. The morphology of bacteria growing on 

 this medium is readily determined also, and is sometimes 

 characteristic. 



Starch jelly — chiefly used for appreciation of diastatic action. 



Silicate jelly : Colin' s and Uschinky's solutions — are chiefly used in 

 the study of certain bacteria which grow best or exclusively on 

 such media ; and in the search for synthetic media applicable to 

 all bacteria. 



TECHNIQUE OF HANGING DROP. 



Secure to a slide with thick Canada balsam, softened by heat, 

 a ring of glass, celluloid, rubber, metal, etc., about £4" diameter, 

 1/16" wide, 1/8" thick. Vaseline the free edge of the ring. Mix 

 on a coverslip one drop of sterile water with one drop of the 

 material to be examined, as directed for staining, but instead of 

 drying, turn the coverslip down over the ring, so that it sticks to 

 the vaselined edge with the drop downward within the ring. Focus- 

 ing carefully on the drop from above, the bacteria, etc., in the drop 

 will be seen. 



TECHNIQUE OF SIMPLE STAIN. 



Place on a clean slide with an inoculating needle one drop 

 of sterile water. Mix with this one drop of a broth culture, of 

 milk, of water or other liquid containing bacteria; dry over a flame 

 gently (without boiling), spreading with the needle from time to 

 time to secure a smooth, even " smear." When dry, and still hot, 

 place two or three drops of 95% alcohol on the smear and allow 

 it to dry. This alcohol treatment may be omitted in the case of 

 milk, and should be omitted when the material to be examined is 

 from a culture on solid media (agar, gelatin, potato, etc.) 



