MEDIA. 49 
Egg-yolk.—This is poured into test-tubes and solidified in a slanting position 
by heat (80° C.), or the egg may be boiled hard and the yolk cut with a sharp 
knife and transferred to sterile Petri dishes. If desired, the yolk and white may 
be mixed before solidifying, z. e., by shaking the egg vigorously before breaking 
the shell. 
SYNTHETIC MEDIA AND OTHER SPECIAL, MEDIA. 
The student should try the following media. He should also invent media to 
suit special cases. "The kinds of media I have in mind are the opposite of universal, 
Fig. 45.* 
to-wit, such as will favor the growth of some organisms while preventing that of 
others. ‘The acid phosphates and many other substances are useful for this purpose. 
The field is comparatively new, and much is to be learned by careful experimenting. 
1. Dunham’s solution. / 
2. Peptone-water (1 or 2 per cent) with addition of various carbohydrates, 
acids, etc. 
3. Sugar-free beef bouillon with Witte’s peptonum siccum (for the indol test). 
4. Cohn’s solution. 
5. Uschinsky’s solution. 
*Fig. 45—Oven for solidifying and sterilizing blood-serum, nutrient starch-jelly, silicate-jelly, 
etc., at temperatures below 100° C. When in use the temperature is controlled by means of a 
Tollen’s thermo-regulator (see fig. 35). 
