TECHNICAL APPENDIX. 187 



placed in a third tube, and the contents are then 

 poured into three dry sterilized plates, lifting the 

 cover briefly and gently inclining the plate to and 

 fro, in order that the gelatin may be distributed as 

 uniformly as possible. In making inoculations from 

 one tube to another it is advisable to hold them in an 

 inclined position in order to guard against the en- 

 trance of foreign germs. The plates are then placed 

 in the culture chamber at- a constant temperature of 

 22 (or they may be kept at the temperature of the 

 room) and at the end of two to three days the indi- 

 vidual colonies which have developed are observed 

 macroscopically and also microscopically with low 

 (50) magnifying powers. As a general thing only 

 two of the three plates are serviceable for observa- 

 tion, one at least is sown too thick or too thin. 



2. If we wish to ascertain the number of colonies, 

 for example, in a specimen of water, we place in three 

 test tubes of melted gelatin, 1 c.c., 0.5 c.c., and 0.1 c.c. 

 of the water, shake and pour into three dishes. To 

 ascertain the number of germs, we use Wolff hugel's 

 counting apparatus if very many germs have devel- 

 oped. If the germs are few the following plan is 

 simpler: The plate is laid upside down (upon the 

 cover) , the bottom is divided with ink into sextants, 

 and each visible colony is marked with a dot. Plates 

 upon which the number of germs in drinking-water 

 are to be ascertained must be counted several times 

 (on the second, third, and fifth days). When the 

 fluid is very rich in germs (for example, sour milk, 

 ditch water, etc.), 1 c.c. is first placed in 100 c.c. of 

 sterilized water and the mixture then manipulated as 

 described above. Solid bodies are first rubbed up in 



