MEDIA EMPLOYED, ETC. 11 



merited with the production of gas, fermentation tubes were used 

 for all the sugars and mannite. If no gas was formed in the glucose, 

 the straight tubes were inoculated. The sugars and mannite were 

 used in a 1-per-cent solution in sugar-free bouillon. 



{d) Rcaetion of media. — The reaction of cultures is determined 

 as it appears on the fifth day in the different media, unless otherwise 

 stated. The medium in the open arm is used to determine the re- 

 action in the fermentation tube. Beginning with a reaction of -|-1.5 

 to phenolphthalein, or slightly alkaline to litmus, the detection of an 

 increase in acidity is not difficult. But inasmuch as the production 

 of an alkali is very frequently small in degree, cultures are often in 

 this paper recorded alkaline in reaction when probably the reaction 

 has not changed. 



(e) Fermentation with the production of gas. — Gas may be formed 

 in such small quantities as not to be observed as such, but to be en- 

 tirely absorbed by the medium. Whenever gas formation is men- 

 tioned as a character, visible gas is meant. The analysis of the gas 

 was made in the usual manner by absorbing a portion with potassium 

 hydrate (KOH) and testing the remainder with the flame. The 

 amount absorbed by potassium hydrate (KOH) is referred to as 

 carbon dioxid (CO,) and the remainder, if an explosion is obtained, 

 as hydrogen (H). This is, naturally, only approximately correct. 

 Since the gas formula may vary from day to day, too much value 

 must not be given to the exact proportion. It is well to observe 

 whether the proportion of hydrogen to carbon dioxid is greater or 

 less than 1. 



(/) Agar. — One per cent agar is used. The description of the 

 growth on this medium is made from the appearance as seen on the 

 surface of an agar slant. The description is usually very brief, since 

 it has, as a rule, little differential value. 



{g) Acid agar. — This medium is made acid by titrating to +3 to 

 phenolphthalein. The absence or presence, as well as the degree of 

 growth, is noted. 



(A) Serum. — The serum used is taken from the horse, sterilized at 

 55° C. and congealed at 80° C. Deep inoculations are made, and the 

 surface of slanted serum is also inoculated. The degree of growth is 

 usually noted. Cultures are observed for 6 weeks to 2 months. The 

 presence or absence of liquefaction is the chief character sought for. 

 Since room temperature varies so greatly, the time at which liquefac- 

 tion begins varies, and little differential value, therefore, can be given 

 to the exact time of this phenomenon. 



(«') Potato. — The composition of potato varies so markedly that a 

 description of a culture on this medium may differ materially from 

 that which is observed on another tube of the same medium. It is the 

 aim to omit for the most part the observed variations due to the 

 composition of the different potatoes. 



