CULTURE MEDIA WITH WIDE RANGE OF HYDROGEN 329 



media according to accepted methods, i.e., sterilized them after 

 the adjustment of reaction, has found that an acidity of 2 to 

 2.5 per cent normal HCl or a pH concentration of approximately 

 4 to 3.5 is the lunit of soHdification of agar. Alkalis in related 

 proportions in the presence of heat have been found to exert 

 a similar action on the jellifying power of agar. Fellers (1917), 

 however, found that this range of jellifying power for 2 per cent 

 agar could be extended to 5 per cent normal HCl or 5 per cent 

 KOH if the acid or alkali were added while the agar was boiling 

 hot and it was not subsequently sterilized. These highly acid 

 or alkahne media were furthermore employed by him (1916) 

 in studies on soil flora, since appropriate quantities could be 

 transferred by means of a sterile pipette to sterile Petri dishes. 

 When one permits the media to cool before adding the acid or 

 alkah as was done in our studies, and as is indicated in Fellers' 

 work, the range of sohdification may be extended very much 

 farther. The appHcation of the principles involved herein are 

 beheved to make it possible both to simphfy the making of 

 media and to improve methods for investigation on the influence 

 of hydrogen ion concentration on microorganisms. Reference to 

 two recent papers one by Webb (1919) on the influence of reaction 

 on the germination of fungous spores and the other by Fred and 

 Davenport (1918) on the growth of nitrogen assimilating bacteria, 

 will illustrate the possibihties which may come in similar studies 

 from the use of very acid or very alkaline solid media. 



In routine work it will be found to be advantageous to flask 

 and sterihze the media in 200 cc. quantities for the reason that 

 the addition of 1 or 2 drops of strong acid or alkah to this quantity 

 will bring about a change in concentration of about pH 0.2. 

 When acid is added to agar in flasks at 50° to 60°C. it may be 

 thoroughly agitated by whirUng, 10 cc. portions may be removed 

 for comparison in reaction with the color standards of Clark and 

 Lubs (1917), and when the usual precautions against contamina- 

 tion are observed the material in the flasks may be kept sterile, 

 while the adjustment to the desired pH concentration is being 

 made. The agar may then, before it has had time to solidify, 

 be poured into sterile test tubes or sterile Petri dishes, where- 



