1919] 



ALLEN — AZOTOBACTER CHROOCOCCUM 17 



It seems that the Beijerinck solution prepared with tap 

 water (i. e., the true Beijerinck solution) is superior to Ash- 

 by 's solution when equal amounts of colloidal Fe 2 C>3 are added 

 to each. The loss of culture 23 is very unfortunate, but the 

 notes describe cultures 2o and 24 as rapid and vigorous 

 growths, as indicated first by turbidity, then zoogloea forma- 

 tion at junction of liquid surface with walls of glass flask, and 

 later by the formation of a finely flocculent precipitate at 

 the bottom of the flask. 



The growth in cultures 20 and 21, while far below that in 

 the regular Beijerinck solution, is really greater than we had 

 reason to expect. This indicates that needs of the organism 

 for mineral nutrients, aside from sodium, potassium, phos- 

 phorus, and iron, must be very low indeed and were partly 

 covered by the impurities carried by the constituents of the 



medium. 



It is interesting to note in this connection that it is more 

 or less generally recognized by bacteriologists that tap water 

 is superior to distilled water for the preparation of regular 

 media. The objection to its universal use is its inconstancy of 

 composition and the consequent varying results which attend 

 its use in different laboratories or even in the same labora- 

 tory at different times. Now, soil biologists recognize the 

 significance of this factor and moreover that in many cases 

 soil extract is superior to tap water. In the case of Azoto- 

 bacter, for instance, it is quite generally known that Ashby's 

 soil extract medium is superior to the regular Ashby medium 

 in which distilled water is used, and to that end we used Ash- 

 by^ soil extract agar to some extent in the propagation of our 

 stock culture. It is also universally recognized by chemists, 

 and to a less extent by biologists, that distilled water is not 

 free from dissolved substances. Inorganic salts are carried 

 over mechanically entrained in the vapor, and volatile or- 

 ganic compounds are with difficulty completely destroyed. It 

 seemed worth while in this work to remeasure the magnitude 

 of the differences resulting from the use of tap, distilled, and 

 redistilled water. The stock laboratory distilled water was 

 prepared by an electric still with a preheating device, and was 



