Bacterial Flora of the Semi -Desert Region of New Mexico. 223 



Still another factor may enter more or less prominently, 

 though indirectly, to modify the number of bacteria to be 

 found in the air; this factor is sunlight. That direct sun- 

 light is a powerful germicide is well known. Its effect then 

 is to decrease the actual number of bacteria that might other- 

 wise find their way into the air. It is not possible to make a 

 reasonably accurate estimate of the force of this factor, but 

 some idea may be obtained from a number of experiments 

 made in testing the effect of direct sunlight on two of the air 

 bacteria — A 2 and A 5 . Both of these bacteria are micrococci, 

 and hence do not form spores. A 2 is killed by direct sunlight 

 in about 30 minutes (March 3, 1900, 12. 30-1. 00 p. m.) and A 5 

 in 20 to 25 minutes (January 21, 1900, 11.30- 11.50 a. m.) 



The potato bacillus is not materially killed out in less than 

 60 minutes and the spores survive a much longer period. 

 It is possible that the air bacteria can survive the effect of 

 sunlight much longer than other forms which might be abun- 

 dant in the air but for this fact. Professor H. L. Russell, in 

 exposing an agar plate of B. campestris (cabbage blight) to sun- 

 light for 30 minutes showed that this organism is completely 

 killed out in that length of time on the sunny portion with 

 heavy growth in the shaded part.* Probably the killing- 

 out time was even less than 30 minutes. Further remarks in 

 this line are reserved for a subsequent paper. 



Perhaps further discussion along this line may be post- 

 poned with advantage until we have recorded a number of 

 similar experiments made under modified conditions and 

 which were made with special objects in view. Several ex- 

 periments were made to show the relation between the num- 

 ber of bacteria in the air in the residence and business dis- 

 tricts of the city of Albuquerque. 



Exp. 18 — October 20, 1899, 6.00 p. m. Three agar plates 

 were exposed in private yard on South Arno Street in the 

 residence portion of town for 20, 40, and 60 minutes. No wind. 



October 27: 



PI. 20 mim — 529 bacteria and 3 molds. 

 PI. 40 min. — 575 bacteria and 9 molds. 

 PI. 60 min. — Worthless, due to molds. 

 Average per 10 min., 276 bacteria. 



* Wis. Agrl. Exp. Sta. Bui. No. 65, p. 19. 



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