Atmospheric Requirements of Bacteria. 309 



prisingly meager in the amount and character of the work done. 

 Such investigations are limited to those of Wolf 1 and Weigert, 2 

 who working with different concentrations of media found definite 

 limits of growth for various organisms. No investigation as to 

 the effect of water vapor tension on surface colonies on a solid 

 medium has apparently ever been made. 



The method used for obtaining preliminary data as to the 

 influence of atmospheric moisture on surface growth consisted in 

 using either two tops or two bottoms of ordinary petri dishes which 

 are placed edge to edge and held together with adhesive tape 

 around the entire circumference giving practically an air-tight 

 capsule. In the upper half, media was poured to a depth of 2 mm. ; 

 in the lower half, solutions of various dehydrating agents were 

 placed to a depth of 5 mm.; about 2 cm., as a rule separating the 

 two surfaces. 1/10 c.c. of a dilution of broth culture of the or- 

 ganism investigated, was spread on the surface of the poured 

 medium and evenly distributed with a bent rod over the entire 

 surface. 



The organisms investigated were B. coli, B. subtilis, Staphylo- 

 coccus aureus j and Streptococcus hemolyticus. The medium used 

 was glucose infusion agar, Ph 7.0, and cultures were usually in- 

 cubated 18 hours at 37. 5 0 C. The dehydrating solutions used 

 were glycerin, 50 per cent, glycerin, 50 per cent, calcium chloride 

 (saturated) and 25 per cent, calcium chloride, giving initial relative 

 humidities of o, 25, 35, and 75 per cent, respectively at the start 

 of the experiment. The rate of change of the water vapor ten- 

 sion with addition of water, of the glycerin solutions is, however, 

 so much larger than that of the calcium chloride solutions, that 

 with the conditions as above stated the drying action of the cal- 

 cium chloride is usually somewhat more energetic than that of the 

 glycerin. 



Marked inhibition of growth of all organisms investigated 

 was shown to take place under these conditions, the action being 

 greater on the more delicate organisms and roughly proportional 

 to the extent of the dehydration which occurs. This phenomenon 

 is obvious in colo nies as soon as they can be distinguished by the 



1 Wolf, L., Archiv.f. Hyg., 1896, xxxiv, 200-209. 



1 Weigert, R., Centbl. f. Bakt., I, Orig., 1904, xxxvi, 112-121. 



