CERTAIN CHROMOGENIC BACTERIA 71 



e) Shake this carefully by rolling the tube between 

 the palms of the hand, or stir with the platinum needle, 

 so as to avoid air-bubbles. 



/) Transfer 4 or 5 loopfuls of this agar-tube (2) to 

 the second agar-tube (3), and mix as above. 



g) If thought necessary, more tubes may be treated 

 in the same way, resulting in still higher dilutions. In the 

 meantime the inoculated tubes should be replaced in 

 the water-bath, so as to keep them liquid. 



h) Pour the contents of the tubes, one after the other, 

 into sterile Petri dishes. 



i) Tip the Petri dishes very carefully, so as to dis- 

 tribute the medium evenly over the bottom. 



k) Label them 2nd, 3d, dilution, etc., with the name 

 of the organism and the date. 



/) Set aside on a level place to solidify. 



m) When solidified, place them bottom up in the 

 thermostat, in order to avoid moistening of the surface 

 by the condensation water dropping down from the 

 cover. 



NOTE. If the surface were moistened, the colonies would 

 run together and the characteristic appearance be destroyed. Gela- 

 tin plates, on the contrary, are placed cover up. Condensation 

 water does not form on these plates as gelatin may be liquefied 

 by the organisms. The liquefied part would then fall from the 

 medium on the cover and ruin the plate. 



The plates prepared in the above manner should be 

 studied after 24 hours, or, if not sufficiently developed, 

 after 48 hours, according to directions in Chap. X. 



EXERCISE II. STUDY OF PIGMENTS 



On the sixth day take agar-slant or potato-cultures 

 of the four chromogenic bacteria, and proceed as follows : 



