12 C. A. LUDWIG 
however, that no attempt will be made to describe them fully or to 
mention the many variations observed. In gas concentrations of 
10 percent and less the development was normal. In 25 percent it 
was sometimes normal but more often slightly retarded and the color 
rendered less brilliant. In 50 percent gas the metallic sheen was 
usually nearly lacking, the color considerably lighter, and the rate of 
growth considerably less, so that it took 2 or 3 days longer for it to 
reach its maximum than it took in air. In still higher concentrations 
these changes were progressively more noticeable until pure gas was 
reached, in which surroundings development was very slight and the 
colony colorless or whitish, with only occasionally a trace of pink. 
In the gas-air mixtures the color was usually variable, ranging from 
deep red to light pink or whitish and usually with purple shades; 
and a number of these tints usually occurred in the same streak. The 
weak colonies which developed in gas of a high concentration grew 
vigorously and developed pigment when returned to the air, but the 
pigment never reached the depth of color shown by a colony grown in 
air from the start. 
The organism was carried continuously through 5 transfers in 
5 percent, 10 percent, 25 percent, and 85 percent gas, 3 transfers in 
50 percent, ii transfers in 75 percent, and 10 transfers in pure gas. 
Cultures in the air inoculated from cultures carried for several transfers 
in pure gas grew rapidly and developed abundant but not normal 
pigment, although the pigment production returned to normal after 
a few transfers in the air. Where the cultures had been carried in gas 
for only two transfers the color was normal at the first recultivation 
in air. 
The examination of stained preparations failed to show any 
striking differences between the treated and untreated bacteria. In 
air culture the organism tends to show shorter, smaller, more coccus- 
like rods as the culture grows old. In gas the juvenile shape seems to 
be maintained for a longer period of time, probably owing to the 
slowing down of the development. 
Bacillus rubidus. — Part of the cultures of B. ruhidus in illuminating 
gas were made on autoclaved potato plugs. On this medium a clear 
orange color is produced which makes development easy to detect. 
The organism grew well in 0.5 percent and 5 percent illuminating 
gas; but did not grow in a strength of approximately 85 percent, 
except possibly in one trial, although it developed promptly when 
