134 DE. A. J. EWART ON THE EVOLUTION OP 
some time, all of the colour-forming Bacteria may often show s 
distinct trace of coloration. Hence probably in all cases the 
living bacterium is colourless. If the pigment is excreted as 
such, at any given moment the Bacteria should contain a slight 
amount, though this might be insufficient to give them a per- 
ceptible coloration when examined singly. On the other hand, 
it is quite possible that the pigment is excreted as a colourless 
chromogen, later altering perhaps by a process of oxidation and 
becoming coloured. If oxygen is deficient or absent, the pigment 
is not formed, and in the stab-canal of an agar-culture the 
pigment develops more slowly than on the surface. 
It is worthy of note that no perceptible change of coloration 
takes place in preparations which have evolved all the oxygen 
they are capable of giving off. Schneider, however, finds that 
deoxidizing agents, such as zine dust+acetic acid, decolorize 
the pigment from Bacillus janthinus and Sarcina aurantiaca, 
partly decolorize that from S. rosea, and do not at all affect that 
from Micrococcus agilis and Sarcina lutea. Strong oxidizing 
agents on the other hand, such as HNO,, in all the 9 colour- 
forming Bacteria rapidly decolorize the pigment. 
From the above it is clear that the phenomenon is not a vital 
but a physical one, the excreted pigment having the power of 
absorbing oxygen from the air, and then, when the partial pressure 
is lowered, slowly evolving it again. Hence a preparation in 
hydrogen in the gas-chamber which has ceased to evolve oxygen; 
after being re-exposed to the air for some time, may again show à 
distinct evolution of oxygen ; whereas, however long it may remain 
in hydrogen in the closed chamber, once the evolution of oxygen 
has completely ceased, it never recommences. 
The further study of the problem was continued by analytical 
methods, using the improved form of Bonnier and Maugin’s 
apparatus for the estimation of CO, and O described by Aubert *. 
Cultures of the Bacteria grown on bouillon are introduced into 
small bulbs with a bent neck, in which a little air is left. The 
open ends of the tubes are immersed in Hg covered by a film of 
water, and after a given length of time the gas is collected ove 
Hg and examined. 
The following are some of the results thus obtained with 
7 to 14 days old cultures of Bacillus brunneus, at 20° C.:— 
* Aubert, in Rey. gén. de Bot., Mars 1891. 
