Bacteria, their Nature and Function. 
377 
of bacteria there are which produce pigments of scarlet, red, orange, 
yellow, yellow-green, green, greenish-blue, blue, violet, or pink 
colour. The nature of these pigments and the meaning and object 
of their formation are still shrouded in a good deal of mystery, 
though Erdmann and Schrotter showed long ago that many points of 
similarity exist between some of these pigments and certain aniline 
colours. Bacillus prodigiosus is the more common of the chromo- 
genic bacteria, being occasionally present in water and in air. The 
pigment is soluble in alcohol, though only to a limited degree. 
I can only make a brief reference to another remarkable group 
of bacteria, which comprises several species, all having the power to 
produce luminosity of themselves and the medium in which they 
grow. These phosphorescent bacteria have been long known 
(P Auger) to be concerned in the production of the phosphorescent 
condition of decomposing sea fish, but within recent times Ludwig, 
Fischer, Katz, and particularly Beyerinck, have studied more in 
detail the conditions under which these bacteria grow, and have 
identified and cultivated several species. Dr. Beyerinck has sent me 
one species of these phosphorescent bacteria, the elements of which 
are short oval rods, often dumb-bells ; they grow in fish broth, and 
when the growth becomes conspicuous to the unaided eye it is 
luminous when viewed in the dark. Some cultures were exhibited 
which, when placed in the dark, showed a beautiful phosphorescent 
appearance. The phosphorescence is more or less limited to the 
surface layer, that is the one in contact with the oxygen of the air ; 
in the depth it is absent, but when shaking the flask the phos- 
phorescence appears also in the depth. 
Fkrmkntation. 
I have mentioned, in connexion with a previous group, bacterial 
species which have the power by hydration to change urea into 
ammonium carbonate — a change which is called a fermentative 
action. Changes similar to these are caused by micro-organisms in 
many processes playing an important part in industries. Amongst 
these changes I may mention one in particular, the souring of milk. 
There are a good many others, the viscous or mannite fermentation, 
the butyric fermentation, the indigo fermentation, the dextran fer- 
mentation, the acetic acid fermentation, and others ; but we must 
confine ourselves to the description of one, viz. the common Bacterium 
lactis. It is a minute oval bacterium, which multiplies with great 
rapidity, and which, introduced into milk, turns this sour in 12 to 
24 hours at the ordinary temperature ; when sterile milk is inocu- 
lated with this bacterium and kept in a warm place at a temperature 
of 60° to 65° F., the milk is found solid and curdled before 20 or 24 
hours are over, and in this curdled milk large numbers of the 
Bacterium lactis are present either as dumb-bell ovals or as short 
chains. When a needle is dipped first into such curdled milk and 
then into normal milk, the same coagulation with the same appear- 
