76 
PRESIDENT S ADDRESS. 
Api:., 1889. 
oxygen; that is, the microbe produces a chromogenous 
substance which is oxidised into a pigment; some of them 
can be forced to grow in the absence of oxygen, and in that 
case the characteristic colour is not produced. 
Bacteria require for their growth carbon, oxygen, 
hydrogen, nitrogen, and minute quantities of various minerals. 
Many of them require free oxygen ; others can obtain it from 
the oxidised compounds of the medium in which they live ; 
the first kind are called aerobic, the second anaerobic, but the 
distinction is not an absolute one. The nitrogen can be 
assimilated either from albuminous substances, or from 
ammonia and ammonium salts. They can grow in Pasteur’s 
solution, and thus are capable of obtaining their nitrogen 
from inorganic substances. The carbon they obtain from 
sugar, glycerine, or from more complex organic bodies. 
Water is also essential to their growth, but desiccation does 
not necessarily kill them, although it does the comma- 
bacillus of Asiatic cholera. One of the reasons why sugar 
preserves from putrefaction is that it combines greedily with 
the water of the preserved substance, and thus hinders any 
germs that may be present from developing by depriving them 
of one of the essentials of their growth. Nine or ten kinds 
of Bacteria are now known which are phosphorescent, and in 
fact are the cause of the phosphorescence of putrid fish. 
Cultivated on a plate, in the manner hereafter described, the 
little colonies shine in the dark like stars in a midnight sky. 
They can even be photographed by the light they emit. 
It has been shown that direct sunlight is fatal to the 
putrefactive Bacteria, and even to some of the pathogenic 
species. But on the other hand several species are known 
which flourish better in the light than in the dark. 
Engelmann has investigated these, and gave to the one on 
which his chief observations were made the name of 
Bacterium photometricum. The Beggiatoa previously men¬ 
tioned, which is common in some stagnant pools, is another 
species. They all belong to the sulphur Bacteria, which, in 
the presence of free hydro sulphuric acid, become filled with 
sulphur granules. Some of these sulphur Bacteria are 
colourless, but others are characterised by a peculiar peach- 
purple pigment, diffused in the protoplasm, which is called 
Bacterio-purpurin, and is capable of acting somewhat in the 
same way as chlorophyll. When they are exposed to light 
there is found to be an evident proportion between the 
amount of light absorbed, and the physiological effect 
produced. In the absence of light they ultimately perish. 
The peculiar effect of light upon them is due to its direct 
