MICRO-ORGANISMS. . 153 
but, practically, the anaérobic fermentation is only partial, 
and the aérobic is a very mixed fermentation. 
The bacteria of the sewage are by the aérobic treatment 
not reduced in numbers; they are rather increased, and no 
diminution of the pathogenic and intestinal bacteria seems 
to occur. What does happen is that the crude sewage, when 
properly treated, becomes sufficiently purified to withstand 
a further putrefactive fermentation; and it can, therefore, 
be discharged into a large volume of river-water without dis- 
comfort to the inhabitants of the neighbourhood. 
It must not be forgotten that the anaérobic treatment 
may be unnecessary, if the sewage is contained for any 
length of time in the sewers, where it rapidly undergoes a 
fermentation practically similar to that which goes on in the 
anaérobic chambers. 
The Chemical Laboratory. 
Although the chemical laboratory is one of the last places 
where one would expect to find microorganisms, yet even 
there many can be found. Every chemist has probably 
noticed how soon an aqueous solution of litmus becomes de- 
colourised. This is due to the reducing action of bacteria, 
especially the gelatineliquefying kinds. Solutions of 
organic acids and salts need to be continually filtered from 
the growth of moulds within the bottles. One of the earliest 
researches in mycology was that of Pasteur’s upon the selec- 
tive action of an unknown mould upon one of the consti- 
tuents of ammonium racemate, the dextro-tartrate being 
utilised, and the laevo-isomer ignored. To obviate the con- 
tinual filtration of the solutions, the addition of an inert 
antiseptic, such as chloroform or thymol, is recommended. 
The selective action of bacteria, yeasts, &c., can be advan- 
tageously employed by the chemist. For example, mannite 
can be separated from the common sugars by fermenting 
out the latter with yeast. Certain yeasts can ferment cer- 
tain synthetic sugars, but are unable to attack the stereo 
chemical isomers of the same. Again, bacteria, ¢.g., Bact. 
Hartlebu, can be employed to separate the aldoses from the 
ketoses, while others prefer levulose to dextrose. 
The brilliant red colour of Bact. prodigiosum is only pro- 
duced in the presence of traces of magnesium sulphate. 
One of the most delicate tests for arsenic is made by placing 
a drop of the suspected fluid, or portion of solid, on a potato 
culture of Penicillium brevicaule, and incubating the culture 
for 24 hours at 379, when a marked odour of arseniuretted 
hydrogen is evolved. Such minute traces (0-00001- 
