MICROSCOPIC ORGANISMS IN INTESTINAL CANAL. 241 
seems to lie in the reaction of the medium, an acid reaction 
repressing, and an alkaline one favouring, their presence and 
development. At the same time, however, certain forms of 
alkalinity, associated with the excessive development of other 
organic forms, are almost as repressive as acidity ; but, as a rule, 
there can be no doubt that alkalinity of the medium is one of 
the necessary conditions for their presence in any considerable 
numbers, and that any excessive acidity is most destructive to 
them. 
This alone is sufficient to account for the extent to which 
their presence in the excreta in health has escaped notice. In 
health the excreta, as a rule, present a faintly acid reaction when 
perfectly fresh, but the degree of acidity mcreases so rapidly 
that within a short time the medium becomes quite unadapted 
to the requirements of the zoospores, which consequently die 
and disappear. Even where the materials, when fresh, are 
neutral, or, as is sometimes the case, exhibit a mixed reaction 
with alkalinity more permanent than acidity, the rapidity of the 
development of an intensely acid condition is very great, so that, 
unless examined at once, they may show no traces of zoospores. 
There is also another circumstance which must be regarded as 
probably accounting in part for the rarity with which these 
organisms have been detected in Europe, uamely, that a depres- 
sion in the temperature of the medium, as is the case with many 
other organisms, exerts a most rapid and prejudicial effect on 
them. During the hot weather months in India this influence 
exerts hardly any appreciable effect, but in the cold season it 
comes into play more or less distinctly. 
While considering the subject of the influence of the condi- 
tion of the medium on the vitality of the zoospores, it may be 
well to examine a little more closely the phenomena attendant 
on the decomposition of normal alvine excreta in this country. 
These phenomena exhibit a wonderful uniformity, as shown by 
the records of very numerous observations conducted at all 
times of the year, and at intervals of several years’ duration. 
When exposed to rapid drying, comparatively little change 
beyond increased acidity has time to take place. When, on the 
other hand, the materials retain their moisture, as, for example, 
when they are reserved in an isolated moist chamber, a definite 
series of phenomena manifest itself with great regularity. The 
first change appreciable consists, as before said, in a very rapid 
increase in acidity, so that the material, after the lapse of twenty- 
four hours, shows an intense and permanent acid reaction. 
This condition is associated with a change in the colour of the 
basis, specially when exposed to the air, a darkening and red- 
dening being more or less distinctly manifested. At the close 
