THE ACTION OF TANNIN. 143 
ina mortar. If afew drops of this are occasionally added to the 
trough containing the Infusoria they may be kept satisfactorily 
for a length of time. The small trough I have has been so kept 
for more than four months. 
In trying the effect of various chemicals on Infusoria—princi- 
pally Paramecium aurelia—I was led to use a solution of tannin, 
or tannic acid ; and I was surprised to find that the immediate 
action of this chemical was to render the cilia visible without any 
manipulation of the light. It may have been noticed that when 
these Infusoria have been killed by ordinary means, such as 
heating the water in which they are contained, the cilia are very 
difficult to observe, probably owing to their great transparency, so 
that no correct idea has, I think, been obtained of their size or 
quantity. 
On placing, however, a drop of water containing Paramecia on 
a slip side by side with a minute quantity of a solution of tannin, 
and making a junction of the two, it will be seen that the instant 
the Farameca approach the mixed fluids their motion is arrested, 
of course in a greater or less degree according to the strength of 
the tannin. They are generally rendered perfectly quiescent, and 
the cilia begin to appear and continue to develope, until the body 
of the animalcule appears entirely surrounded by them. ‘The 
symmetry of the cilia depends upon the strength of the solution ; 
if it is too weak, it seems as if the animal had had time to slightly 
move the cilia, by struggling, as it were, as they appear crossed 
and crumpled; but if the solution of tannin happens to have 
mixed with the water in a better proportion, the cilia are more 
rapidly developed, and stand out almost parallel, hardly one being 
seen to overlap another. 
To bring out the best appearance of the cilia over the whole of 
the surface of the Paramecium, the parabola is required ; the 
animal then appears as if it were supported on the slip by its cilia. 
If the tannin solution is strong the Faramecum is almost 
instantly rendered motionless, and the cilia appear to be entirely 
removed, remaining in a more or less confused state at the 
extremity. 
I have shown this action to several microscopists, and so con- 
trary is the remarkable development of the cilia to received ideas, 
that on nearly every occasion I have been met with the remark 
that they were not cilia but fungoid growths. This is, however, 
entirely disproved by the fact that they are developed, as it were, 
instantaneously. 
The action of the tannin on the cilia I believe to be analogous 
to its action on gelatine, rendering them leathery, and conse- 
quently opaque. It does not appear to kill the Paramecum 
itself—at least for some little time, unless the solution is very 
