32 THE AMERICAN NATURALIST. [VOL XXXIII 
Unfortunately, for the success of the experiment, I have 
been unable to obtain in Cambridge, Paramzecia which showed 
decided geotactic reaction. My results may, nevertheless, serve 
as a basis for further experiment by those more fortunate in 
the material at their command. 
The Method. 
The specific gravity of the organism was to be obtained by 
finding the density of a solution in which the animal, either 
dead or paralyzed, remained suspended without rising or fall- 
ing. This known, the movements of the living infusorian in 
solutions of the same, of greater, and of less specific gravity 
were to be recorded. It was, therefore, necessary to find a 
substance in solutions of which Infusoria will live, and a rea- 
gent that will kill or paralyze the animal with little apparent 
change of form. 
I found that in solutions of salt, sugar, and glucose, of suffi- 
cient density to be serviceable, Infusoria do not live, but in 
dying increase their specific gravity, probably owing in great _ 
part to loss of water. Gelatine was not satisfactory, since even 
weak solutions tend to become stiff when cold. After fail- 
ing with the above-mentioned substances, I found gum arabic 
admirably adapted to my purpose. Solutions surpassing the 
specific gravity of protoplasm are easily obtained, and Para- 
mzecia or Spirostoma live for hours in solutions heavier than 
their specific gravity, not only surviving, but also multiplying, 
when kept for days or weeks in solutions of less weight. 
To exclude the small error that might arise from water con- 
tained in the gum, the specific gravity of the solution has not 
been determined by adding a known weight of the gum toa 
given volume of water, but in each case the solution has been 
weighed and compared directly with the weight of the same 
volume of water. 
Spirostomum. 
_ Although Spirostoma frequently lie on the bottom of the 
vessel in which they are cultivated, they are also often found 
