34 THE AMERICAN NATURALIST.  [VOL. XXXIII. 
but still living Spirostoma were placed in solutions of differing 
densities with the following result : 
In H,O, every Spirostomum immediately fell to the bottom. 
In gum arabic solutions of sp. grav. 1.0110, all of the Spirostoma fell to 
the bottom within fifteen minutes, except two or three that were 
still falling. 
sp. grav. 1.0165, the Spirostoma were carried by the slight currents. For 
the first five minutes indifferently. They then slowly rose, but not 
to the surface, then fell, but not to the bottom. They were evi- 
dently nearly balanced. 
1.0165, the above experiment was repeated, and the Spirostoma 
remained scattered for twenty to thirty minutes. Then all drifted 
upwards except six which lay on the bottom. 
1.0180, Spirostoma remain still for a long time. 
1.0190, Spirostoma remain still for a short time, and then rise, 
several coming to the surface. 
1.0190, the above experiment was repeated with the same result. 
1.0208, Spirostoma slowly rise. 
1.0230, after ten minutes thirteen Spirostoma were floating on the 
surface, and but two still remained in the solution. Both of these 
were rising. When the solution was shaken, all were scattered, 
and then rose to the surface again. After two hours all fell to 
the bottom. 
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From these experiments it appears that the specific gravity 
of Spirostomum lies between 1.0165 and 1.0180, or very near 
to 1.017. 
Jensen (93) also attempted to obtain the specific gravity of 
an infusorian, Paramzecium, by finding the density of a solution 
in which the infusorian would neither rise nor fall. He chose 
for the experiment, however, potassium carbonate, which imme- 
diately kills the organism. By means of solutions of potassium 
carbonate, Jensen decided that the specific gravity of Paramæ- 
cium was 1.25. Since his conclusions differ so greatly from 
the results obtained with gum arabic and living Spirostoma, I 
repeated the experiment by placing Spirostoma in a solution of 
potassium carbonate of 1.25 specific gravity, and found that 
this infusorian rose at once to the surface, and then fell directly 
back into the solution, where it remained in fact suspended. 
Evidently potassium carbonate of this density rapidly reduces 
_ the protoplasm of the organism to its own weight. Only the 
