EFFECTS OF CARBON DIOXIDE 523 
Preliminary experiments showed the necessity of observing a 
number of precautions. The first of these is that the rate of 
evolution of the carbon dioxide gas shall be approximately the 
same in different experiments which it is desired to compare, 
since it was found that, other things being equal, the slower the 
stream of carbon dioxide passing over the drop the longer the 
animals survive. This is probably due to the fact that in a rapid 
stream the air is removed from the gas chamber and the drop 
more quickly, and the animals have less time to adjust themselves 
to the new conditions than in the case of a slow stream. By 
using all four of the gas chambers in one experiment it was found 
easy to compare a considerable number of forms with this factor 
constant. Frequently, indeed, a number of forms were present 
in the same drop and thus subjected to exactly the same condi- 
tions. In order to be able to compare experiments made on 
different days the attempt was made always to have the gas 
evolved at the rate of approximately 100 cc. per minute. This 
it was found possible to do within the necessary degree of accu- 
racy by proper regulation at the beginning’ of the experiment of 
the apparatus, which is automatic when once started. 
A second and most important point to be considered is the 
temperature, which has a marked effect on the time in which death 
occurs. A preliminary experiment on the three species of Para- 
mecium showed that at 22°C. death occurs in roughly half the 
time in which it does at 12°C. In order that this factor might be 
made constant, all of the experiments here recorded were made 
at, or very near, the first mentioned temperature, which is slightly 
above ordinary room temperature. 
A third point that cannot be neglected is the size of the drop 
containing the animals. Preliminary experiments showed that 
this has an appreciable effect on the results obtained, especially 
when the drop is very small. In one such experiment in a very 
small drop the average time of death of a certain ‘pure’ race of 
Paramecium aurelia was seventeen minutes while the average 
for the same race in a rather large drop was thirty minutes. In 
two medium sized drops in the same experiment the times were 
twenty-eight and twenty-nine minutes respectively. It will be 
