June, ’18] FREEBORN AND ATS ATT: PETROLEUM AND MOSQUITO LARVAE 305 
We then used these evaporated residues to make the films in the 
usual manner and found that the practical toxicity or lethal effect of 
the oils had decreased in proportion to the percentage of the weight 
lost through evaporation, in other words, kerosene, previously giving 
rise to the greatest “lethality” had become the least efficient in its 
larvicidal properties which seemed to indicate that the loss of the 
volatile constitutents brought about a decreased toxicity. 
Having obtained this data, we proceeded with a very definite check. 
Using shell vials of 2 x 5 cm. size, we placed several larvae in equal 
volumes of their natural medium in each vial. Plugs of absorbent 
cotton were then introduced into the neck of each vial and on them 
were poured 3 cc. of the various oils, the lighter ones being colored 
with Sudan III in order to ensure the detection of any oil that might 
run down the inside of the vial to the surface of the water, and the 
vials corked up and sealed. A check, consisting of a similar vial con¬ 
taining larvae, but to whose cotton plug there was applied no oil, was 
also used. 
The results were as follows: 
Vapors confined in 
chambers above water 
Gasoline. 
Kerosene. 
High grade stove distillate. 
Low grade stove distillate. 
Residium. 
Crude. 
Water vapor only (check vial). , 
Average time required 
to kill larvce 
.. 153 minutes 
.. 185 
.. 254 “ 
20 hours 
.. *72 “ 
.. *72 
. *72 “ 
This experiment established the point that the vapors of the various 
volatile petroleum oils were toxic to mosquito larvae even in dilute 
quantities when there was no possibility for the oil as a liquid to come 
in contact with them. As the curve of the time required to kill the 
larvae in this experiment coincides with the curve of their volatility, 
the most volatile killing in the shortest time and also to the curve of 
the time required to kill when the oils were applied as films to the 
surface of the water, we feel justified in saying that the practical 
toxicity of the petroleum oils increases with their volatility. Of what 
these volatile products consist we are unable to state at this time. 
We might call attention also to the fact that although we have sug¬ 
gested that volatility (a physical characteristic) may be taken as an 
index for the practical toxicity of the petroleum oils when used as 
larvicides, it must be borne in mind that the toxicity is entirely due 
to the chemical characteristics of the oils which vary with almost 
* No apparent ill effects at end of 72 hours. 
