PROPHYLAXIS OF MALARIA. 
63 
shows that the oil has in part or wholly disappeared before the end 
of two weeks, it is obvious that reoiling should be done at once. The 
two-week interval was the time adopted by the Army medical officers 
in Cuba and was found to give satisfaction. 
In some regions the use of oil as a larvacide is practically impos¬ 
sible owing to the currents produced by the strong winds of the 
locality or to the development of tropical vegetation in the collec¬ 
tions of water to be oiled, thus preventing the spreading of the oil 
over the water surface. In such instances the use of a larvacide 
that is less easily displaced by winds and that is capable, to some 
extent, of killing vegetation is indicated, but up to the present 
time such an ideal larvacide has not been discovered, the nearest 
approach to it being the one used in the Canal Zone and which will 
now be described: * 
The Canal Zone larvacide .—This preparation has been very largely 
used in the Canal Zone, and all of the reports concerning its action 
are very satisfactory. Darling 23 has described the method of manu¬ 
facture and has conducted valuable experiments determining its 
efficiency when brought in contact with the larvae of both Culex and 
Anopheles mosquitoes. He thus describes the method of preparing 
the larvacide: 
One hundred fifty gallons of crude carbolic acid (specific gravity not greater 
than 0.97 and to contain not less than 30 per cent tar acids) are heated in an 
iron tank having a steam coil with steam at 50 pounds pressure. Two hundred 
pounds of finely crushed and sifted common rosin are dissolved in the heated 
acid, and then 30 pounds of caustic soda, dissolved in 6 gallons of water, are 
added. There is a mechanical stirring rod attached to the tank. The product 
is ready in a few minutes, yielding about 34 barrels. 
Regarding the value of this preparation as a larvacide, he says: 
The resultant emulsion makes a very good disinfectant or larvacide. In fact, 
1 part of it to 10,000 parts of water will kill anopheles larvae in less than half 
an hour, and 1 part to 5.000 parts of water will kill anopheles larvae in from 5 
to 10 minutes or less. This property of killing larvae rapidly is of great im¬ 
portance in ihe Tropics, where continuous rainy periods make crude oil or 
kerosene much less valuable as a larvacide than it is in northern latitudes hav¬ 
ing less rainfall. Also the larvacide acts as an algecide, and thus destroys the 
food and hiding places of anopheles larvae. 
This larvacide is used by spraying an emulsion,. prepared by add¬ 
ing 1 part of the larvacide to 5 parts of water, over the surface 
and along the edges of ponds, streams, or other collections of water 
that serve as breeding places for mosquitoes. 
Used in this 'manner the larvacide will kill all anopheles larvse 
within five, minutes,, and it .possesses the advantage that after a few 
hours, in;case of running streams, the water will be again fit for 
domestic use*.. . 
* This larvacide would appear to be an ideal one for adoption by 
the Army in its antimosquito work, especially in the Tropics where 
