14 REMEDIES AND PREVENTIVES AGAINST MOSQUITOES. 
of low grade, or of the grade known as fuel oil, is the most satisfac- 
tory as regards efficiency and price. 
In choosing the grade of oil two factors are to be considered: First, 
it should spread rapidly; second, it should not evaporate too quickly. 
The heavier grades of oil will not spread readily over the surface of, 
the water, but will cling together in spots and the coating will be 
unnecessarily thick. The rapidity of spread of the film is also im- 
portant. As to quantity, under still conditions, an ounce of kerosene 
to 15 square feet of surface space is about the right proportion, and 
in the absence of wind such a film will remain persistent for 10 days 
or slightly longer. Even after the iridescent scum apparently dis- 
appears there is still an odor of kerosene about the water. In a wind 
the film of kerosene is frequently blown to one side, but with a change 
will go back again, so that larve are destroyed. Not only are larve 
and pup destroyed by the kerosene film, but many adult mosquitoes 
alighting on the surface of the water to drink or to lay their eggs are 
killed by it. In California, Mr. H. J. Quayle has used a combina- 
tion of heavy oil of 18° gravity and a light oil of 34° gravity, in the 
proportion of 4 to 1, respectively. This mixture made an oil that was 
just thin enough to spray well from an ordinary spray nozzle and yet 
was thick enough to withstand very rapid evaporation. It was ap- 
pled by a barrel pump where this could be used, and by an ordinary 
knapsack pump in other regions. A single application was found by 
Mr. Quayle to be effective sometimes up to four weeks. The army of 
occupation in Cuba used oil every two weeks. 
The use of a spray pump has been mentioned. Small ponds can be 
sprinkled out of an ordinary watering pot with a rose nozzle, or for 
that matter pouring it out of a dipper or cup will be satisfactory. 
In larger ponds pumps with a straight nozzle may be used. A straight 
stream will sink and then rise and spread until the whole surface of 
the pond can be covered without waste. The English workers in 
Africa advise mopping the kerosene upon the surface of the water by 
means of cloths tied to the end of a long stick and saturated with 
kerosene. 
In Panama a larvicide is being used which is made as follows: 150 
gallons of carbolic acid is heated in a tank to a temperature of 212° 
F., then 150 pounds of powdered or finely broken resin is poured in. 
The mixture is kept at a temperature of 212° F. Thirty pounds of 
caustic soda is then added and the solution is kept at the same 
temperature until a perfectly dark emulsion without sediment is 
formed. The mixture is thoroughly stirred from the time the resin 
is used until the end. One part of this emulsion to 10,000 parts 
of water is said to kill Anopheles larve in less than half an hour, 
while 1 part to 5,000 parts of water will kill them in from 5 to 10 
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