62 
PROPHYLAXIS OF MALARIA. 
Army, both in this country and in Cuba and the Philippines, with 
excellent results. This method of prophylaxis depends for success 
upon the fact that the larvae of mosquitoes are unable to breathe 
if the surface of the water is covered with a film of oil, and it there¬ 
fore follows, that in selecting petroleum for this purpose an oil 
should be chosen that will spread rapidly when placed upon the sur¬ 
face of water and one that will not evaporate too rapidly. Howard 
recommended to the Army in Cuba the use of the grade of oil known 
as light fuel oil, costing from $2.25 to $3 per barrel, and this was 
used with good results. A thicker oil, composed of 4 parts of oil 
of 18° gravity and 1 part of oil of 34° gravity, is recommended by 
Quale, who used it in California and found that it remained efficient 
for from three to four weeks after application. 
The application of the oil, in the case of large bodies of water, is 
best made with a pump having a straight nozzle. The operator is 
rowed from side to side of the pond and the oil discharged at such in¬ 
tervals that when it rises and spreads the entire surface passed over is 
covered. In smaller collections of water an ordinary watering pot 
may be used with a spray nozzle or the oil may be poured from a 
cup or dipper. The greatest care should be taken to see that there 
are no portions of the surface left uncovered after the oil has spread, 
and all oiled collections of water should be inspected at least every 
third or fourth day, as the layer of oil becomes easily displaced by 
currents in the water, by winds, or by the movements of aquatic 
animals. Whenever unoiled areas appear upon the surface the 
oiling of that particular portion of the pond should be repeated. In 
the case of more or less stagnant streams or ditches the oil is best 
applied with a pump and the application frequently repeated. The 
pumps used for this purpose are the ordinary knapsack Dump or the 
bucket pump. 
The exact time that the oil will have to be renewed varies greatly, 
and in many instances the method is rendered almost useless by 
ignorance regarding the necessity of renewing the oil at frequent 
intervals. . While, in rare instances, one application of oil may be 
efficacious for as long a period as four weeks, in the vast majority 
of cases the application will have to be renewed much within that 
time. Where the temperature is high and the surface of the water 
is exposed to the sun, the oil evaporates rather rapidly and has to 
be reneAved at much shorter intervals than where the temperature 
is low and the collection of water is Avell shaded. 
It would seem that the most sensible rule for reneAving the oil 
would be to reoil every two weeks. We know that the average life 
of the larAue of mosquitoes is from 10 to 14 days, and if the oil be 
reapplied at these intervals we will be sure to destroy the A^ast 
majority of the larvae. HoweA^er, if inspection of the oiled water 
