The spreading of larvicidal oils is hindered by the presence of natural films upon the surface of standing water. The film pressure, which measures the resistance offered to the spreading of oil, can be determined in the field by a simple method depending on the use of standard spreading liquids.

It seemed reasonable to suppose that by making tests on the waters at many different places it should be possible to estimate the optimum spreading properties required in a larvicidal oil before its distribution began in a particular district. An attempt to do this was made in a part of the Gold Coast in 1944 and the investigation showed that in that region, and at that time, the spreadability of an oil with spreading pressure equal to 30 dynes/cm., approximately, would have been adequate for larvicidal purposes. It was also found that the natural films, which could be either visible or invisible, were of two physical types distinguishable as “uniform” and “non-uniform” according to the observed variation of the film pressure at a single site.