It is essential that an oil used for killing mosquito larvae on the surface of stagnant water should cover the whole surface completely with a spread film. Only if the oil spreads completely over the surface can it be certain of reaching all the larvae, penetrating into their respiratory system, and so killing them. Although practically all oils (except the most highly refined mineral oils) will spread on clean water surfaces, most stagnant waters have surfaces so contaminated by surface films, visible or invisible, that their surface tension is reduced much below that of clean water. This hinders, or completely prevents, spreading of oils of low spreading power which would spread on clean water. Therefore, other things being equal, an oil with a high spreading power is preferable to one with a low spreading power; indeed an oil with a low spreading power may, in practice, be of little or no use. A simple method of determining spreading power is thus of considerable importance.