But the application of this remedy must be carefully performed. Some trees may endure it without injury, even undiluted or unmixed; but this is scarcely to be expected, and the oil should therefore be applied in some mixed form. Also, it is important to remember _that a hot sun increases the injurious effect of kerosene_; consequently winter, or cloudy weather, should be chosen for its employment. (_a._) _Pure kerosene._ As just stated, it is probably not advisable to use this. Still, "a young shoot of orange, not more than fourteen days old, was uninjured by an application of pure kerosene which thoroughly wet every leaf;" (Comstock); and _Lecanium hesperidum_ on ivy, similarly treated, was destroyed, without injury to the plant (ibid.). (_b._) _Kerosene and milk._ An excellent mixture, if milk can be obtained cheap (Riley; Hubbard; Comstock). It must be applied in the form of an "emulsion," sprayed over the tree or brushed on the bark. Hubbard gives the following directions for use: Heat the milk nearly to boiling-point and mix with double the quantity of kerosene; churn violently from ten minutes to half an hour, according to temperature, until a creamy thick fluid is obtained; dilute this with nine or ten times the quantity of water. The mixture is of course purely a _mechanical_ one, as far at least as the water is concerned, and it must be kept constantly stirred, to prevent the substances from separating from the water. For evergreen trees impel the mixture on leaves and branches _in the finest possible spray_. Sour milk is as useful as fresh. The object of the milk is not only to lessen the injurious qualities of the kerosene, but also to induce it to mix more freely with the water; but it is the oil alone which destroys the insects _and their eggs_. (_c._) _Kerosene and soap._ When milk is not obtainable, or too dear, nothing is so excellent as this mixture. Soap itself (see below) is a useful insecticide, and in combination with kerosene includes the good qualities of both substances. The cheapest possible qualities of soap will do. The mixture, which is, even more than the last, purely _mechanical_, must be made first of all an "emulsion." The American experiments result in the following recipe and method of using:-- Formula: Common soap 1/2lb. Kerosene 2 galls. _Soft_ water 1 gall.