386 THE MONTHLY BULLETIN. 



that oil and water can be agitated until a momentary emulsion is formed that 

 can be applied to the plant before it breaks into oil and water again. Formerly 

 this method was extensively used, but now its application is limited to the 

 spraying of olives for black scale and for a few other uses. 



Most emulsions used for spraying are more or less permanent in character. 

 This permanence is brought about by the addition of a third substance to 

 the oil and water. The simple emulsions in which the drops of oil are merely 

 entangled in the minute particles of some finely divided insoluble substance 

 have been called by Pickering quasi-emulsions. The limoid or calcium 

 hydrate emulsion is a good example. There is a simple gradation through 

 the use of various emulsifiers or third substances from the simple quasi-emul- 

 sion to the highly complex true emulsions. 



It might be well at this place to define a true emulsion. In this country the 

 study of emulsification has been led by Prof. Wilder D. Bancroft of Cornell 

 University. From his several articles and summaries in the Journal of Physi- 

 cal Chemistry, the following brief statement has been devised. A true 

 emulsion must have three components. There must be two nonmiscible or 

 partially nonmiscible liquids such as oil and water. There must be a third 

 component which is commonly called the emulsifier. One liquid occurs in the 

 form of drops and is said to be in the dispersed phase. The other is the 

 matrix liquid and is said to be in the dispersing or continuous phase. The 

 function of the emulsifier is to form a layer or pellicle around the drops of the 

 liquid in the dispersed phase to keep these drops from coalescing. How the 

 emulsifier performs this function is still a matter of theoretical conjecture. 

 The laws of surface tension account for the formation of the drops and for 

 the pellicles that inclose the drops. The static electric charges on the drops 

 are supposed to hinder coalescence by causing the drops to repel each other. 

 The writer has found that there is heat absorbed in the formation of an emul- 

 sion which would indicate a molecular rearrangement. This molecular 

 rearrangement is also shown in the fact that in a true emulsion where there 

 is a maximum of oil emulsified in a minimum of water the viscocity of the 

 resulting emulsion is always greater than that of its component liquids. 



There are certain facts that limit the selection of a true emulsifier. The 

 emulsifier must be colloidally soluble in the dispersing liquid. If the emulsion 

 Is to be the common one of oil dispersed in water, then the emulsifier must 

 be colloidally soluble in water. The petroleum insecticides belong to this 

 group, as the oil is in the dispersed phase and the water is the continuous 

 liquid. Soap, the usual emulsifier, is colloidally soluble in water. On the other 

 hand it Is perfectly feasible to get an emulsion of water in oil. All that is 

 necessary is that the emulsifier shall be colloidally soluble in the oil. The oil 

 companies are greatly troubled by an emulsion of water in oil that forms 

 when they pump crude oil. In this case it is the colloidal substances dis- 

 solved in the crude oil, asphalt, etc., that acts as the emulsifier. There is a 

 long list of substances that can act as emulsifiers of petroleum in water. Our 

 choice of an emulsifier of oil for insecticidal purposes is by no means limited 

 to soaps. Colloids as different as iron hydroxide, soluble silica, zinc sulphid, 

 and gelatin gave excellent emulsions. In making a spray emulsion, the emulsi- 

 fier is added to the water and then the oil added in gradually. If the emulsion 

 Is to be very much diluted the oil can be put in all at once. It is necessary to 

 follow this general scheme in order that the emulsion shall have the oil as 

 drops and not the reverse phase. 



For spraying purposes an emulsion approaches perfection as the drops 

 become smaller and more uniform in size. One advantage is that the emulsion 

 is much more stable with small even drops. Also under these conditions the 

 application of the oil is more uniform which increases the insecticidal power 

 of the spray. The injury to the plant Is decreased because no drops of free 

 oil gather to cause burning. 



It is often desirable to know what is the correct amount of a certain emulsi- 

 fier to add to a given oil. Up to a certain limit the addition of more emulsififr 

 has the advantage of decreasing the size of the drops. After this limit is 

 reached no addition of emulsifier will cause the drops to get any smaller. The 

 minimum size of the drops seems to be a function of the oil. If there is 

 insufficient emulsifier the size of the drops vary and are generally larger than 

 the minimum. 



