86 DDT— Killer of Killers 



But, no use speculating upon something that can't be. If 

 we can't dissolve it in water, we can at least disperse it in 

 water, and that is some help. We can make two types of 

 dispersions: We can mix up a dry DDT powder with water, 

 or we can dissolve the DDT in some organic solvent to make 

 a concentrated, 25 or 30% solution, and then we can mix 

 the solution with water. We'll talk about the first type of 

 dispersion, known as a suspension, in a few minutes. But 

 first, let's take up the second type, known as an emulsion. 

 This is a dispersion of one liquid in another. 



An emulsion has two phases: a dispersed phase (in this 

 case, a solution of DDT in a solvent) and the dispersion 

 medium (in this case, water). We can't use kerosene or 

 fuel oil as the solvent for DDT in the preparation of emul- 

 sions because the DDT is not soluble enough in these sol- 

 vents. To get 25 or 30% DDT in solution, a material like 

 xylene, a coal-tar hydrocarbon, must be used. Furthermore, 

 we must use a third substance, known as an emulsifying 

 agent, in order to give a product which is relatively stable. 

 You know the old saying that "oil and water don't mix." 

 This is not strictly correct. It is true that oil will not dis- 

 solve in water, but under the proper conditions you can get a 

 mixture that is so uniform that it looks like a single sub- 

 stance. You can try this yourself: Shake up some water 

 and kerosene together. See how milky it looks? But now 

 let it stand without shaking. Notice how the little particles 

 of kerosene coalesce and rise to the top? All you have left 

 is a layer of kerosene on top of a layer of water. 



Now add a little soap and shake up the water and kero- 

 sene again. See how much longer it takes for the kerosene 

 to separate out? The soap is the emulsifying agent. Once 

 upon a time soap was about the only emulsifying agent we 



