Fractional Distillation is a term used in chemistry for the separation of a mixture into its component parts or fractions, such as separating chemicals by the boiling points. The separation of crude oil from other products, is an example of fractional distillation as is the separation of liquid oxygen, liquid nitrogen,and concentrated argon from air.
In John Steinbeck's Of Mice and Men, there exists a figurative fractional distillation. First of all, the setting of the Great Depression separates men from their families as they must become itinerant workers because their farms have dried up in the Dust Bowl or because they have lost their jobs in some financial disaster. The "bindle stiffs" who seek work in California's fertile soil are solitary seekers who vie for jobs against other strangers.
At the ranch where George and Lennie secure work, all these separated men are suspicious of the others. Even Slim, with his calm "God-like eyes," asks George why he travels with Lennie. The others conflict constantly with another, separating themselves fromeach encounter. For instance, Whitson breaks the bond between Candy and his old dog; and, just as the men seem to have established some peace in the bunkhouse, Curley's wife appears, causing enough tension to severe any harmony. When Curley himself jealously appears, his fractious nature causes him to attack Lennie and wreak havoc into any fraternity which may have begun among the men. With the stable buck, Crooks, too, there is great separation as he is racially segregated and forced to live in the barn with the mules.
Indeed, it is this figurative "fractional distillation" of the men that causes their aggressive and cruel behavior which, in turn, leads to the destruction of Lennie and, with him, the end of the dream which has unified Lennie, George, Candy, and even momentarily Crooks in a hopeful brotherhood as the separate Whitson and Curley search for Lennie to kill him for accidentally breaking Curley's wife's neck.