The missile from the air pistol struck the center of Jerome Coffern's chest.
Instantly a puff of grayish vapor arose. It was as though a small cloud of cigarette smoke had escaped from the chemist's body at that point. No sound of an explosion accompanied the phenomena, however. There was only the dull impact of the air-gun missile striking.
The grayish vapor increased in volume. It had a vile, oily quality. Close to Jerome Coffern's body, it was shot through and through with tiny, weird flashes. These were apparently of an electrical nature. (From The Land of Terror by Lester Dent, writing as Kenneth Robeson)
The Smoke of Eternity, used by the villain Kar in The Land of Terror, is characterized as a grayish vapor with electrical sparks. It is composed from an unidentified element discovered by Kar during his expedition to Thunder Island with Jerome Coffern. It is capable of dissolving anything it comes into contact with the exception of a specific metal alloy that is used to contain the material.
The element, an ore found only on Thunder Island, is mined and chemically processed to produce the Smoke of Eternity.
Kar uses it to have his minions dispose of the body of Jerome Coffern, to attack Doc Savage (on several occasions), dissolve the back of Doc's safe, dissolve a wall and a bank vault door, and more. It is a very dangerous material that Doc wishes to keep out of the hands of other criminals.
Kar developed an air pistol to shoot a projectile of the stuff at a target. It was also used as a bomb in one attempt to kill doc Savage.