“Now,” Grover told the absorbed patrolmen, and a detective who had come, by Police Chief’s order, from Headquarters, “here is a dodge that some police departments have tried, and it will interest you.”
Roger assembled on the interviewing desk his heater for a great lot of the wax, held in a crucible over the electric stove. In a large glass container he mixed, according to a formula dictated by Grover, nitric acid and other chemicals, which discretion suggests should not be mentioned here.
“The purpose of this experiment,” Grover said, “is to learn which hand, if any among us, held, and discharged the weapon. That seems to be the simplest way to narrow down investigation. Once we know our culprit, he must reveal where Astrovox is, what happened.”
The very modern experiment, the police saw, was based on the fact that the charges used in modern pistol projectiles form, during combustion, gases which leave marks on any hand discharging the bullet.
Grover explained his procedure.
“The gases blow back sufficiently to mark the hand,” he stated. “If our test is made within five days after such an occurrence, the test will reveal it.
“I will be first. Roger will take the wax, properly softened, and at a temperature around one hundred and fifteen degrees, Fahrenheit, not hot enough to scald, will pour it over and will mould it around my hand.”
Roger carried out the action as it was described.
“The paraffin, now cooling, at a point where it is hard enough to hold its shape, is taken off.”
This, also, Roger carried out carefully, securing a sort of cast with the shape of the hand moulded inside it.