A hideous traffic was going on among these nightmare ghouls, whereby illustrious bones were bartered with the calm calculativeness of schoolboys swapping books; and from what was extorted from this centuried dust there was anticipated a power and a wisdom beyond anything which the cosmos had ever seen concentrated in one man or group. They had found unholy ways to keep their brains alive, either in the same body or different bodies; and had evidently achieved a way of tapping the consciousness of the dead whom they gathered together. There had, it seems, been some truth in chimerical old Borellus when he wrote of preparing from even the most antique remains certain "Essential Saltes" from which the shade of a long-dead living thing might be raised up. There was a formula for evoking such a shade, and another for putting it down; and it had now been so perfected that it could be taught successfully. One must be careful about evocations, for the markers of old graves are not always accurate.