Of the four nails found in the place where the Cross was buried, one was said to have been sent to Rome. Another the Empress Helena threw into the Gulf of Venice, to allay a storm; while the other two were sent by her to Constantine, who welded one of them to his helmet, as an amulet, and affixed the other to his horse's headstall. Among the classic peoples, symbols of their gods were used by physicians in writing prescriptions for material remedies, as invocations or charms, and were credited with the same wonderful healing powers which were ascribed to holy relics, blessed medals and amulets, and in later times to many purely superstitious remedies.[167:1] The worship of relics naturally afforded a strong impulse to visit sacred places, and especially Palestine. Generally speaking, the prized relic, a piece of the true cross, whether possessed by a church, a crowned head or a private individual, is a minute speck of wood, scarcely visible to the naked eye, set sometimes on an ivory tablet, and always inclosed in a costly reliquary. M. Rohault de Fleury, who calculates that the total volume of the wood of the original cross must have been somewhere about 178,000,000 cubic millimetres, has made a list of all the relics of which he can find any record, and the sum of their measurements amounts to only 3,941,975 cubic millimetres, or about one forty-fifth of the amount of wood necessary to reconstruct the original cross. In the United States there is not an authenticated relic of the cross as large as half a lead-pencil, and some are so minute as to be visible only through the aid of a microscope. The Church of St. Francis Xavier in New York has a fragment which is exposed for veneration on Easter Sunday, as is the custom in European churches possessing a relic. Another fragment, at the Cathedral, is shown on Good Friday. This relic is in a crystal and gold casket, set with precious stones, which form the centre of a handsome altar cross. The French Church of St. Jean Baptiste, in East Seventy-sixth Street, also possesses a relic of the cross.[168:1] The powder obtained by scraping the tombstones of saints, when placed in water or wine, was in great repute as a remedy. The French historian, Gregory of Tours (544-595), was said to have habitually carried a box of this powder, when travelling, which he freely dispensed to patients who applied to him.