But there never yet was an adept in any art or science who freely communicated to his pupil the full amount of his own knowledge; something for experience to gather, or for ingenuity to discover, is always kept in reserve, and the instructions of Roger Bacon stopped short at one point. He was himself engaged in the prosecution of that chemical secret which he rightly judged to be a dangerous one, and, while he experimented with the compound of sulphur, saltpetre, and charcoal, he kept himself apart from his general laboratory, and wrought in a separate cell, to which not even Hubert had access. To know that the friar had a mysterious occupation, which, more than the making of gold or the universal medicine, engrossed him, was enough of itself to rouse the young man's curiosity; but when to this was added the fact, that, from time to time, strange and mysterious noises were heard, accompanied by bright corruscations and a new and singular odor, penetrating through the chinks close to which his eyes were stealthily riveted, Hubert's eagerness to know all that his master concealed had no limit. He resolved to discover the secret, even though he should perish in the attempt; he feared that there was good reason for the accusation of dealing in the Black Art, which, more than all others, the monks of Bacon's own convent countenanced, but this apprehension only stimulated him the more. For some time Hubert waited without an opportunity occurring for gratifying the secret longing of his heart; at last it presented itself. To afford medical assistance to the sick, was, perhaps, the most useful practice of conventual life, and the monks had always among them practitioners of the healing art, more or less skillful. Of this number, Roger Bacon was the most eminent, not only in the monastery to which he belonged, but in all Oxford. It was about the hour of noon on a gloomy day toward the end of November, in the year 1282, while the Friar and his pupil were severally employed, the former in his secret cell, and the latter in the general laboratory, that there arrived at the gate of the Franciscan convent a messenger on horseback, the bearer of news from Abingdon, that Walter de Losely, the sheriff of Berkshire, had that morning met with a serious accident by a hurt from a lance, and was then lying dangerously wounded at the hostelry of the Checkers in Abingdon, whither he had been hastily conveyed. The messenger added, that the leech who had been called in was most anxious for the assistance of the skillful Friar, Roger Bacon, and urgently prayed that he would lose no time in coming to the aid of the wounded knight.