_Testimony of Expert, how Impeached._—Sometimes, on cross-examination or otherwise, the fact becomes known that the witness who is proposed as an expert has expressed an opinion on the subject in hand contrary to that which he has given upon the witness-stand, and the question has been raised as to establishing that fact at the outset and before his testimony goes before the jury, in order to enable the trial judge to determine whether he is competent. The rule in that case is that the testimony as to his prior expression of opinion is not to be received at that time, but will come properly up as rebuttal, he having been asked upon his cross-examination, giving time and place, whether he has made the statements attributed to him. An expert witness may in other respects be impeached like any other witness, that is, by the oaths of persons who know him and have known his reputation, and who testify that his reputation for truth and veracity is bad and that they would not believe him under oath. He may also be impeached by producing witnesses to prove that his special knowledge or technical skill is not reliable or adequate to the undertaking which he has assumed. But this testimony must be from personal knowledge of the man and not from general reputation. Wharton on Evidence, Sec. 437; Le Rose _v._ Commonwealth, 84 Pa. St., 200.[183] _General Rule as to Required Amount of Skill and Experience Stated._—The general rule may be stated thus, as derived from these and other authorities: The extent of the previous study and investigation, and the amount of skill and information which must be shown, will depend upon the facts of each particular case. But some special and peculiar knowledge or skill must be established, the amount of it to be determined by the trial judge in his discretion. The possession of such knowledge and skill is presumed in medico-legal cases if the witness is a licensed practitioner.