As Physicians, Surgeons, and others conversant in medicine and chemistry, are constantly called upon to give testimony in Courts of Justice, it is necessary for us to enter upon this subject of the law of evidence, so far as it immediately affects the medical witness; it is proper that he should understand when he is bound to appear, and on what terms, and it may be useful for him to be prepared, by some previous knowledge of the usual course of examination, for the difficulties and objections which may arise in the progress of it. A scientific witness, fully acquainted with the subject in dispute, and by his particular knowledge well qualified to inform the Court on the most important points, is too frequently rendered miserable in himself, and absolutely ineffective to the ends of justice, by the diffidence which a man of real acquirement generally feels, when impressed at once with the novelty of his situation, a sense of the importance of the duty which he is about to perform, and a consciousness that the truths which he is about to utter, may be obscured, suppressed, or perverted, by technicalities for which he is unprepared with any defence; we do not mean to arraign the present forms of examination in general, when we assert that some abuse in practice too frequently places the witness in as painful a situation, as if he were himself a criminal. Some knowledge of the law of evidence is the best security against this inconvenience; we propose therefore to lay down a few general rules on the points most likely to occur, and to refer our readers for more particular information to those works which expressly or incidentally treat on this subject.[230] It is necessary in the first place to consider how the attendance of witnesses is to be compelled by process, under what terms they must appear, their liabilities if they fail to appear, and their duties when in Court. The writ of Subpœna _ad testificandum_, is the ordinary process of the Courts for compelling the attendance of witnesses; by this the intended witness is required to appear at the trial at a fixed time and place, to testify what he knows in the cause, under the penalty of £100 to be forfeited to the king. Four witnesses may be included in one subpœna, but a ticket containing the substance of the writ (which is to be shewn at the same time) is as effectual service as the writ itself, (5 _Mod._ 355). The service must be upon the witness in person, (_Cro. Eliz._ 130) and within reasonable time, before the trial, respect being always had to the residence and circumstances of the party.