As attendance is more burthensome on a professional man than on others, so also it is more frequently called for; men in general can only be summoned as witnesses when they have, or are reasonably supposed to have, cognisance of the particular facts in question; and he may therefore deem himself peculiarly unfortunate or imprudent, who is often present at such scenes as give rise to criminal investigation; but the medical practitioner, in addition to his liability of being called in for his assistance, and so becoming acquainted with facts, may also be summoned on matters of opinion; those therefore who stand highest in public estimation as men of science and research, will be most frequently burthened with the execution of painful and unprofitable duties; we do not believe that they will shrink from the performance of them when necessary, but we may express a hope that they may be rendered as little burthensome as their nature will allow. Great difficulties must always arise in the examination of a medical or chemical witness, where the examining party is uninformed or at least very partially acquainted with the science in question; for it is next to impossible for Counsel so to frame their examination of a scientific witness, as to elicit the whole truth unless they are, by previously acquired knowledge, acquainted with the bearings of each answer upon the case which they are maintaining; and though there are a few instances of persons of such superior talent, that they can collect from the mere information of their briefs, so much knowledge as will enable them to perform this duty, with credit to themselves and satisfaction to their clients and the public; yet such instances are rare, and even those most gifted will admit that there is a most material difference between examining a witness on matters of fact of which all persons who have applied themselves to the laws and nature of evidence may be competent judges, and the examination of abstract opinions, and speculations of philosophy or physics, where the examiner can as little follow the reasoning of a witness as if he spoke some foreign and unknown language. For it is impossible within the compass of any ordinary _viva voce_ examination to elicit all the points on which explanation may be necessary, or to remove all the doubts which may give occasion to future controversy; hence questions of this kind are seldom determined at the first hearing, but are repeatedly brought before the Courts in the form of new trials; the cases of _Severn, King & Co._ against several Fire Insurances Offices, which in part suggested the undertaking of the present work, may serve as an elucidation of this point. The causes were conducted by professional men of the first eminence, the Judge who presided well known for his love of science, and from having attained more knowledge in several branches of natural philosophy, than can usually be acquired by those whose time is engrossed by severer studies;