It is not necessary that the physician should have made the particular disease involved in the enquiry a specialty; medical men of practice and experience are experts, and their opinions are admissible in evidence upon questions that are strictly and legitimately embraced in their profession and practice. If one has made the matter in question a specialty, doubtless his opinion will be of more value than if he has not; and it has been said, that one who has devoted himself exclusively to one branch of his profession cannot give evidence as an expert on another [277]. For example, one not an oculist may speak as to the cause of injuries to an eye; one who has not made diseases of the mind a special study may give his opinion as to the existence of insanity; one not a practical chemist or analyst, but understanding the practical details of chemistry and the means of detecting poisons, may testify as to the tests in the chemical analysis of a stomach, and as to the tests usually applied to detect poison [278]. The law will even allow a physician to speak as to the length of time a mule has been suffering from a disease [279]. But one who has had no experience as to the effect upon health of illuminating gas cannot testify in relation thereto as an expert [280]. Nor can one who has for thirty years been exclusively treating the insane be permitted to testify, as an expert, on |113| the mental capacity of a person in the last stages of disease, who has not been previously insane [281]. To render the opinion of a witness competent evidence, he must, in general, be in some way peculiarly qualified to speak on the subject, and have knowledge not possessed by the mass of persons of ordinary experience and intelligence [282]. Upon this principle, a priest who had studied physiology and psychology, in order that he might pass upon the mental conditions of communicants in his church, and who had so to decide daily, was permitted to speak as to the mental state of a woman whom he had attended in her last illness [283]. It is a question of fact to be decided at the trial, by the Court, whether a witness offered as an expert has the necessary qualification [284]. And the matter cannot be referred to the decision of the jury. The decision of the Judge at the trial will not be interfered with by the Court, except in a clear and strong case [285]. Any one offered as an expert who cannot establish the fact of special knowledge or skill, in the particular department which he is called upon to illuminate, will be rejected. A Court before permitting an expert to testify may examine him, or hear evidence, to satisfy itself that the witness is really what he assumes to be [286]. |114|