Nevertheless, in that case the evidence of chemists who had examined the wadding, and had discovered the marks on it which were said to be powder-marks, and upon analysis had determined that they were powder-marks, or that they were marks of powder which had exploded, would have been clearly admissible. The subjects concerning which medical men may be called upon to testify as experts are as numerous as the diseases, injuries, mental and physical conditions of the human race which fall within the range of the practice of medicine and surgery. It is therefore practically impossible to give them in detail.[185] _Practical Suggestions and Admonitions Embodied in Rules._—It is deemed advisable that the following practical suggestions and admonitions to physicians, concerning their duties as expert witnesses, shall here be given. First: A physician should refuse to testify as an expert unless he is conscious that he is really qualified as an expert. Second: After accepting the responsibility, his first duty should be to make a diligent examination and preparation for his testimony, unless it is upon a subject with which he is familiar and which he is satisfied that he has already exhausted, by reading the best authorities that he can find, and by careful reflection upon particular questions as to which his opinion will be asked. Third: Where he is to make an examination of facts, such as the post-mortem examination of a body, a chemical analysis or an examination of an alleged insane person, he should insist upon having plenty of time and full opportunity for doing his work thoroughly. He should take particular pains to make his examination open and fair, and, if possible, should invite opposing experts to co-operate with him in it. Fourth: He should be honest with his client before the trial in advising him and giving him opinions, and upon the trial should preserve an absolutely impartial attitude, concealing nothing, perverting nothing, exaggerating nothing. Fifth: On the preliminary examination as to his qualifications as a witness he should be frank and open in answering questions. He should state fully the extent and the limits of his personal experience and of his reading upon the subject, without shrinking from responsibility, yet without self-glorification. Sixth: He should be simple, plain, and clear in his statement of scientific facts and principles, avoiding the use of technical language, and trying to put his ideas in such form that they will be grasped and comprehended by men of ordinary education and intelligence.