The advancement of the sciences and the progress of research in special fields of knowledge have made expert testimony of large importance during the present century. The basis of its admission is the fact that there are certain processes of reasoning which an ordinary jury is incapable of performing, even with the assistance of courts and lawyers. Oftentimes in the administration of justice in our courts, proof is given of circumstances which although admitted would have little or no significance in the mind of an ordinary juror, and which he would be unable to contrast and compare with other facts, successfully, without the aid of those more familiar with scientific matters and the inductive process of reasoning than he is. In such cases it is necessary that the jury should be specially enlightened by persons who have, through training, skill and experience, acquired the power to enlighten them. A common instance and illustration of this matter is to be found in the case of homicide by poisoning. A human body is found dead; externally there may be no _indicia_ to show positively the cause of death. Under such circumstances the laws of all civilized countries permit what is called a post-mortem examination by skilled physicians, who, finding no external evidences of the cause of death, are permitted by the officers of the law to remove the internal portions of the body for special and careful examination. If this discloses traces of inflammation or of lesions of an abnormal character, further power is vested in the authorities to have at the expense of the State a chemical examination of the internal organs. If this examination, which is necessarily long and excessively technical, results in the discovery of any poisonous substance, such as would produce death, and if it is found in sufficient quantities to produce death, these persons who made the post-mortem examination and discovered the outward indications of the administration and effects of the poison, and the chemists who discovered the poison itself in the tissues of the body, in sufficient quantities to produce death, are called as experts before the jury. The post-mortem examiners explain what the appearance of the body was, as distinguished from the appearances of the body of an individual who had died from natural causes. The chemist describes his course of experimentation, the various deductions which he made from his experiments, the tests which he applied in his investigation in discovering poison, and is then allowed to testify that the poisonous substance was found in sufficient quantities to produce the physical appearances which the post-mortem examiners have described, and to accomplish the death of the human being in whose body the poison was found. It is obvious that the power of observation and the skill, which the skilled chemists and physicians used as the basis of their reasoning in this case, were such as an