It is therefore not to be wondered at that juries give but little consideration to the knowledge or professional standing of expert witnesses. It is, in fact, notorious that the medical autocrat of the village, who has superintended the entrance of the majority of the jurymen into this troublous world, is a more important witness than the most renowned special student of the branch: indeed, the chief value of the real expert often rests on his ability to influence the local physician.[14] At the late Wharton-Van Ness trial the defence desired to show that the work of the chemist employed by the prosecution was unreliable, because the analyses made by him in a previous case had "been condemned by the united voice of the whole scientific world." The court was not able to see the _relevancy_ of this, and refused to allow the professional ability or standing of an expert to be called in question. The witness thus adjudged competent brought no results into court; had kept no laboratory notes; relied solely on a memory so deficient that although he had been teaching for thirty-five years, he could not tell the shape of a crystal of tartar emetic, the poison in question; and upon the stand made a statement different from one which he had furnished officially to the district attorney of Baltimore fourteen months before. There are principles of toxicology which ought to have legal force and recognition, and ought to govern expert testimony in the same way that the principles of evidence govern ordinary testimony. Without presuming to enumerate these, I will cite two or three for illustration. Certain substances, the so-called irritant poisons, such as arsenic, tartar emetic and the like, induce their toxic effects by causing irritation and inflammation of the alimentary canal. All authorities agree that poisoning by these substances cannot be proved, or even rendered, very probable, by symptoms alone--that chemical evidence, the discovery of the poison in the food, dejections, or in case of death the body, is absolutely essential for making out a case. Irritation and inflammation of the alimentary canal occur so often and so suddenly from natural causes, which are sometimes apparent, but often hidden, that no especial weight can be attached to them. In the case of the so-called neurotic poisons, those which act upon the nervous system, the symptoms are so closely simulated by natural disease that even when they agree in the most absolute manner with those usually developed by any such poison they only render poisoning highly probable, not certain.[15] When in any case the symptoms diverge from the typical array, poisoning becomes improbable just in proportion to the amount of divergence.