As a Thief in the Night by R. Austin Freeman

I was rather sorry for Dimsdale. His position was a very disagreeable one and he fully realized it. His patient had been poisoned before his very eyes and he had never suspected even grave illness. In a sense, the death of Harold Monkhouse lay at his door and it was pretty certain that every one present would hold him accountable for the disaster. Indeed, it was likely that he would receive less than justice. Those who judged him would hardly stop to reflect on the extraordinary difficulties that beset a busy medical man whose patient is being secretly poisoned; would fail to consider the immense number of cases of illness presented to him in the course of years of practice and the infinitely remote probability that any one of them is a case of poison. The immense majority of doctors pass through the whole of their professional lives without meeting with such a case; and it is not surprising that when the infinitely rare contingency arises, it nearly always takes the practitioner unawares. My own amazement at this incredible horror tended to make me sympathetic towards Dimsdale and it was with some relief that I noted the courteous and considerate manner that the coroner adopted in dealing with the new witness.

"I think," the former observed, "that we had better, in the first place, pursue our inquiries concerning the medicine. You have heard the evidence of Dr Randall and Dr Barnes. This bottle of medicine, before any was taken from it, contained twelve and a half grains of arsenious acid, in the form of just over three fluid ounces of Fowler's Solution. Can you suggest any explanation of that fact?"