As a Thief in the Night by R. Austin Freeman

"We now," Thorndyke resumed, "enter the final stage of the inquiry. Hitherto we have dealt with purely scientific evidence which would have had to be communicated to the jury and which they would have had to take on trust with no convincing help from their own eyes. We had evidence, conclusive to ourselves, that Monkhouse had been murdered by means of a poisoned candle. But we could not produce the candle or any part of it. We had nothing visible or tangible to show to the jury to give them the feeling of confidence and firm conviction which they rightly demand when they have to decide an issue involving the life or death of the accused. It was this something that could be seen and handled that I sought, and sought in vain until that momentous evening when I called at your chambers to return your diary.

"I remember that as I entered the room and cast my eyes over the things that were spread out on the table, I received quite a shock. For the first glance showed me that, amongst those things were two objects that exactly fulfilled the conditions of the final test. There was the wax mould -- a part, and the greater part, of one of the suspected candles; and there was the tress of hair -- a portion of the body of the person suspected to have been poisoned. With these two objects it was possible to determine with absolute certainty whether that person had or had not been poisoned with arsenic, and if she had, whether the candle had or had not been the medium by which the poison was administered."