"Let us have some fresh blood," he said, digging a long bodkin into his finger, and drawing off the resulting drop of blood in a chemical pipette. "Now, I add this small quantity of blood to a litre of water. You perceive that the resulting mixture has the appearance of pure water. The proportion of blood cannot be more than one in a million. I have no doubt, however, that we shall be able to obtain the characteristic reaction."
What does characteristic reaction?