There was no truly reliable method, for example, of distinguishing a spot of rust from a genuine bloodstain, not to mention telling human from animal blood. As Holmes laboured in the chemistry lab at Bart’s, the idea of distinguishing different types of human blood was still decades off. Back in the 1860s, a Swiss chemist called Christian Friedrich Schönbein had noted that if a stain foamed on contact with hydrogen peroxide, it likely contained haemoglobin and so could be assumed to be blood. But it was not an easy test to run. However, according to Holmes, his new method could identify blood in a solution where the proportion of blood was not more than one in a million. This was truly breathtaking stuff.