“Never mind,” said he, chuckling to himself. “The question now is about hœmoglobin. No doubt you see the significance of this discovery of mine?”
“It is interesting, chemically, no doubt,” I an- swered, “but practically—”
“Why, man, it is the most practical medico-legal discovery for years. Don’t you see that it gives us an infallible test for blood stains. Come over here now!” He seized me by the coat-sleeve in his ea- gerness, and drew me over to the table at which he had been working. “Let us have some fresh blood,” he said, digging a long bodkin into his fin- ger, 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.” As he spoke, he threw into the vessel a few white crystals, and then added some drops of a trans- parent fluid. In an instant the contents assumed a dull mahogany colour, and a brownish dust was precipitated to the bottom of the glass jar.
“Ha! ha!” he cried, clapping his hands, and looking as delighted as a child with a new toy.
“What do you think of that?”
“It seems to be a very delicate test,” I remarked.
“Beautiful! beautiful! The old Guiacum test was very clumsy and uncertain. So is the microscopic examination for blood corpuscles. The latter is val- ueless if the stains are a few hours old. Now, this appears to act as well whether the blood is old or new. Had this test been invented, there are hun- dreds of men now walking the earth who would long ago have paid the penalty of their crimes.”
“Indeed!” I murmured.
“Criminal cases are continually hinging upon that one point. A man is suspected of a crime months perhaps after it has been committed. His linen or clothes are examined, and brownish stains discovered upon them. Are they blood stains, or mud stains, or rust stains, or fruit stains, or what are they? That is a question which has puzzled many an expert, and why? Because there was no reliable test. Now we have the Sherlock Holmes’ test, and there will no longer be any difficulty.”

A Study In Scarlet, Chapter 1