I turned on a low-powered light in an alcove and got to work.
Excited as I was, it was still necessary to follow the rules. I turned on the fan which would exhaust any fumes from the hood which covered the work area. More than one chemist in days gone by had, while conducting the Marsh test, sniffed at his apparatus and died in agony.
I pulled the silver medallion from my pocket. Fortunately I had remembered to wrap it in a bit of cellophane for protection before tying it back into a knot in my handkerchief.
As I set up the required glassware, I was possessed by the old familiar thrill. Like the vicar in the run-up to the consecration, I was about to witness a transformation at my very fingertips, to be glorified by the gods of chemistry.
The Marsh test is not only simple and elegant, but also the most theatrical of the chemical procedures. How many sleuths in fact and fiction have hunched tensely over that telltale flame?
It is that hushed moment just before the final curtain when all the world seems to hold its breath: the moment when nothing more than a tiny, flickering, and nearly invisible flame will either send the accused to the gallows or set him free.
It was James Marsh, the ordnance chemist of the Royal Arsenal at Woolwich, who recognized that nascent (meaning newly generated) hydrogen, whenever brought into contact with any compound of arsenic and oxygen, will produce water and arseniuretted hydrogen, otherwise known as arsine, an extremely poisonous gas with the chemical formula ASH3 and the odor of garden-fresh garlic.
His test is so sensitive that it is able to detect as little as two parts in a million of arsenic.
These days, of course, the idea of newborn hydrogen having superior powers is generally pooh-poohed, and it is now believed that age does not wither its ability to finger a felon.
Oxygen is oxygen is oxygen they say, although Dogger, being old-fashioned, doubts this.
I dropped a bit of zinc into the bottom of the U-shaped tube, then filled it somewhat more than half full with sulfuric acid.
With a small twist of surgical cotton, I wiped off about a quarter of the shiny black tarnish from the medallion and dropped the swab into the left side of the tube, sealing it with a glass stopper. The swab began to char and turn black as it was carbonized by the sulfuric acid.
The zinc at once began to bubble in the acid. Hydrogen was being born!
And, if my hunch was right, arsine.
The stoppered right side of my U-shaped container led off through a slender glass tube which ended in an upturned tip.
I waited for about thirty seconds … lit a match … held it to the tip of the exhaust tube and …
Poof!
A flame … burning red, burning orange, burning blue …
I reached for an unglazed pottery dish, flipped it over, and held its underside to the flame, much as a freezing schoolboy home for the holidays holds his bottom to the family fireplace.
A circular dark patch began to form around the outer edges of the flame, brownish at first, but quickly turning black and shiny.
An arsenic mirror, in which, if I were any judge, the image of a murderer would soon be reflected.
This wasn’t the end of it by any means. First, I needed to place a few drops of sodium hypochlorite in solution on part of the newly formed mirror. If the sooty deposit was soluble, and vanished, it was arsenic; if it remained, it was arsenic’s cousin, antimony.
And then, of course, I needed to repeat the experiment with clean, uncontaminated glassware and a fresh and untreated swab. This would be my control, or reference, and should result in no formation of an arsenic mirror.
I leaned back from the little pool of light to think about what I had discovered: about what it would mean for me—and for several others. Once I made my findings known, Miss Bodycote’s Female Academy would never be the same again.
It was at that moment that a voice from out of the darkness behind me said: “Very clever.”