When you look at the whole problem some things become self evident
During a new construction at 221 Baker Street, an amazing discovery was made. A small cabinet was found containing previously unknown documents. They revealed that Dr. Watson kept notes about his adventures with Mr. Sherlock Holmes into the 1950s. An interesting, but incomplete story read as follows:
....and was able to spring into a cab and drive to Baker Street, half afraid that I might be too late to assist at the dénouement of the little mystery. I found Sherlock Holmes alone, however, half asleep, with his long, thin form curled up in the recesses of his arm-chair. A formidable array of bottles and test-tubes, with the pungent smell of hydrochloric acid, told me that he had spent his day in the chemical work which was so dear to him. It was obvious to me that my companion had already examined the carefully closed metal box we had found in a recess behind a sliding panel just above the right bell-pull in poor Browning's sitting-room.
'No doubt the murderers were after this box,' said he. 'They searched the house from cellar to garret. I would have been at a loss myself if I had not met Irene Adler some time ago...'
My attention was captured by the open box on the mantelpiece. It was empty.
'It is far better to keep the contents of the box in mineral oil,' Holmes explained and showed me a bottle. 'This will keep it safe from air but also makes it more flammable.'
The yellowish liquid in the bottle covered a few thumb-sized pellets.
'Is this a dangerous poison?' I asked.
'Not at all, Watson. Have you ever seen a poison in so big a pellet? It would hardly be healthy to swallow, but that is not the point. Now look at this.'
He took out a pellet, dried it with great care, and dropped it into a bowl of water. Instead of slowly dissolving or sinking, the pellet began a strange dance on the surface of the water, hissed ominously, gave out bubbles and some malodorous product. The acrid fumes took me by the throat and set me coughing.
'Holmes, this will kill us both,' I screamed.
'You should have seen the reaction with hydrochloric acid. Anyway, I told you it is not particularly poisonous,' said Holmes coughing. With dramatic suddenness he struck a match, and as he held the match nearer, the bubbles caught fire and gleamed with the most beautiful crimson flame I have ever seen.
'Magnificent, is it not? One ounce of this substance when reacting with water or hydrochloric acid gives more than three cubic feet of gas. To be precise, 3.068 cubic feet at 80.0 degrees and atmospheric pressure.'
'You measured this?' I cried.
'Of course I measured it,' said Holmes with an impatient gesture. He took a small bottle labelled phenolphthalein and put a few drops of its contents into the bowl of water, which turned pink immediately, its colour resembling the gleam of the flames.
'Is this why this substance is so precious?'
'Not really,' murmured Holmes. 'The Powers of Evil created these pellets, Watson, or I am very much mistaken. The murder of Browning was nothing but a trifle in this case.'
Half crazy with fear, I looked at the marble-like pellets in the bottle.
'I do not really understand, Holmes.'
'I made accurate measurements. I dissolved exactly one ounce of this substance in water, then boiled away the water. The remaining white solid I could not dry completely, so I re-dissolved it in water and added some hydrofluoric acid until the colour of phenolphthalein was gone. I boiled away the water again, and drying the white residue was not a problem this time. Its mass was precisely three and one eighth ounces. Three and one eighth. Do you see, Watson?'
'I am still in the dark,' I answered with some embarrassment.
'I do not wish to make a mystery,' said he, laughing. 'The matter is elementary; simplicity itself. You remember our little adventure with Professor Urey?'
The notes end here. Sherlock Holmes uses imperial units of measurement: 1 foot equals 30.48 cm, 1 ounce is 28.350 g, the atmospheric pressure has been constant (101325 Pa) over the last few centuries. The temperature is measured in degrees Fahrenheit (°F): 0 °C equals 32 °F, whereas 100 °C is 212 °F.
Help Watson figure out what was in the box. What could it possibly have been intended for?