But it all hangs together, and if we find any poisonous matter in the sugar, it will be reasonable to assume that we are right.

The sugar is the Experimentum Crucis.

If you will hand it over to me, we will go up to the laboratory and make a preliminary test or two." I took the lump of sugar from my pocket and gave it to him, and he carried it to the gas-burner, by the light of which he examined it with a lens.
"I don't see any foreign crystals on the surface," said he; "but we had better make a solution and go to work systematically.

If it contains any poison we may assume that it will be some alkaloid, though I will test for arsenic too.

But a man of Weiss's type would almost certainly use an alkaloid, on account of its smaller bulk and more ready solubility.