Poisonious pills

"Could you lay your hand upon those pills?”
“I have them,” said Lestrade, producing a small white box; “I took them and the purse and the telegram, intending to have them put in a place of safety at the police station.
“Give them here,” said Holmes. “Now, Doctor,” turning to me, “are those ordinary pills?”
They certainly were not. They were of a pearly gray colour, small, round, and almost transparent against the light. “From their lightness and transparency, I should imagine that they are soluble in water.”