SoR 7 – The white powder

Sherlock was lying on the sofa with his eyes closed. His fingertips touching he was deeply lost in his mind palace when Billy Wiggins burst back into the living room of 221B Baker Street.
»Mr. Holmes!«, Billy panted. Sherlock partly sat up and glared at him. He had sent Billy downstairs to ask Mrs. Hudson for tea & biscuits.
»Sorry, Mr. Holmes, but I’d never have thought…«, Billy continued and hurried to explain the reason for his excitement as Sherlock’s look was darkening.
»I just wanted to tell Mrs. Hudson about the tea and the door of her kitchen stood ajar… and so through the gap I could see… and you won’t believe it, Mr. Holmes… she gave white powder into the teapot and poured boiling water over it! I’ve never seen anyone prepare coke tea?! Or do you think it might even be heroin?«
With an irritated sigh Sherlock fell back into his cushion and told the baffled Billy:
»When you’ll go down again now to finally order tea & biscuits as I had asked you to you’ll find that Mrs. Hudson doesn’t drink this infusion but actually pours it down the sink or toilet. I’ll leave it to her to explain to you what the white powder is all about. I just hope she’ll rinse the teapot throroughly before she prepares my tea in it.«

Do you know what Billy observed Mrs. Hudson doing? What is the white powder called which she used?

The name consists of two words. You will need the fourth letter of the first word for the final solution. Further informations about the summer giveaway… SoR