Nevertheless, the whole affair was profoundly mysterious.  As to the change in the woman’s appearance, there was little in that.  The coarse, black hair might be her own, dyed, or it might be a wig.  The eyebrows were made-up; it was a simple enough proceeding and made still more simple by the beaded veil.  But how did she come to be there at all?  How did she happen to be made-up in this fashion at this particular time?  And, above all, how came she to be provided with a lump of what I had little doubt was poisoned sugar?

I turned over the events of the day, and the more I considered them the less comprehensible they appeared.  No one had followed the omnibus either on foot or in a vehicle, as far as I could see; and I had kept a careful look-out, not only at starting but for some considerable time after.  Yet, all the time, Mrs. Schallibaum must have been following.  But how?  If she had known that I was intending to travel by the omnibus she might have gone to meet it and entered before I did.  But she could not have known:  and moreover she did not meet the omnibus, for we watched its approach from some considerable distance.  I considered whether she might not have been concealed in the house and overheard me mention my destination to Thorndyke.  But this failed to explain the mystery, since I had mentioned no address beyond “Kensington.”  I had, indeed, mentioned the name of Mrs. Hornby, but the supposition that my friends might be known by name to Mrs. Schallibaum, or even that she might have looked the name up in the directory, presented a probability too remote to be worth entertaining.

But, if I reached no satisfactory conclusion, my cogitations had one useful effect; they occupied my mind to the exclusion of that unfortunate draught of tea.  Not that I had been seriously uneasy after the first shock.  The quantity that I had swallowed was not large—­the tea being hotter than I cared for—­and I remembered that, when I had thrown out the lump of sugar, I had turned the cup upside down on the table; so there could have been nothing solid left in it.  And the lump of sugar was in itself reassuring, for it certainly would not have been used in conjunction with any less conspicuous but more incriminating form of poison.  That lump of sugar was now in my pocket, reserved for careful examination at my leisure; and I reflected with a faint grin that it would be a little disconcerting if it should turn out to contain nothing but sugar after all.

On leaving the tea-shop, I walked up Sloane Street with the intention of doing what I ought to have done earlier in the day.  I was going to make perfectly sure that no spy was dogging my footsteps.  But for my ridiculous confidence I could have done so quite easily before going to Endsley Gardens; and now, made wiser by a startling experience, I proceeded with systematic care.  It was still broad daylight—­for the lamps in the tea-shop had been rendered necessary only by the faulty construction of the premises and