Nayland Smith spread his hands in a characteristic gesture.

“The green mist, Petrie, can be explained in several ways. 
Remember, we have only one man’s word that it existed. 
It is at best a confusing datum to which we must not attach
a factitious importance.”

He threw the wrappings on the floor and tugged at a twine loop in the lid of the square box, which now stood upon the table.  Suddenly the lid came away, bringing with it a lead lining, such as is usual in tea-chests.  This lining was partially attached to one side of the box, so that the action of removing the lid at once raised and tilted it.

Then happened a singular thing.

Out over the table billowed a sort of yellowish-green cloud—­ an oily vapor—­and an inspiration, it was nothing less, born of a memory and of some words of my beautiful visitor, came to me.

“Run, Smith!” I screamed.  “The door! the door, for your life! 
Fu-Manchu sent that box!” I threw my arms round him. 
As he bent forward the moving vapor rose almost to his nostrils. 
I dragged him back and all but pitched him out on to the landing. 
We entered my bedroom, and there, as I turned on the light,
I saw that Smith’s tanned face was unusually drawn,
and touched with pallor.

“It is a poisonous gas!” I said hoarsely; “in many respects identical with chlorine, but having unique properties which prove it to be something else—­God and Fu-Manchu, alone know what!  It is the fumes of chlorine that kill the men in the bleaching powder works.  We have been blind—­I particularly.  Don’t you see?  There was no one in the sarcophagus, Smith, but there was enough of that fearful stuff to have suffocated a regiment!”

Smith clenched his fists convulsively.

“My God!” he said, “how can I hope to deal with the author of such a scheme?  I see the whole plan.  He did not reckon on the mummy case being overturned, and Kwee’s part was to remove the plug with the aid of the string—­after Sir Lionel had been suffocated.  The gas, I take it, is heavier than air.”

“Chlorine gas has a specific gravity of 2.470,” I said; “two and a half times heavier than air.  You can pour it from jar to jar like a liquid—­if you are wearing a chemist’s mask.  In these respects this stuff appears to be similar; the points of difference would not interest you.  The sarcophagus would have emptied through the vent, and the gas have dispersed, with no clew remaining—­except the smell.”

“I did smell it, Petrie, on the stopper, but, of course, was unfamiliar with it.  You may remember that you were prevented from doing so by the arrival of Sir Lionel?  The scent of those infernal flowers must partially have drowned it, too.  Poor, misguided Strozza inhaled the stuff, capsized the case in his fall, and all the gas—­”

“Went pouring under the conservatory door, and down the steps, where Kwee was crouching.  Croxted’s breaking the window created sufficient draught to disperse what little remained.  It will have settled on the floor now.  I will go and open both windows.”