"The door swung open as we ascended the front steps, and I was ushered into a cheerful dining-room where a meal lay already spread. Mr. Jones was a brilliant talker, and throughout the meal he kept up a flow of interesting conversation, without, however, once hinting at the nature of the business which he had brought me there to discuss. It was only when we had adjourned to the smoking-room, with one detective patrolling the gravel walk in front of the windows and another keeping watch in the passage outside the door, that he placed his hand in his pocket and produced a small sheet of paper.
"'Did you write that?' he asked in a conversational tone.
"I nodded, wondering what was coming next. For the thing was merely one of the letters that I had sent to the Ministry of Munitions, suggesting a quite minor and unimportant modification of the formula of one of the stock explosives. But before I could frame the question that was in my mind, he turned the sheet over and pointed to some chemical symbols scribbled in pencil on the back.
"'And this too, I presume?' he went on, watching me keenly the while.
"I took the paper in my hand and read: C4H7N3O2. C6H12O6. C216H338N51S5O68. C12H14O4(NO3)6. C3H5(NO3)3. There was a sixth combination of symbols, but this I must not divulge, even to you; so, for the purpose of this narrative, I will refer to it simply as the 'X Formula.'
"In a flash I realized what had happened. I must have been jotting down some notes respecting my experiments, and I had inadvertently used the same sheet of paper on which to write my letter to the ministry.
"'Yes,' I was forced to admit, 'that is my handwriting. But I certainly had no idea that there was anything on the back of that sheet when I sent that letter to you.'
"'I can well believe that!' Mr. Jones smiled somewhat grimly. "It's extremely fortunate that the communication did not fall into other hands. However, I have not brought you here to call you over the coals for being so careless. It is rather to ask you for a friendly explanation of what was in your mind when you made those notes.'
"'You know the meaning of the formulæ?'
"Mr. Jones nodded his gray head. 'Naturally, in these days, when every newspaper is full of the spy peril, we should not allow a set of mysterious-looking letters and figures to pass through our hands without wanting to know the meaning of it. Within an hour of its receipt, that letter was in the hands of a government analyst. But his report only seemed to deepen the mystery. He states that the first three formulas respectively represent creatine, inosite, and albumen—three organic substances which are to be found in every human body; while the last three combinations of symbols represent gun-cotton, nitro-glycerin, and the newly invented devastite—three of the most powerful explosives known to science.'
"'Yet the same chemical elements occur in each!' I said slowly. 'Carbon, hydrogen, nitrogen and oxygen—combined in certain proportions they form substances, not only innocuous in themselves, but substances that are absolutely vital to the human organism. Combine the same elements in different proportions, and you have the deadliest explosives!'
"'My God!—you mean to say——'
"'That every human being is a potential living bomb!'
"The effect of my words was electrical.
