At these words the younger man stopped short.  “You know—­you?” he interrupted.

“Yes, I know,” replied the old detective, with the evident satisfaction of a man who has gained his revenge.  “They invented a way at the time of that affair at the Maison Blanche, last December.”

“I recollect.”

“Ah! well, on the snow in the courtyard there was a footprint that attracted a detective’s attention.  He said that the whole evidence depended on that mark alone, that it was worth more than ten years’ hard work in following up the case.  Naturally, he desired to preserve it.  They sent for a great chemist—­”

“Go on, go on.”

“I have never seen the method put into practise, but an expert told me all about it, and showed me the mold they obtained.  He explained it to me precisely, on account of my profession.”

Lecoq was trembling with impatience.  “And how did they obtain the mold?” he asked abruptly.

“Wait:  I was just going to explain.  They take some of the best gelatine, and allow it to soak in cold water.  When it becomes thoroughly softened, they heat it until it forms a liquid, of moderate consistency.  Then when it is just cool enough, they pour a nice little covering of it upon the footprint.”

Lecoq felt the irritation that is natural to a person who has just heard a bad joke, or who has lost his time in listening to a fool.

“Enough!” he interrupted, angrily.  “That method can be found in all the manuals.  It is excellent, no doubt, but how can it serve us?  Have you any gelatine about you?”

“No.”

“Nor have I. You might as well have counseled me to pour melted lead upon the footprints to fix them.”

They continued their way, and five minutes later, without having exchanged another word, they reentered the Widow Chupin’s hovel.  The first impulse of the older man would have been to rest to breathe, but Lecoq did not give him time to do so.

“Make haste:  get me a dish—­a plate—­anything!” cried the young detective, “and bring me some water; gather together all the boards and old boxes you can find lying about.”

While his companion was obeying him, Lecoq armed himself with a fragment of one of the broken bottles, and began scraping away furiously at the plastered wall that separated the two rooms.

His mind, disconcerted at first by the imminence of this unexpected catastrophe, a fall of rain, had now regained its equilibrium.  He had reflected, he had thought of a way by which failure might possibly be averted—­and he hoped for ultimate success.  When he had accumulated some seven or eight handfuls of fine plaster dust, he mixed one-half with a little water so as to form a thin paste, leaving the rest untouched on the side of the plate.

“Now, papa,” said he, “come and hold the light for me.”

When in the garden, the young man sought for the deepest and most distinct of the footprints, knelt beside it, and began his experiment, trembling with anxiety.  He first sprinkled upon the impression a fine coating of dry plaster, and then upon this coating, with infinite care, he poured his liquid solution drop by drop.