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