For answer the guard dampened the flap of the envelope, sealed it, thrust it into his pocket and passed on.  The secret agent sat down again, and sipped his milk meditatively.

One hour later Mr. Grimm, accompanied by Johnson, came out of a photographer’s dark room in Pennsylvania Avenue with a developed negative which he set on a rack to dry.  At the end of another hour he was sitting at his desk studying, under a magnifying glass, a finished print of the negative.  Word by word he was writing on a slip of paper what his magnifying glass gave him and so, curiously enough, it came to pass that Miss Thorne and Chief Campbell of the Secret Service were reading the hidden, milk-written message at almost the identical moment.

“Johnson got Petrozinni’s letter from the postman,” Mr. Grimm was explaining.  “I opened it, photographed it, sealed it again and remailed it.  There was not more than half an hour’s delay; and Miss Thorne can not possibly know of it.”  He paused a moment.  “It’s an odd thing that writing such as that is absolutely invisible to the naked eye, and yet when photographed becomes decipherable in the negative.”

“What do you make of it?” Mr. Campbell asked.  The guileless blue eyes were alive with eagerness.

“Well, he’s right, of course, about not being in danger,” said Mr. Grimm.  “If he came with credentials as special envoy this government must respect them, even if Senor Alvarez dies, and leave it to his own government to punish him.  If we were officially aware that he has such credentials I doubt if we would have the right to keep him confined; we would merely have to hand him over to the Italian embassy and demand his punishment.  And, of course, all that makes him more dangerous than ever.”

“Yes, I know that,” said the chief a little impatiently.  “But who is this man?”

“Who is this man?” Mr. Grimm repeated as if surprised at the question.  “I was looking for Prince Benedetto d’Abruzzi, of Italy.  I have found him.”

Mr. Campbell’s clock-like brain ticked over the situation in detail.

“It’s like this,” Mr. Grimm elucidated.  “He has credentials which he knows will free him if he is forced to present them, but I imagine they were given to him more for protection in an emergency like this than for introducing him to our government.  As the matter stands he can’t afford to discover himself by using those credentials, and yet, if the Latin compact is signed, he must be free.  Remember, too, that he is accredited from three countries—­Italy, France and Spain.”  He was silent for a moment.  “Naturally his escape from prison would preserve his incognito, and at the same time permit him to sign the compact.”

There was silence for a long time.

“I believe the situation is without precedent,” said Mr. Campbell slowly.  “The special envoy of three great powers held for attempted—!”

“Officially we are not aware of his purpose, or his identity,” Mr. Grimm reminded him.  “If he escaped it would clarify the situation tremendously.”