the leading hotels in Chicago, which in practically all cases were members of the American Protective League. Senor Otero was found to have reserved a large suite at the Blackstone, and had made the time of his arrival known. From that time on, he was in the hands of the American Protective League, although he never knew it. The boy who took his bag at the door was an A. P. L. operative, the bellhop who responded to his summons was an A. P. L. operative, his waiter at table was A. P. L., his night taxicab driver was A. P. L. In fact, the A. P. L. put Senor Otero to bed and woke him up in the morning, followed his activities during the day and knew what he was doing all night. It was not discovered that he was engaged in any plot against the peace of the United States, but was apparently active in the more pleasant task of spending some money he had gotten hold of in Mexico. If relatives or friends of the Senor Otero would be pleased to know how he spent it, the nature of his associations in Chicago by day—or night—and if they can persuade the Department of Justice to advise them, they can find the entire record of his stay in Chicago. Had he been engaged in any suspicious acts against this country, his return to Mexico might not have been so peaceful.
If an A. P. L. man knew the chemistry of any synthetic or invisible ink, he would not make the secret public any more than would M. I. D. Many devices for making and using these inks, however, are very generally known, although it is believed that Great Britain and France have gone farthest in classifying and developing them. A piece of a necktie has been taken from one German, a corner of which, snipped off and put in a glass of water, would make an invisible ink. A shoestring has been known to do the same thing, a small piece of it making enough for a letter or more. A shirt-stud has been described by a foreign operative, which, when unscrewed and dropped into a glass of water, would do the same thing and leave no trace. With what chemicals were these articles treated in order to make the ink? Ah, that is another matter. If the author knew, he could not tell. One thing is sure, it is not likely that the most inventive writer of "detective" stories could imagine anything more ingenious or
