ROBERT ALEXANDER.--This gentleman, who is well known to the citizens of Chicago, has held several positions of responsibility and trust, and has ever been a consistent, earnest, devoted advocate of the Union. So intensely Republican in sentiment is he, that the attempt to introduce him into the Sons of Liberty, called forth such opposition that it was thought we should fail in the attempt, and he finally, was only admitted, after he and his sponsor (the writer) had been told, in plain words, accompanied with an oath, that if he proved false to them, _both should die_. For months he bore the opprobrium of a Copperhead, and suffered extreme annoyances in sustaining the role it was his duty to assume. Conscientious, earnest, persevering, patient, with keen perception, and a remarkable power of reading human character, with the experience of an excellent police officer, Mr. Alexander brought to his post of duty high qualifications, and was a valuable, ready and willing assistant. It should be remarked that Mr. Alexander had been informed in May, 1864, that he had been appointed First-Lieutenant in the 53d U.S. Infantry, and supposed he was in the service of the U.S. Government at the time of joining this great undertaking, but the information, though coming from a high source, proved incorrect, and this is one additional reason why the writer made choice of Mr. Alexander. While we know that loyal men will appreciate Mr. Alexander's valuable services, we have yet to learn that he has, thus far, experienced any other satisfaction than the approval of his own heart, and the sincere gratitude of the writer, for his hazardous undertaking, and the able manner in which he performed his duty.