“With your permission I should like to retain possession of them temporarily. But first, I would like you to put on them some private mark so that you would be able to swear to them in case it becomes necessary.” “You will be very careful of the letter to Miss Doane?” “Certainly.” “I should hate to have it lost.” In pursuance of the detective’s desire, he put a private mark on each of the letters, and then they were carefully placed in the detective’s pocket. Thanking him, Nick Carter left the house. He went by the back way, and having got downstairs he began looking about him as he went. Finally he appeared to see what he wanted and took possession of it. It was a scrap of paper, and on it were some drops of blood. There was a peculiar smile on his face as he put this in his pocket, and through his mind this thought was running: “I would like to be as sure of getting ten thousand dollars as I am that the verdict of the microscope will be--not human blood!” CHAPTER XX. AT THE CHEMIST’S. Nick Carter went downtown. He turned his steps toward the East Side after leaving the cars, and finally ran up the steps of a house. In response to his ring a servant came to the door, of whom he inquired: “Is the professor in?” “I don’t know, sir. If you will step into the hall, and give me your name, I will go and see.” The detective entered the hall, then said: “Now, please be kind enough to tell me positively if the professor is in?” “I don’t know.” “Yes, you do.” “I say I don’t.” “And I know better. If he is in I promise you he will see me, while if he is out I will not waste my time waiting for your red tape.” The servant was rather taken aback by this plain talk, and for a moment hardly knew what to say. Then the half-laughing reply was made: “While I cannot say positively, I think he is.” “Are you not quite sure that he is?” “Yes.” “Very well. Now, go ahead and don’t waste any time, for I have none to spare.” The servant departed, and soon returning, escorted Nick into the professor’s office. The person alluded to as the professor was one of New York’s most celebrated chemists. He came forward, with a smile, to meet Nick Carter, explaining heartily: “Glad to see you, Carter. It is all of two months since I have set eyes on you.” “So am I glad to see you! Yet I would not be here if it were not that business made it necessary.” “Business, eh?” “Yes.” “You are the greatest fellow to have your hands full of business I ever saw. Well, what is it this time?” “I’ve got something here that I want you to look at.” “Let’s see it.” Nick took out the paper that was spotted with blood. This he quietly handed to the professor, saying: “Take a look at that through the microscope.” “Blood? What do you want to know?” “I want to know if you can say positively what animal’s blood that is?” “Animal’s blood?” “Yes.” “You don’t suspect it of being human blood?”