"Why think of unpleasant things?" he said with a smile. Then they went into the library, where Professor Wentworth added his congratulations. "But I'm afraid I didn't accomplish much," said Jim, explaining about the pictures. "Let me see them," said the professor. Jim handed them over. For a moment or two Professor Wentworth examined them intently, holding them this way and that. "They indeed appear to be extremely over-exposed," he admitted at length. "Your Fire Ants are doubtless radio-active to a high degree. The results could not have been much worse had you tried to photograph the sun direct." "I thought as much," said Carter, gloomily. "But possibly the damage isn't irreparable. Suppose we try re-developing a few of these negatives." He led the way to his study, which since the destruction of the observatory had been converted into a temporary laboratory. * * * * * Ten minutes later, Professor Wentworth had his re-developing bath ready in a porcelain basin and had plunged some of the negatives into it. "This process is what photographers call intensification," he explained. "It consists chemically in the oxidation of a part of the silver of which the image is composed. I have here in solution uranium nitrate, plus potassium ferricyanide acidified with acetic acid. The latter salt, in the presence of the acid, is an oxidizing agent, and, when applied to the image, produces silver oxide, which with the excess of acetic acid forms silver acetate." "Which is all so much Greek to me!" said Carter. "At the same time, the ferricyanide is reduced to ferrocyanide," the professor went on, with a smile at Joan, "whereupon insoluble red uranium ferrocyanide is produced, and, while some of the silver, in being oxidized by this process, is rendered soluble and removed from the negative into the solution, it is replaced by the highly non-actinic and insoluble uranium compound." The process was one quite familiar to photographers experienced in astronomical work, he explained. In fifteen minutes they should know what results they were getting. But when fifteen minutes passed and the negatives were still as black as ever, Jim's hope waned. Not so Professor Wentworth's, however. "There is a definite but slow reaction taking place," he said after a careful examination. "Either the over-exposure is even greater than I had suspected, or the actinic rays from your interesting subjects have formed a stubborn chemical union with the silver of the image. In the latter event, which is the theory I am going to work on, we must speed up the reaction and tear some of that excess silver off, if we're ever to see what is underneath." "But how are you going to speed up the reaction?" asked Jim. "I thought that uranium was pretty strong stuff by itself."