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254 black color. In parchment this color is permanent, but in paper it is only temporary. Old parchment chronicles in the Nuremberg Museum which have been treated in this way are now as legible as when first they were written, though before the application of the process all color had faded out of the ink. The rationale of the process is, that by the action of the ammonia sulpho-hydrate, the iron of the ink is changed into a black sulphuret.

An Optical Illusion.—St. Simon, in his famous "Mémoires," describing the personal appearance of the twelfth Duke of Albuquerque, characterizes his hair as "coarse and green." The question here arises, Was the duke's hair really of this color, or was St. Simon the victim of an optical illusion? That the latter was in all probability the fact, is shown in a communication made to the Paris Academy of Sciences by the venerable M. Chevreul, "the oldest student in France." On the day when the Duke de St. Simon saw Albuquerque, the latter wore a bullock's-blood coat of coarse cloth, with buttons of the same, and his hair hung down on his shoulders. "Now," says Chevreul, "if we take hairs of a certain color, and arrange them on a red ground in parallel lines, making a small ribbon of them, and place beside them exactly similar hairs on a white ground, the former relatively to the latter will appear green. If for white we substitute orange, the hairs on the red ground will assume a bluish tint; if violet, a yellow tint; if green, a ruddy tint; if blue, an orange tint; if violet, a greenish yellow; and, finally, if we substitute black for the white ground, the hairs on the red ground will become whitened. In short, if we look at a broad surface of one simple color, we see it and appreciate it absolutely. If we see it in Juxtaposition with another color, or, still better, at the centre of a broad surface of another color, we see it relatively, and the sensation produced by it will be quite different."

A Rat in the Telegraph Service.—A telegraph-inspector in England recently pressed into his service a rat under the following peculiar circumstances: It was necessary to overhaul a cable of wires inclosed in iron tubes. A certain length of the cable had to be taken out of the tube, and the men commenced hauling at one end without having taken the precaution to attach to the other a wire by which it might be drawn back into the tube after inspection and repairs. The question arose, how the cable was to be restored to its proper place; and here the ingenuity of the inspector was manifested. He invoked the aid of a rat-catcher, and, provided with a large rat, a ferret, and a ball of string wound on a Morse paper drum, he repaired to the opening in the tube. The "flush-boxes" were opened, and the rat, with one end of the string attached to his body, was put into the pipe. He scampered away at a racing pace, dragging the twine with him until he reached the middle of the length of pipe, and there stopped. The ferret was then put in, and off went the rat again until he sprang clear out of the next flush-box. One length of the cable was thus safe, and the same operation was commenced with the other; but the rat stopped short a few yards in the pipe and boldly awaited the approach of the ferret. A sharp combat here commenced, and it was feared that one or both of the animals would die in the pipe. But, after sundry violent jerks had been given to the string, the combatants separated; the ferret returned to his master, and the rat, making for the other extremity of the pipe, carried the string right through, and so relieved the inspector from his anxiety.

Behavior of Metals with Hydrogen.—From researches carried on conjointly by Messrs. Troost and Hautefeuille, and reported to the French Academy of Sciences, it appears that potassium, sodium, and palladium, combine with hydrogen, while a considerable number of other metals merely dissolve this gas. Iron, nickel, and manganese, offer striking analogies in their behavior with hydrogen at different temperatures. The facility with which they absorb or give off hydrogen gas depends greatly on their physical condition. An ingot of pure nickel gave out in a vacuum, at a red heat, one-sixth of its volume of hydrogen. Pulverulent nickel gave out 100 times its volume, and remained pyrophoric after the