Every one has heard of the power which bleaching gas (chlorine) possesses in taking away color, so that a red rose held over its fumes will become white. The lecturer, referring to this fact, exhibited two pieces of paper; upon one was inscribed, in large letters, the word "PROTEUS;" upon the other no writing was visible; although he assured us the same word was there inscribed. He now dipped both pieces of paper in a solution of bleaching-powder, when the word "Proteus" disappeared from the paper upon which it was before visible; while the same word instantly came out, sharp and distinct, upon the paper which was previously a blank. Here there appeared another contradiction: the chlorine in the one case obliterating, and in the other reviving the written word; and how was this mystery explained? Easily enough! Our ingenious philosopher, it seems, had used indigo in penning the one word which had disappeared; and had inscribed the other with a solution of a chemical substance, iodide of potassium and starch; and the action which took place was simply this: the chlorine of the bleaching solution set free the iodine from the potassium, which immediately combined with the starch, and gave color to the letters which were before invisible. Again--a sheet of white paper was exhibited, which displayed a broad and brilliant stripe of scarlet--(produced by a compound called the bin-iodide of mercury)--when exposed to a slight heat the color changed immediately to a bright yellow, and, when this yellow stripe was crushed by smartly rubbing the paper, the scarlet color was restored, with all its former brilliancy. This change of color was effected entirely by the alteration which the heat, in the one case, and the friction, in the other, produced in the particles which reflected these different colors; and, upon the same principle, we may understand the change of the color in the lobster-shell, which turns from black to red in boiling; because the action of the heat produces a new arrangement in the particles which compose the shell.