As the film of the compound augments, it undergoes the various changes of color which take place in all transparent films, thus affording a means of ascertaining the absolute thickness obtained in different circumstances, when it would be difficult to detect the slightest difference in weight by means of the most delicate balance. The depth of this coating may be ascertained when either the index of refraction of the compound itself is known, or if the angle of polarized light is given by means of the law discovered by Sir David Brewster, between the tangent of the angle of polarization, and the index of refraction. The most convenient way which occurred to me of performing these experiments, was the employment of a bell-glass within which some iodine is fixed at the top; this apparatus being placed over the metal to be acted on, the experiment may be watched in all its progress, and the action can be retarded or accelerated at pleasure by varying the interval of the iodine from the metal, or by interposing at some distance from its surface a disc of paper so as to cause the vapors of iodine to pass through it. Bromine may be made use of likewise by pouring a few drops of it over some carded cotton, and using it in a similar manner with the iodine. In respect to chlorine, it is most convenient to disengage it slowly by dropping a little sulphuric acid upon some chlorinated lime.
In illustration of the objects of this mode of experimenting, I will aduce some of the results it has given me with various metals. Some of the experiments below were performed before I had the idea of watching the progress of the combination through a transparent medium; they are therefore less exact than they might otherwise have been: but I have preferred stating them as I had inserted them in my note-book before I had conceived any idea as to their probable utility in the elucidation of chemical affinity, and when I intended them for other purposes, which I shall hereafter explain.
1st change. Silver--pale gold. " Copper--assumes a darker red. " Silver--blue. 2nd do. Copper--blue. " Silver--white. 3rd do. Copper--white. " Silver--yellow. 4th do. Copper--yellow more extended than on the silver. " Silver--Orange. 5th do. Copper--Red. " Silver--blue, bluish-red. " Copper--red, with a tinge of green on some parts. " Silver--greenish blue. " Copper--red, tinged with green. " Silver--green. " Copper--orange. " Silver--yellowish green. " Copper--orange tending to red. " Silver--yellowish green. " Copper--orange-red. " Silver--red. " Copper--dull green. " Silver--red. " Copper--green. " Silver--deep green. " Copper--dull red.