films were superposed, one upon the other, the color was very perceptibly darkened; even when subjected to a magnifying power of 1,000 diameters the films retain their continuous character, though the brilliance of the green color is of course diffused. The dimensions of the waves of light, when decomposed by the prism, have been carefully measured. There are 47,000 green waves in the space of an inch; dividing the estimated thickness of our gold film by this number, we find that the thickness of the film is less than 120 part of a single undulation of green light.
In the course of an examination made by the writer upon "The Practicability of assaying Metals used in Coinage, by means of Spectrum Analysis, made in and for the Assay Department of the United States Mint at Philadelphia,"[1] it was noticed that a large number of very powerful electric flashes might be passed between two slips of metal without any apparent loss, and an important query suggested itself, viz., whether the amount of metal vaporized by each spark was not too infinitesimal to be determined. In order to ascertain this point the following experiments were tried: Having weighed small electrodes averaging 18 milligrammes each with the greatest possible accuracy upon the gold assay-balance of the mint, which is sensitive to 120 of a milligramme, or 11295 grain troy, and having arranged a spark-register, it was found that 1,000 sparks might be passed between these poles, each spark showing the spectrum of the metal distinctly, and yet the loss in weight was too small to be made the base of calculation. Thus a gold pole lost in weight, after passing 1,000 sparks, 11000 of a grain; this gives for each spark 11000000 of a grain of gold, producing a bright spectrum.
The number was then increased to 3,000 sparks as the test. The loss of weight depends, of course, upon the electric volume, and in the experiments tabulated an endeavor was made to keep the latter constant. The tables (marked A and B) show that the loss in weight is marvelously small, averaging less than 710 of a milligramme of gold for 3,000 sparks. To give the amount for each spark, this must be divided by the number of sparks; thus, in round numbers, an electrode loses 11000 of a grain after passing 3,000 sparks; or, for 1,000 sparks, 11000 of a grain; or, for each spark, 11000000 of a grain. The experiments made by M. Cappel to determine the minimum amount of each element that would show a spectrum have been published in tabular form. His method was to volatilize "solutions of the metallic salts between the poles of a small induction-coil in Mitscherlich's glass tubes with platinum wicks. A series of solutions, each one-half the strength of the preceding one, was prepared from a number of metallic chlorides. The spectrum in connection with the positive pole was continually observed, while increasingly-concentrated solutions were
- ↑ Published in the "Proceedings of the American Philosophical Society," vol. xiv., p. 162.
