Popular Science Monthly/Volume 11/May 1877/On the Wonderful Divisibility of Gold and Other Metals
|ON THE WONDERFUL DIVISIBILITY OF GOLD AND OTHER METALS.|
ASSAY LABORATORY, UNITED STATES MINT, PHILADELPHIA.
IT is both curious and interesting to notice how frequently original investigators, working from different standpoints, and with entirely dissimilar objects in view, will, independently of each other, accumulate a mass of observations corroborative of some one physical law, but which require to be collated in order to reveal their mutual relations.
The motive of this paper is to collect together several observations illustrating the divisibility of gold (made either as the direct object of experiment, or as incidental to other investigations), and to present them in a condensed form to the readers of The Popular Science Monthly.
Some of these experiments (notably those of Faraday) present the curious anomaly of revealing to the physical sense of sight particles of matter which are almost too infinitesimal for the mind's eye to conceive, thus seeming to reverse the order of scientific investigation which usually prolongs the mental vision far beyond the region of possible physical revelation.
The experiments to be described have been arranged in the following order:
1. On the natural dissemination of gold.
2. Beating into thin leaves.
3. Faraday's researches.
4. Depositing by the galvanic battery.
5. Vaporization by the electric spark.
Some years since a very interesting series of experiments was made by the late Mr. J. R. Eckfeldt, then chief-assayer of the mint at Philadelphia, and his associate, Mr. W. E. DuBois (the present incumbent), upon the "Natural Dissemination of Gold." The results were presented to the American Philosophical Society, in the form of a paper, by Mr. DuBois, and published in their "Proceedings" of June 21, 1861.
The precious metal was found disseminated in marvelously fine division through a number of substances where its existence had not been previously suspected.
In the clay of which the Philadelphia bricks are made, gold was found in the proportion of about forty cents' worth to the ton. Each brick contains a sufficient amount of gold to make a glittering show of two square inches, if brought to the surface in the form of leaf.
An estimate of the thickness of the bed of clay under the city revealed the startling fact that more gold lies securely locked up in it than has been procured, according to the statistics, from Australia and California. A specimen of galena from Buck's County, Pennsylvania, yielded gold in the proportion of one part of gold in six million two hundred and twenty thousand (6,220,000) parts of ore; not quite ten cents to the ton. The report of these experiments concludes as follows: "Of this we may be confident, that the atoms of gold are homogeneously and equably dispersed through the clay, or other matrix; but by what natural process or for what final cause these fine particles should be thus diffused, seems quite beyond the reach of human philosophy."
The remarkable malleability of fine gold was a property well known to the ancients. Homer refers to the art of gold-beating, and Pliny mentions that an ounce of gold was beaten into seven hundred and fifty leaves, "each leaf being four fingers square," or about three times thicker than our ordinary gold-leaf. On the coffins of the Theban mummies, gold-foil has been discovered of extraordinary thinness. The ancient Peruvians covered the walls of their temples with very thin sheets of gold, and the rude specimens of gilding on the palace of Tippo Saib, at Bungalore, prove that the art of gold-beating was practised in India. We also have Biblical authority for the antiquity of the art.
Experiments made in modern times have shown that a single grain of gold may be beaten out so as to cover a space of seventy-five square inches; the thickness of the leaf is then only the three hundred and sixty-seven thousand six hundred and fiftieth (367650) part of an inch, or about twelve hundred times thinner than an ordinary sheet of printing-paper.
Faraday states in his researches on "The Experimental Relations of Gold (and other Metals) to Light" that a leaf of beaten gold occupies an average thickness of no more than 5 to 8 part of a single wave of light. He reduced the thickness of gold-leaves at pleasure, by spreading them upon glass plates and gradually dissolving the metal by means of a weak solution of cyanide of potassium. "By this means," he says, "I think fifty or even one hundred might be included in a single progressive undulation of light."
Faraday's researches upon the nature of thin films of gold and other metals, and upon the size of finely-divided particles of gold diffused through various liquids, are of a most interesting and refined character. Availing himself of the well-known reducing power of phosphorus, he floated small particles of it upon the surface of weak solutions of chloride of gold. In the course of twenty-four hours he found that the surfaces of the liquids were covered with films of metallic gold, which were thicker near the pieces of phosphorus "possessing the full golden reflective power of the metal," but becoming so thin by gradations as to be scarcely perceptible. "They acted as thin plates upon light, producing the concentric rings of colors round the phosphorus at their first formation, though their thickness then could scarcely be the 100, perhaps not the 500, of a wave-undulation of light."
By treating very dilute solutions of gold with phosphorus he obtained the metal diffused through the liquid in extremely fine particles, producing a beautiful ruby-color. These particles, when in their finest state, often remain unchanged for months, and have all the appearance of solutions, but they never are such, containing, in fact, no dissolved but only diffused gold. The particles are easily rendered evident by gathering the rays of the sun (or a lamp) into a cone by a lens, and sending the part of the cone near the focus into the fluid; the cone becomes visible, and though the illuminated particles cannot be distinguished, because of their minuteness, yet the light they reflect is golden in character and seen to be abundant in proportion to the quantity of gold present. Portions of gold so dilute as to show no trace of gold by color or appearance can have the presence of the diffused solid particles rendered evident by the sun in this way. . . . The state of division of these particles must be extreme; they have not as yet been seen by any power of the microscope."
Faraday further tells us that he endeavored to obtain an idea of the quantity of gold in a given ruby fluid, and for this purpose selected a plate of gold ruby glass, of good full color, to serve as a standard, and he compared the different fluids with it, varying their depth until the light from white paper, transmitted through them, was apparently equal to that transmitted by the standard glass. Then, known quantities of these ruby fluids were evaporated to dryness, the gold converted into chloride, and compared, by reduction on glass and otherwise, with solutions of gold of known strength. "From these considerations it would appear that one volume of gold is present in the ruby fluid in about 750,600 volumes of water; and that, whatever the state of division to which the gold may be reduced, still the proportion of the solid particles to the amount of space through which they are dispersed must be of this extreme proportion. This accords perfectly with their invisibility in the microscope; with the manner of their separation from the dissolved state; with the length of time during which they can remain diffused; and with their appearance when illuminated by the cone of the sun's rays."
While all the statements of this profound investigator were so carefully kept within the limit of actual observation, he tells us, in conclusion, that he not only believed the gold to be diffused in solid metallic particles, but that he also believed "there may be particles so fine as to reflect very little light indeed, that function being almost gone."
The art of gold-plating has become so extensive in its application to a great variety of ornamental objects, that it seemed to the writer an interesting question "How thick a film is required to produce a fine gold-color?" He was unable to find, on inquiry, that any careful notes to determine this point have been recorded, and he recently made some experiments with the following results: A sheet of copper was rolled down to an average thickness of 1000 inch. Two plates were cut from this strip, 4 by 22 inches, having a metallic surface of twenty square inches each. These plates were boiled in alkali, to remove grease, polished, and accurately weighed on a balance sensitive to the twentieth of a milligramme, or the 1295 of a grain.
A gold "blush" of sufficient thickness to produce a fine gold color was then deposited by the battery. The plates were washed in distilled water, dried, and reweighed without rubbing, and were found to have each gained in weight exactly one-tenth (10) of a grain. It thus appears that one grain of gold may be distributed, by the galvanic deposit, over the surface of two hundred square inches, as contrasted with seventy-five square inches by beating. In other words, the metal is more than two and a half times thinner in the former case than in the latter, or 980400 compared with 317650 inch.
A still thinner deposit of gold could, of course, be detected on the delicate assay-balance, but, owing to the transparency of the film, it would not possess the true gold-color. It seemed important to ascertain whether the gold was evenly distributed over the copper surface, or whether it was deposited in spots. The strips were accordingly examined under a microscope.
A careful examination showed that there were no exposed surfaces of copper, and the gold appeared to have a fine, bright, smooth surface. This, however, was not considered sufficient proof, and several expedients were tried to obtain the gold films free from the copper plate, in order that they might be examined by transmitted light. Owing to their extreme thinness this was difficult to accomplish. One method, which was partially successful, was to heat the copper plates to a cherry red in the annealing muffle of an assay-furnace. On cooling, the gold film peeled off in flakes with a thin backing of oxide of copper; these flakes were pressed between two plates of glass, and nitric acid allowed to flow in by capillary action. The acid dissolved the copper; leaving a film of free gold, The difficulty was, that the bubbles of gas formed perforated the film of gold. Another plan was then tried. The gold-plating was removed from one surface of the copper plates by means of fine emory-paper. Pieces about one inch square were immersed for several days in very weak nitric acid. The copper was entirely dissolved, and detached films of gold were found floating intact on the surface of the liquid; these were carefully collected on strips of glass, washed with distilled water, and dried; they then firmly adhered to the glass.
When examined by reflected light they retain their brilliant gold color and lustre, but when viewed by transmitted light they are bright green and very transparent; the color is an even shade, having none of the mottled appearance of gold-leaf when seen by transmitted light, caused by its very uneven thickness. This monotone appears to be a positive indication of uniform thickness, for, when two 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 20 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," 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 20 of a milligramme, or 1295 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, 1000 of a grain; this gives for each spark 1000000 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 10 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 1000 of a grain after passing 3,000 sparks; or, for 1,000 sparks, 1000 of a grain; or, for each spark, 1000000 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 brought in succession into the action of the spark until the lines of the substance were clearly visible," If a skeptical person refuse to believe the results of Cappel, who tells us that 1 of a milligramme (1 of a grain) of nickel will just write the signature of that metal, what will he say when, glancing at Table B, appended hereto, he finds the statement that 1 of a milligramme (1 of a grain) will sign its name in brilliant characters! And yet the writer does not hesitate to say that even a smaller amount of this metal will show a spectrum, for in these experiments a much stronger spark was used than was necessary to show a visible spectrum. When reduced to a minimum, by means of a miniature Leyden jar, improvised out of a test-tube, which still gave a distinct spectrum, the loss in weight, after passing 3,000 sparks, was absolutely inappreciable on the balance. The tables show another curious and unexpected result, viz., that the loss in weight of the volatile metals very slightly exceeds, and in some cases does not equal, the loss of the less volatile metals. Thus, in three different experiments of 3,000 sparks each, copper loses but 1 milligramme, while gold loses 5 milligramme."
In one experiment the number of sparks was increased to 10,000, and the loss in weight was nearly proportioned to the increased number. In this case the sparks were passed at the rate of about 250 per minute.
The first column shows weight of metallic electrodes (in milligrammes) before passing the sparks.
The second column shows weight after passing 3,000 sparks.
The third column shows total weight of metal volatilized (in fractions of a milligramme).
The fourth column shows the amount of metal volatilized by each spark (in fractions of a milligramme).
The fifth column shows the amount of metal volatilized by each spark (in fractions of a grain troy).
|Upper||"||gold and copper||24||23.4||.6||1||1|
|Upper||"||gold and copper||21||20.4||.6||1||1|
While it is probable that (with the exception, perhaps, of Faraday's researches) we have not here indicated the smallest particles of metal which it would be possible to determine by the delicate means at our disposal, it is thought that the experiments recorded may prove interesting, as showing what has been, and what may be, accomplished in this direction; and, lest any incredulous reader should fancy that, when we speak of weighing the three million eight hundred and eighty-thousandth part of a grain of metal, we are toying with imaginary fractions, we would refer him to Sir William Thomson's estimate of the size of the final molecules; compared with which this unit is as large as the famous Philadelphia cobble-stones compared with grains of sand upon the sea-shore. In conclusion, we are led to appreciate the wisdom as well as the wit of the distich—
And these again have lesser fleas,—and so ad infinitum."
- "And they did beat the gold into thin plates" (Exodus xxxix. 3).
- "Philosophical Transactions," 1857.
- Faraday's "Researches in Chemistry and Physics."
- It is stated that when a cylindrical bar of silver is coated with gold and drawn into the fine wire used in embroidered housings, etc., a grain of gold will cover 345.6 feet of wire.
- Published in the "Proceedings of the American Philosophical Society," vol. xiv., p. 162.
- "The Spectroscope in its Application to Mint-Assaying." Journal of the Franklin Institute, October, 1874. Reprinted in the Quarterly Journal of Science, January, 1875.
- He fixes the limit between the 1 and the 1 of an inch, and says that they are "pieces of matter of measurable dimensions, with shape, motion, and laws of action; intelligible subjects of scientific investigation."