Alexander Graham BELL, Ph.D.,
[Read before the American Association for the
Selenium - In the year 1817, Berzelius and Gottlieb Gahn made an examination of the method of preparing sulphuric acid in use at Gripsholm? During the course of the examination they observed in the acid a sediment of a partly reddish, partly clear brown color, which under the action of the blow-pipe gave out a peculiar odor, like that attributed by Klaproth to tellurium.
As tellurium was a substance of extreme rarity, Berzelius attempted its production from this deposit, but he was unable after many experiments to obtain farther indications of its presence. He found plentiful signs of sulphur mixed with mercury, copper, tin, zinc, iron, arsenic and lead, but no trace of tellurium.
It was not in the nature of Berzelius to be disheartened by this result. In science every failure advances the boundary of knowledge as well as every success ; and Berzelius felt that if the characteristic odor that had been observed did not proceed from tellurium, it might possibly indicate the presence of some substance then unknown to the chemist. Urged on by this hope he returned with renewed ardor to his work.
He collected a great quantity of the material and submitted the whole mass to various chemical processes. He succeeded in separating successively the sulphur, the mercury, the copper, the tin and the other known substances, whose presence had been indicated by his tests ; and after all these had been eliminated, there still remained a residue, which proved upon examination to be what he had been in search of - a new elementary substance.
The chemical properties of this new element were found to resemble those of tellurium in such a remarkable degree that Berzelius gave to the substance the name of "selenium" from the Greek word selhum, the moon, ("tellurium," as is we'll known, being derived from tellus, the earth). Although tellurium and selenium are alike in many respects, they differ in their electrical properties ; tellurium being a good conductor of electricity, and selenium, as Berzelius showed, a non-conductor.
When selenium is rapidly cooled from a fused condition it is a non- conductor. In this, its " vitreous " form, it is of a dark brown color, almost black by reflected light, having an exceedingly brilliant surface. In thin films it is transparent, and appears of a beautiful ruby red by transmitted light.
When selenium is cooled from a fused condition with extreme slowness, it presents an entirely different appearance, being of a dull lead color, and having throughout a granular or crystalline structure and looking like a metal. In this form it is opaque to light even in very thin films. This variety of selenium has long been known as " granular " or " crystalline " selenium ; or as Regnault called it, " metallic " selenium. It was selenium of this kind that Hittorff found to be a conductor of electricity at ordinary temperature.
He also found that its resistance to the passage of an electrical current diminished continuously by heating up to the point of fusion ; and that the resistance suddenly increased in passing from the solid to the liquid condition. (3)
It was early discovered that exposure to sunlight (4) hastens the change of selenium from one allotropic form to another; and this observation is significant in the light of recent discoveries.
Although selenium has been known for the last sixty years, it has not yet been utilized to any extent in the arts, and it is still considered simply as a chemical curiosity. It is usually supplied in the form of cylindrical bars. These bars are sometimes found to be in the metallic condition, but more usually they are in the vitreous or non-conducting form.
It occurred to Willoughby Smith that, on "account of the high resistance of crystalline selenium, it might be usefully employed at the shore-end of a submarine cable, in his system of testing and signaling during the process of submersion. Upon experiment the selenium was found to have all the resistance required ; some of the bars employed measuring as rnuch as 1400 megohms - a resistance equivalent to that which would be offered by a telegraph wire long enough to reach from the earth to the sun! But the resistance was found to be extremely variable. Efforts were made to ascertain the cause of this variability, and it was discovered that the resistance was less when the selenium was exposed to light than when it was in the dark !
This observation was first made by Mr. May (5) - (Mr. Willoughby Smith's assistant, stationed at Valentia) - was soon verified by a careful series of experiments, the results of which were communicated by Mr. Willoughby Smith (6) to the Society of Telegraph Engineers, on the 17th of February, 1873. Platinum wires were inserted into each end of a bar of crystalline selenium, which was then hermetically sealed in a glass tube through the ends of which the platinum wires projected for the purpose of connection. One of these bars was placed in a box, the lid of which was closed so as to shade the selenium, and the resistance of the substance was measured.
Upon opening the lid of the box the resistance instantaneously diminished. When the light of an ordinary gas burner n (which was placed at a distance of several feet from the bar,) was intercepted by shading the selenium with the hand, the resistance again increased ; and upon passing the light through rock salt, and, through glasses of various colors, the resistance was found to vary according to the amount of light transmitted. In order to be certain that temperature had nothing to do with the effect, the selenium was placed in a vessel of water so that the light had to pass through a considerable depth of water in order to reach the selenium. The effects, however, were the same as before. When a strong light from the ignition of a narrow band of magnesiuin was held about nine inches above n the water, the resistance of the selenium immediaitely fell more than two-thirds, returning to the normal condition upon the removal of the light. The announcement of these results naturally created an intense interest among scientific men, and letters of enquiry regarding the details of the experiment soon appeared in the columns of Nature, from Harry Napier Draper (7) and Lieut. M. L. Sale (8), which were answered in the next number by Willoughby Smith.(9)
Sale and Draper were soon able to corroborate the statements that had been made by Willoughby Smith.
Sale (10) presented his researches to the Royal Society on the 8th of May, 1873, and in the following November, Draper (11) presented his results to the Royal Irish Academy in the shape of a joint paper by himself and Richard J. Moss.
Draper and Moss gave in their paper an admirable summary of the condition of our knowledge regarding selenium at that time. They confirmed Hittorff's observation that the temperature of minimum resistance of granular selenium was somewhere about 210° C, and that at 217° C. (the fusing point), the resistance suddenly increased. They carried the temperature to a still higher point than Hittorff had done, and found that the resistance again diminished, reaching a second minimum at 250° C.
During the course of their experiments they produced a variety of granular selenium not different in appearance from other specimens but having different electrical properties. In this form the resistance became greater instead of less when the temperature was raised.
They also used thin plates of selenium instead of the cylindrical bars formerly employed, and found great advantage from the increased sensitiveness of the former to light.
Sale found upon exposing selenium to the action of the solar spectrum that the maximum effect was produced just at or outside the extreme edge of the red end of the spectrum at a point nearly coincident with the maximum of the heat rays, thus rendering it uncertain whether the effect was due to light or to radiant heat.
In the winter of 1873-4 the Earl of Rosse (12) attempted to deccide this question by comparing the selenium effects with the indications of the thermopile. He exposed selenium to the action of non-luminous radiations from hot bodies, but could produce no effect; whereas, a thermopile under similar conditions gave abundant indications of a current.
He also cut off the heat rays of low refrangibility from luminous bodies by the interposition of glass and alum between the selenium and the source of light without materially affecting the result; but when the thermopile was employed the greater portion of the heat-effect was cut off.
Later, Prof. W. G. Adams, (13) of Kings College, took up the question, and his experiments seemed to prove conclusively that the action was due principally, if not entirely, to those rays of the spectrum which were luminous and that the ultrared or the ultra-violet rays had little or no effect.
This conclusion was supported by the marked effect produced by the light of the moon, and by the apparent insensitiveness of selenium to rays passed through a solution of iodine in bisulphide of carbon. He found that the maximum effect was produced by the greenish-yellow rays, and showed that the intensity of the action depended upon the illuminating power of the light, being directly as the square root of that illuminating power.
Professor Adams and Mr. R. E. Day (14) continued these researches, and among other interesting and suggestive results, discovered that light produces in selenium an electromotive force without the aid of a battery.
The most sensitive variety of selenium that has yet been produced was obtained in Germany by Dr. Werner Siemens, by continued heating for some hours at a temperature of 210° C, followed by extremely slow cooling.
Dr. C. W. Siernens, (15) in a lecture delivered before the Royal Institution of Great Britain, on the 18th of February, 1876, stated that his brother's modification of selenium was so sensitive to light that its conductivity was fifteen times as great in sunlight as it was in the dark.
In Werner Siemens' (16) experiments special ,arrangements were made for reducing the resistance of the selenium.
For this purpose two fine platinum wires were coiled into a double flat spiral and were laid upon a plate of mica, so that they did not come into contact with one another. A drop of melted selenium was then placed upon the platinum wire arrangement and a second sheet of mica was pressed upon the selenium so as to cause it to spread out and fill the spaces between the wires. Each cell was about the size of a silver dime. The cells were then placed in a paraffine bath and annealed.
other arrangements of apparatus for reducing the resistance. In the form known as
"Siemens' Grating," the two wires, instead of being coiled together, were
arranged in a zig-zag shape, forming a sort of platinum gridiron.
(1) Trans. Roy. Irish
Acad. (1839), xix, 147; also Phil. Mag. III, xvi, 185.