going. The question is sometimes asked, Of what use is astronomy? and the reply generally made is that it has conferred great benefits on navigation and on commerce, since it is by means of his astronomical knowledge that the sailor determines the position of his ship on the ocean. There is a truth in this reply, but it is only partial. The great value of astronomy is that it is really a science, and that it has broken the path and led the way through which all branches of science must pass if they ever become scientific. It is the spirit of honest, unrelenting criticism, and of impartial examination, that finally eliminates error and awards to every one his just due, that makes astronomy honorable and attractive; and it is by cultivating this spirit that astronomy confers its chief benefit, for it is this that shall break in pieces and destroy all false assumptions in science and in philosophy.

PAPERS READ.

ON THE PRODUCTION AND REPRODUCTION OF SOUND BY LIGHT. By ALEXANDER GRAHAM BELL, of Washington, D. C.

In bringing before you some discoveries made by Mr. Sumner Tainter and myself, which have resulted in the construction of apparatus for the production and reproduction of sound by means of light, it is necessary to explain the state of knowledge which formed the starting point of our experiments.

I shall first describe that remarkable substance "selenium," and the manipulations devised by previous experimenters; but the final result of our researches has widened the class of substances sensitive to light vibrations, until we can propound the fact of such sensitiveness being a general property of all matter.

We have found this property in gold, silver, platinum, iron, steel, brass, copper, zinc, lead, antimony, german-silver, Jenkin's metal, Babbitt's metal, ivory, celluloid, gutta-percha, hard rubber, soft vulcanized rubber, paper, parchment, wood, mica, and silvered glass; and the only substances from which we have not obtained results are carbon and thin microscope glass.1

We find that when a vibratory beam of light falls upon these substances they emit sounds, the pitch of which depends upon the frequency of the vibratory change in the light. We find further, that when we control the form or character of the light-vibrations on selenium (and probably on the other substances), we control the quality of the sound, and obtain all varieties of articulate speech. We can thus, without a conducting wire as in electric telephony, speak from station to station wherever we can project a beam of light. We have not had the opportunity of testing the

1 Later experiments have shown that these are not exceptions.

limit to which this photophonic effect may be extended, but we have spoken to and from points 213 metres apart; and there seems no reason to doubt that the results will be obtained at whatever distance a beam of light can be flashed from one observatory to another. The necessary privacy of our experiments hitherto has alone prevented any attempts at determining the extreme distance at which this new method of vocal communication will be available.

I shall now speak of selenium.

Selenium. In the year 1817, Berzelius and Gottlieb Galin made an examination of the method of preparing sulphuric acid in use at Gripsholm. During the course of this examination they observed in the acid a sediment of a partly reddish, partly clear brown color, which, under the action of the blowpipe, 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 further 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 Berzelins gave to the substance the name of "selenium," from the Greek word ask, the moon ("tellurium," as is well 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.

Knox2 discovered in 1837, that selenium became a conductor when fused; and Hittorff, in 1851, showed that it conducted at ordinary temperatures when in one of its allotropic forms.

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 temperatures.

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.4

It was early discovered that exposure to sunlight 5 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

2 Trans. Roy. Irish Acad. (1839), Vol. XIX, p. 147; also Phil. Mag. (3d ser.), Vol. XVI, p. 185.

Pogg. Ann., LXXXIV, 214; also Phil. Mag. (4th ser.), Vol. III, p. 546.

*See Draper and Moss in Proc. Roy. Irish Acad., Nov., 1873 (2nd ser.), Vol. I, p. 529. Gmelin's Handbook of Chemistry (1849), Vol. II, p. 235; see also Hittorff in the Phil. Mag. for 1852 (4th scr.), Vol. III, p. 547.

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 much 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 first made by Mr. May 6 (Mr. Willoughby Smith's assistant, stationed at Valentia) was soon verified by a careful series of experiments, the result of which was communicated by Mr. Willoughby Smith 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 (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 magnesium was held about nine inches above the water, the resistance of the selenium immediately 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

See lecture by Siemens, in Proc. Roy. Inst. of Great Britain, Vol. VIII, p. Cs. 7 Jour. of Soc. of Teleg. Eugin., Vol. II, p. 31 (1873); Nature, Vol. VII, p. 303; Amer. Jour. of Science and Arts (3d ser.), Vol. V, p. 301.