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lege, London.*

The selenium was exposed not only to radiations from different sources, but to light which had been transmitted through various absorbing media, such as colored glass, solutions of colored salts, plates of rock salt, quartz, mica, alum, and other appropriate substances. These experiments showed convincingly that light was the chief agent in inducing the change in the electrical properties of the selenium, inasmuch as these properties were scarcely affected either by the ultra-red or by the ultra-violet rays. The maximum effect was obtained in the yellowish-green portion of the spectrum. Under the influence of moonlight the resistance of the selenium was sensibly reduced. On the whole it was clear that light and not heat was the agent to which Mr. Willoughby Smith's phenomenon was due. In fact, it is now a well-established fact that while light increases the conducting power of crystalline selenium, heat diminishes it.

While these investigations were being conducted in this country, Dr. Werner Siemens was independently engaged upon the same subject in Berlin.† He devised an ingenious form of selenium cell, which was prepared in the following manner. Two opposite spirals, or two parallel zigzags, of thin platinum wire were laid upon a sheet of mica, and united by a drop of molten selenium, which, before solidifying, was squeezed out into the form of a thin film by pressure of a second plate of mica. The current was caused to enter the cell through one of the wires, then to traverse the selenium, and finally to pass out through the opposite wire. With cells of this construction, a great number of experiments were made by Dr. Siemens in conjunction with Dr. Obach. As long as the selenium remained in the amorphous condition, the current was

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unable to pass, but on heating it to 100° C., and then allowing it to cool, it became a feeble conductor, and its conductivity was increased by the action of light. If, however, the selenium disk were exposed to a temperature of about 210° C., or nearly to its melting-point, and then gradually cooled, the substance passed into a second modification, in which it was a much better conductor of electricity, and was extremely sensitive to luminous rays.

For the purpose of detecting variations in the strength of the current under varying conditions of illumination, all experimentalists who had worked on this subject had naturally made use of galvanometers. It occurred, however, to Mr. Graham Bell, that his telephone might be advantageously used in such experiments. It is obvious that if a telephone were introduced into a circuit which included a cell of crystalline selenium, the telephone would be affected at every admission of light to the sensitive material, and again at every exclusion. But, in each case, the effect would be only of momentary duration. Consequently, in order to throw the diaphragm of the telephone into a state of vibration, so as to produce distinct sounds, the light must be intermitted with great rapidity. Let the selenium be subjected to a quick succession of exposures and eclipses, and the corresponding changes in the conductivity of the material would keep the disk of the telephone in a state of oscillation, and thus sound would be produced by the action of light. The light would act upon the selenium, and the telephone would audibly respond.

Foreseeing the possibility of thus evoking sound by the action of light, Professor Bell, in the course of a lecture which he delivered at the Royal Institution in 1878, ventured to express his opinion that when light which had fallen upon selenium was intercepted, it would be possible, by proper arrangements, to hear the shadow. And only a few days afterward, Mr. Willoughby Smith announced that he had actually heard, through the telephone, the effect of the fall of a ray of light upon a piece of sensitive selenium.

Practically, however, it was found that the very great resistance which the sele

nium offered to the passage of the current rendered it unmanageable. But Mr. Bell, working conjointly with his friend, Mr. Sumner Tainter, has completely overcome this difficulty, and has prepared, by very simple means, selenium cells which offer only a moderate resistance, and are, therefore, suitable for telephonic experiments. No fewer than fifty different forms of apparatus have been devised by these experimentalists for the purpose of actuating the telephone by varying the illumination of the selenium. One of the most simple of these forms consists merely of a small flexible mirror, upon which a beam of light is concentrated. The mirror may be made of a piece of very thin glass, or of a disk of mica silvered on one side. Upon such a mirror a beam of lightpreferably sunlight, by reason of its intensity is concentrated by means of a lens. The light reflected from the mirror is passed through another lens so as to form a beam of parallel rays, and this beam is projected to the distant station, where it is received upon a parabolic mirror. The mirror concentrates the light upon a cell of sensitive selenium which is placed in the focus, and is connected in a local circuit with a telephone and a galvanic battery.

If a speaker at the transmitting station now directs his voice against the back of the little flexible mirror, the mirror is thrown into a state of vibration, and the agitation is necessarily communicated to the beam of reflected light. When, therefore, this light reaches the receiving station, it falls upon the selenium as an "undulatory beam"-in other words, although it may shine continuously upon the selenium, its intensity is yet subject to rapid variations. These variations produce equally rapid changes in the electric current which traverses the selenium, and every rise or fall in the conductivity of the selenium is thus transmitted to the telephone, where it manifests itself audibly by throwing the diaphragm into a similar state of vibration. It is obvious, therefore, that every sound produced at the back of the transmitting mirror must evoke a corresponding sound at the distant receiving station, Words uttered at one end are thus faithfully reproduced at the other, though the bond between

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the two stations is nothing more than a beam of light.

No sooner had the photophone been constructed in the form which has just been described than it was destined to undergo an extraordinary modification. It may fairly be supposed that when light falls upon the selenium, it must set up some kind of molecular disturbance upon its sensitive surface. Accordingly, Mr. Bell argued that if such a movement of the molecules really does take place, there was the bare possibility that it might be heard with the unaided ear. Removing then the telephone and battery, Mr. Bell applied his ear directly to the selenium disk. The early experiments were not successful, but ultimately he had the satisfaction to find that the crystalline selenium, under proper conditions, did actually emit distinct sounds. Far more remarkable, however, than this fact, was the unexpected discovery that such an emission of sound, under the influence of varying illumination, is not confined to selenium. The first material in which Professor Bell distinctly observed this phenomenon was a piece of hard rubber, and a great variety of other substances were then tested with more or less success. Antimony and hard rubber were found to emit the loudest sounds, paper and mica the weakest, while the only substances which remained silent in the course of these experiments were carbon and thin glass. The inventors of the photophone feel warranted in stating, as the result of their studies, that sounds can be produced by the action of a variable light upon substances of all kinds, provided they be used in the suitable form of thin diaphragms. Mr. Bell's experiments have therefore resulted not only in the invention of a new acoustical instrument, but in the discovery of the fact that matter in general is susceptible of molecular change, under the influence of light, to an extent and in a way which had not previously been suspected.

In delivering the Presidential Address to the British Association at the recent meeting at Swansea, Professor Ramsay fave publicity to some geological observations which had recently been made by Professor Geikie in the north-west of Scotland, and which, if they bear the

interpretation that has been put upon them, are undoubtedly of the deepest interest to the physical geologist." The President's announcement was immediately followed by the publication of Professor Geikie's own account of the observations.t

For many years past the order of succession of the old rocks in the north of Scotland has been placed almost beyond dispute. Mr. Peach's discovery of Lower Silurian fossils at Durness long ago settled the age of the limestones and white quartzites of Sutherlandshire, and thus afforded a starting-point for the determination of the age of the unfossiliferous rocks in this region. Beneath the Silurian rocks, in the north-west of Scotland, are enormous masses of dark red or purple sandstones and conglomerates, which rise at places into conical mountains upward of three thousand feet above the level of the sea. The late Sir Henry James and Professor Nicol showed that these sandstones are separated by a strong unconformity from the overlying Silurian rocks; and Sir Roderick Murchison, recognizing their higher antiquity, referred them to the Cambrian formation. But far older than these Cambrian strata, and separated from them in turn by another unconformity, is a series of highly metamorphosed crystalline rocks, consisting chiefly of contorted gneiss. This gneiss occurs in the outer Hebrides, and is occasionally known, from its occurrence in the Isle of Lewis, as Lewisian gneiss; it also stretches along the coast of the opposite mainland from Cape Wrath, with more or less interruption, as far south as Loch Torridon. Finding in this pre-Cambrian gneiss a representative of the most ancient stratified rocks in the country, Murchison bestowed upon it the name of the Fundamental gneiss-a name which was intended to suggest that it formed the floor of the British islands, upon which the later formed deposits had been spread. When the investigations of Sir William Logan and his colleagues had clearly shown that there existed in Can

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Along the western margin of the counties of Sutherland and Ross the Laurentian gneiss presents a peculiar type of scenery, which has been graphically described by Professor Geikie. The gnarled gneiss forms a succession of bosses, hummocks, and ridges, peculiarly rounded in contour, and wellnigh destitute of vegetation. The mammillations of the surface suggest that the rocks have been worn down and rounded by the passage of moving ice; and it needs but little examination to recognize the smoothing, the polishing, and the striation which speak so unmistakably of glacial action. At first sight it might naturally be assumed that these effects were due to erosion by ice during that comparatively modern period which is known as the Glacial Age. Yet it is strange that the neighboring sandstones, quartzites, and schists, over which the ice of that period must also have travelled, fail to exhibit equally marked traces of glacial erosion. Nor can it be said that the unyielding nature of the gneiss has enabled it to retain with persistence the evidence of icework, while such evidence has been obliterated from many of the neighboring rocks; for in the Scottish Highlands, where gneissose rocks of younger age have been exposed to the action of ice during the glacial period, the contours and general characters of the rocks are quite different from those of the Laurentian gneiss. How then can the geologist hope to explain the peculiarities in the erosion of the venerable gneissose rocks of the north-west of Scotland?

Probably the explanation is to be found in the recent observations of Professor Geikie. In examining the iceworn surfaces of Laurentian gneiss, he has been able to trace their rounded out

lines passing distinctly beneath the overlying Cambrian rocks. This was the

case, for example, on both sides of Loch Torridon, and again on the west side of Loch Assynt.

The conclusion is thus

forced upon the observer that the old gneiss must have received its smooth flowing contours, to some extent at least, before the Cambrian sandstones were deposited. Can it be, then, that we have evidence in these rocks of a glacial period dating back to early paleozoic times?

This suggestion appears to receive some support from Professor Geikie's observations in the neighborhood of Gairloch, where he found the undulating surface of gneiss to be capped in places by a coarse unstratified breccia, containing angular fragments of the Laurentian gneiss, sometimes as much as five feet in length, standing on end and at all angles. Such a breccia obviously bears a suspicious resemblance to a modern moraine.

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Since Professor Ramsay, in 1855, brought before the Geological Society the evidence upon which he had satisfied himself as to the existence of glaciers during the Permian period, he has naturally been interested in any traces of the recurrence of glacial phenomena, especially among the earlier rocks. To To him, therefore, Professor Geikie's observations were peculiarly acceptable, and he received them without hesitation as evidence of the action of "ancient glaciers of Cambrian age. There was already a body of facts tending to show that glacial conditions must have prevailed in certain parts of the world during a portion of the Silurian period; but if the early glaciation of the Laurentian gneiss be admitted, we may now carry the glacial phenomena a stage further back in the earth's history. It is only fair, however, to remark that Professor Geikie himself speaks most guardedly as to the conclusions to be drawn from his observations, and in referring to the rounded surfaces of the gneiss is content to remark that "they have certainly been ground by an agent that has produced results which, if they were found in a recent formation, would without hesitation be ascribed to land ice." If this ascription be warranted in the case of the old Scottish gneiss, that rock presents us with vestiges of glacial action far older than anything of the kind hitherto

known to geologists in any part of the world.

When Sir Charles Lyell, in preparing the first edition of his Principles of Geology," now nearly half a century ago, addressed himself to the task of classifying the Tertiary strata, he introduced a principle of arrangement founded upon the varying proportions of living species which occur among the fossil shells in the several beds. Since that time the number of Tertiary species of mollusca known to palæontologists has vastly increased, and the percentages originally suggested by Lyell have not been strictly adhered to, though his divisions and their well-known names-Eocene, Miocene, and Pliocene --still hold their place in our geological systems. There can be no doubt that the quantivalent expressions have ceased to convey the ideas which they originally expressed; and Professor Boyd Dawkins,* holding that the classification is not in harmony with our present knowledge, has accordingly proposed a new method of arrangeîment. For this purpose he uses the mammalian remains instead of the mollusca. Not that he seeks to displace the Lyellian names, or to propose a new set of divisions. But he holds that the fossil mammalia of Europe present stages of specialization which coincide with the old geological divisions, and are more useful for classificatory purposes than are the mollusca, or indeed any invertebrate forms, or even the lower vertebrates. If his views referred only to certain points of classification, they might be left to the attention of the technical geologist; but, as a matter of fact, they possess a wide and popular interest in consequence of their bearing upon the probable period at which the earliest remains of man may be expected to occur.

The Eocene, or oldest group of the Tertiary formations, originally included all those strata which contained only a very small proportion of recent species of mollusca. But if the paleontologist fastens his attention upon the mammalia,

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he finds that the Eocene period was characterized by the appearance of representatives of living orders and families of placental mammals, but not of living genera, much less of species. In this country, for instance, we have representatives of the Ungulata, or great group of hoofed quadrupeds, both in the oddtoed division (Perissodactylia) and in the even-toed section (Artiodactylia). There are also representatives of the Rodentia and-what is of far more importanceof lemurine forms of the order Primates, which is the highest order of mammalia, including the lemurs, the apes, and man. It is important to remember that it is only the placental mammals which are used as the basis of Professor Dawkins's classification. For if the paleontologist descends to the marsupials, he finds that even in the Eocene period there were representatives of at least one living genus. Thus the Woolwich-and-Reading beds of Suffolk have yielded an opossum (Didelphys). Marsupial mammals are known to have existed throughout the secondary period, and it is therefore only probable that they should have attained in Eocene times to a more advanced stage of evolution than that reached at the same period by the higher mammalia. But, so far as the placental mammals are concerned, all the fossils found in the Eocene rocks are referred to extinct genera, and consequently the Eocene fauna is not likely to have contained man. "To seek for highlyspecialized man in a fauna where no living genus of placental mammal was present would be," in Professor Dawkins's opinion, an idle and hopeless quest.

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In the Miocene, or middle stage of the Tertiary strata, the proportion of recent species of mollusca is larger than in the Eocene beds, but still the extinct forms are dominant. Professor Dawkins would define the Miocene as that period in which living genera of the placental mammalia first make their appearance. Although the Miocene mammalia are represented in Britain only by the hoglike Hyopotamus, yet on the continent, where the Miocene strata are strongly developed, there is a rich mammalian fauna of this period. The Miocene fauna includes representatives of a large number of existing genera, and Professor Dawkins's studies lead him to the

conclusion that certainly as many as twenty-three living genera date their earliest appearance from Miocene times. During the early stages of this period the opossum might still be found lingering in the European forests; but at the close of the Lower Miocene age the palæontologist bids farewell to this, the last representative of the European marsupials. On the other hand, he finds several representatives of the Primates, more or less allied to the anthropoid apes, yet all apparently belonging to extinct genera. Remains of these apes occur in the Middle Miocene strata of France and Germany, Switzerland and Italy, and in the Upper Miocene deposits in Greece. It is noteworthy that a large ape has left a record of its existence as far north as Eppelsheim in Germany, thus proving that the range of the Simiada in Europe must have extended, during the warm Miocene period, at least fourteen degrees north of the present limit of the Old World apes.

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Whether we regard the apes or any other of the terrestrial mammals of the Miocene fauna, it is a significant fact that we fail to find any trace of a single existing species. Upon this fact Professor Dawkins bases a strong argument against the probability of ever finding any remains of man in strata of Miocene age. Man, the most highly specialized of all creatures, had no place in a fauna which is conspicuous by the absence of all the mammalia now associated with him." Yet it must be remembered that several eminent naturalists in France have confidently expressed their belief in the existence of Miocene man. Some of the evidence upon which this belief is grounded has already been set forth in these pages. It is true that Miocene Europe, with its warm climate and with abundance of food in its luxuriant forests, appears to have offered all the needful surroundings for the development of man. But Professor Dawkins, reasoning on the evolution of the higher mammalia, refuses to include man in the Miocene fauna, and expresses his opinion that "were any man-like animal living in the Miocene age, he might reasonably be expected to be not man, but intermediate between man and something else."

With regard to the chipped flints and incised bones, to which the French an

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