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lege, London. *

The selenium was ex- unable to pass, but on heating it to rooo posed not only to radiations from differ- C., and then allowing it to cool, it beent sources, but to light which had been came a feeble conductor, and its contransmitted through various absorbing ductivity was increased by the action of media, such as colored glass, solutions light. If, however, the selenium disk of colored salts, plates of rock salt, were exposed to a temperature of about quartz, mica, alum, and other appropri- 210° C., or nearly to its melting-point, ate substances. These experiments and then gradually cooled, the substance showed convincingly that light was the passed into a second modification, in chief agent in inducing the change in the which it was a much better conductor of electrical properties of the selenium, in- electricity, and was extremely sensitive asmuch as these properties were scarcely to luminous rays. affected either by the ultra-red or by the For the purpose of detecting variaultra-violet

rays. The maximum effect tions in the strength of the current unwas obtained in the yellowish-green por- der varying conditions of illumination, tion of the spectrum. Under the influ- all experimentalists who had worked on ence of moonlight the resistance of the this subject had naturally made use of selenium was sensibly reduced. On the galvanometers. It occurred, however, whole it was clear that light and not heat to Mr. Graham Bell, that his telephone was the agent to which Mr. Willoughby might be advantageously used in such Smith's phenomenon was due. In fact, experiments. It is obvious that if a telit is now a well-established fact that while ephone were introduced into a circuit light increases the conducting power of which included a cell of crystalline crystalline selenium, heat diminishes selenium, the telephone would be affect

ed at every admission of light to the While these investigations were being sensitive material, and again at every exconducted in this country, Dr. Werner clusion. But, in each case, the effect Siemens was independently engaged would be only of momentary duration. upon the same subject in Berlin. He Consequently, in order to throw the devised an ingenious form of selenium diaphragm of the telephone into a state cell, which was prepared in the following of vibration, so as to produce distinct manner. Two opposite spirals, or two sounds, the light must be intermitted parallel zigzags, of thin platinum wire with great rapidity. Let the selenium were laid upon a sheet of mica, and be subjected to a quick succession of united by a drop of molten selenium, exposures' and eclipses, and the correwhich, before solidifying, was squeezed sponding changes in the conductivity of out into the form of a thin film by pres- the material would keep the disk of the sure of a second plate of mica. The telephone in a state of oscillation, and current was caused to enter the cell thus sound would be produced by the action through one of the wires, then to trav- of light. The light would act upon the erse the selenium, and finally to pass selenium, and the telephone would out through the opposite wire. With audibly respond. cells of this construction, a great number Foreseeing the possibility of thus evokof experiments were made by Dr. Sie- ing sound by the action of light, Profesmens in conjunction with Dr. Obach. sor Bell, in the course of a lecture which As long as the selenium remained in the he delivered at the Royal Institution in amorphous condition, the current was 1878, ventured to express his opinion

that when light which had fallen upon * “ The Action of Light on Selenium.” selenium was intercepted, it would be Proc. Roy. Soc. June 17, 1875, vol. xxiii. p. possible, by proper arrangements, to 535 ; Jan. 6, 1876, vol. xxiv. p. 163 ; June 15, hear the shadow. And only a few days 1876, vol. xxv, p. 113.

“ Ueber die Abhängigkeit der electrischen afterward, Mr. Willoughby Smith Leitungsfähigkeit des Selen von Wärme und nounced that he had actually heard, Lichte.” Monatsberichte d. k. preuss. Akad. d. through the telephone, the effect of the Wissenschaften z. Berlin, Feb. 17. 1876 ; June fall of a ray of light upon a piece of 4, 1877. See also a lecture on The Action of Light on Selenium,” by Dr. C. William

sensitive selenium. Siemens, in Proc. Roy. Institution, Feb. 18,

Practically, however, it was found that 1876, vol. viii. p. 68.

the very great resistance which the sele

an

nium offered to the passage of the cur- the two stations is nothing more than a rent rendered it unmanageable. But beam of light. Mr. Bell, working conjointly with his No sooner had the photophone been friend, Mr. Sumner Tainter, has com- constructed in the form which has just pletely overcome this difficulty, and has been described than it was destined to prepared, by very simple means, seleni- undergo an extraordinary modification. um cells which offer only a moderate re- It may fairly be supposed that when light sistance, and are, therefore, suitable for falls upon the selenium, it must set up telephonic experiments. No fewer than some kind of molecular disturbance upfifty different forms of apparatus have on its sensitive surface. Accordingly, been devised by these experimentalists Mr. Bell argued that if such a morement for the purpose of actuating the tele- of the molecules really does take place, phone by varying the illumination of the there was the bare possibility that it selenium. One of the most simple of might be heard with the unaided ear. these forms consists merely of a small Removing then the telephone and batflexible inirror, upon which a beam of tery, Mr. Bell applied his ear directly to light is concentrated. The mirror may the selenium disk. The early experibe made of a piece of very thin glass, or ments were not successful, but ultimately of a disk of mica silvered on one side. he had the satisfaction to find that the Upon such a mirror a beam of light - crystalline selenium, under proper conpreferably sunlight, by reason of its in- ditions, did actually emit distinct sounds. tensity-is concentrated by means of a Far more remarkable, however, than lens.' The light reflected from the mir- this fact, was the unexpected discovery ror is passed through another lens so as that such an emission of sound, under to form a beam of parallel rays, and this the influence of varying illumination, is beam is projected to the distant station, not confined to selenium. The first where it is received upon a parabolic material in which Professor Bell dismirror. The mirror concentrates the tinctly observed this phenomenon was a light upon a cell of sensitive seienium piece of hard rubber, and a great variety which is placed in the focus, and is con- of other substances were then tested with nected in a local circuit with a telephone more or less success.

Antimony and and a galvanic battery.

hard rubber were found to emit the If a speaker at the transmitting sta- loudest sounds, paper and mica the tion now directs his voice against the weakest, while the only substances which back of the little flexible mirror, the mir- remained silent in the course of these ror is thrown into a state of vibration, experiments were carbon and thin glass. and the agitation is necessarily commu- The inventors of the photophone feel nicated to the beam of reflected light. warranted in stating, as the result of When, therefore, this light reaches the their studies, that sounds can be proreceiving station, it falls upon the sele- duced by the action of a variable light nium as an undulatory beam” — in upon substances of all kinds, provided other words, although it inay shine con- they be used in the suitable form of thin tinuously upon the selenium, its in- diaphragms. Mr. Bell's experiments tensity is yet subject to rapid variations. have therefore resulted not only in the These variations produce equally rapid invention of a new acoustical instrument, changes in the electric current which but in the discovery of the fact that mattraverses the selenium, and every rise ter in general is susceptible of molecular or fall in the conductivity of the seleni- change, under the influence of light, um is thus transmitted to the telephone, to an extent and in a way which had not where it manifests itself audibly by previously been suspected. throwing the diaphragm into a similar state of vibration. It is obvious, there- In delivering the Presidential Address fore, that every sound produced at the to the British Association at the recent back of the transmitting mirror must meeting at Swansea, Professor Ramsay evoke a corresponding sound at the dis- fave publicity to some geological obtant receiving station. Words uttered servations which had recently been made at one end are thus faithfully reproduced by Professor Geikie in the north-west of at the other, though the bond between Scotland, and which, if they bear the

interpretation that has been put upon ada a vast series of metamorphic rocks, them, are undoubtedly of the deepest also of pre-Cambrian age and largely interest to the physical geologist. * The made up in like manner of gneiss, it was President's announcement was immedi. but natural to compare the old Scottish ately followed by the publication of Pro- rocks with those of Canada, and thus fessor Geikie's own account of the ob- the " Fundamental gneiss' of Scotland servations.

has come to be generally called nowadays For many years past the order of suc- Laurentian gneiss-the term Laurencession of the old rocks in the north of tian” having been borrowed by Canadian Scotland has been placed almost beyond geologists from the Laurentides, a range dispute. Mr. Peach's discovery of of hills which lie on the north of the valLower Silurian fossils at Durness long ley of the St. Lawrence. ago settled the age of the limestones and Along the western margin of the counwhite quartzites of Sutherlandshire, and ties of Sutherland and Ross the Laurenthus afforded a starting-point for the tian gneiss presents a peculiar type of determination of the age of the unfos- scenery, which has been graphically desiliferous rocks in this region. Beneath scribed by Professor Geikie. The gnarled the Silurian rocks, in the north-west of gneiss forms a succession of bosses, Scotland, are enormous masses of dark hummocks, and ridges, peculiarly roundred or purple sandstones and conglom- ed in contour, and wellnigh destitute of erates, which rise at places into conical vegetation. The mammillations of the mountains upward of three thousand feet surface suggest that the rocks have been above the level of the sea. The late Sir worn down and rounded by the passage Henry Janies and Professor Nicol showed of moving ice ; and it needs but little that these sandstones are separated by a examination to recognize the smoothing, strong unconformity from the overlying the polishing, and the striation which Silurian rocks; and Sir Roderick Mur- speak so unmistakably of glacial action. chison, recognizing their higher an- At first sight it might naturally be astiquity, referred them to the Cambrian sumed that these effects were due to formation. But far older than these erosion by ice during that comparatively Cambrian strata, and separated from modern period which is known as the them in turn by another unconformity, Glacial Age. Yet it is strange that the is a series of highly metamorphosed crys. neighboring sandstones, quartzites, and talline rocks, consisting chiefly of con- schists, over which the ice of that period torted gneiss. This gneiss occurs in the must also have travelled, fail to exhibit outer Hebrides, and is occasionally equally marked traces of glacial erosion. known, from its occurrence in the Isle Nor can it be said that the unyielding of Lewis, as Lewisian gneiss; it also nature of the gneiss has enabled it to restretches along the coast of the opposite tain with persistence the evidence of icemainland from Cape Wrath, with more work, while such evidence has been obor less interruption, as far south as Loch literated from many of the neighboring Torridon. Finding in this pre-Cambrian rocks ; for in the Scottish Highlands, gneiss a representative of the most an- where gneissose rocks of younger age cient stratified rocks in the country, Mur- have been exposed to the action of ice chison bestowed upon it the name of the during the glacial period, the contours Fundamental gneiss—a name which was and general characters of the rocks are intended to suggest that it formed the quite different from those of the Laurenfloor of the British islands, upon which tian gneiss. How then can the geologist the later - formed deposits had been hope to explain the peculiarities in the spread. When the investigations of Sir erosion of the venerable gneissose rocks William Logan and his colleagues had of the north-west of Scotland ? clearly shown that there existed in Can- Probably the explanation is to be

found in the recent observations of * “ Address on the Recurrence of certain Professor Geikie.

Professor Geikie. In examining the icePhenomena in Geological Time," delivered before the British Association, August 25, 1830, has been able to trace their rounded out

worn surfaces of Laurentian gneiss, he P. 17.

+ A Fragment of Primæval Europe." lines passing distinctly beneath the overNature, August 26, 1880, p. 400.

lying Cambrian rocks. This was the case, for example, on both sides of Loch known to geologists in any part of the Torridon, and again on the west side of world. Loch Assynt. The conclusion is thus When Sir Charles Lyell, in preparing forced upon the observer that the old the first edition of his Principles of gneiss must have received its smooth Geology,” now nearly half a century ago, Aowing contours, to some extent at least, addressed himself to the task of classifybefore the Cambrian sandstones were ing the Tertiary strata, he introduced a deposited. Can it be, then, that we have principle of arrangement founded upon evidence in these rocks of a glacial the varying proportions of living species period dating back to early palæozoic which occur among the fossil shells in times ?

the several beds. Since that time the This suggestion appears to receive number of Tertiary species of mollusca some support from Professor Geikie's known to palæontologists has vastly inobservations in the neighborhood of creased, and the percentages originally Gairloch, where he found the undulating suggested by Lyell have not been strictly surface of gneiss to be capped in places adhered to, though his divisions and by a coarse unstratified breccia, contain- their well-known names - Eocene, Mioing angular fragments of the Laurentian cene, and Pliocene -- still hold their gneiss, sometimes as much as five feet in place in our geological systems. There length, standing on end and at all can be no doubt that the quantivalent angles. Such a breccia obviously bears expressions have ceased to convey the a suspicious resemblance to a modern ideas which they originally expressed ; moraine.

and Professor Boyd Dawkins, * holding Since Professor Ramsay, in 1855, that the classification is not in harmony brought before the Geological Society with our present knowledge, has accordthe evidence upon which he had satis- ingly proposed a new method of arrangefied himself as to the existence of glaciers stient. For this purpose he uses the during the Permian period, he has nat- mammalian remains instead of the molurally been interested in any traces of lusca. Not that he seeks to displace the recurrence of glacial phenomena, es- the Lyellian names, or to propose a new pecially among the earlier rocks. 'To

To set of divisions. But he holds that the him, therefore, Professor Geikie's ob- fossil mammalia of Europe present servations were peculiarly acceptable, stages of specialization which coincide and he received them without hesitation with the old geological divisions, and as evidence of the action of “ancient are more useful for classificatory purglaciers of Cambrian age." There was poses than are the mollusca, or indeed already a body of facts tending to show any invertebrate forms, or even the lower that glacial conditions must have pre- vertebrates. If his views referred only vailed in certain parts of the world dur- to certain points of classification, they ing a portion of the Silurian period ; but might be left to the attention of the techif the early glaciation of the Laurentian nical geologist ; but, as a matter of fact, gneiss be admitted, we may now carry they possess a wide and popular interthe glacial phenomena a stage further est in consequence of their bearing upback in the earth's history. It is only on the probable period at which the fair, however, to remark that Professor earliest remains of man may be expected Geikie himself speaks most guardedly as to occur. to the conclusions to be drawn from his The Eocene, or oldest group of the observations, and in referring to the Tertiary formations, originally included rounded surfaces of the gneiss is content all those strata which contained only a to remark that “they have certainly very small proportion of recent species been ground by an agent that has pro- of mollusca. But if the palæontologist duced results which, if they were found fastens his attention upon the mammalia, in a recent formation, would without hesitation be ascribed to land ice.” If * “ The Classification of the Tertiary Period this ascription be warranted in the case by means of the Mammalia.” Quarterly Fourof the old Scottish gneiss, that rock pre

nai of the Geological Society, vol. xxxvi., No.

Early sents us with vestiges of glacial action far Man in Britain, and his place in the Tertiary

143, August 1380, p. 379. See also his older than anything of the kind hitherto Period.” London: Macmillan & Co., 1880.

.

he finds that the Eocene period was char- conclusion that certainly as many as acterized by the appearance of repre- twenty-three living genera date their sentatives of living orders and families earliest appearance from Miocene times. of placental mammals, but not of living During the early stages of this period the genera, much less of species. In this opossum might still be found lingering country, for instance, we have represent in the European forests; but at the atives of the Ungulata, or great group close of the Lower Miocene age the of hoofed quadrupeds, both in the odd- paleontologist bids farewell to this, the toed division (Perissodactylia) and in the last representative of the European even-toed section (Artiodactylia). There marsupials. On the other hand, he finds are also representatives of the Rodentia several representatives of the Primates, and-what is of far more importance- more or less allied to the anthropoid of lemurine forms of the order Primates, apes, yet all apparently belonging to exwhich is the highest order of mammalia, tinct genera.

tinct genera. Remains of these apes including the lemurs, the apes, and man. occur in the Middle Miocene strata of It is important to remember that it is France and Germany, Switzerland and only the placental mammals which are Italy, and in the Upper Miocene deused as the basis of Professor Dawkins's posits in Greece. It is noteworthy that classification. For if the palæontologist a large ape has left a record of its existdescends to the marsupials, he finds that ence as far north as Eppelsheim in Gereven in the Eocene period there were many, thus proving that the range of the representatives of at least one living Simiada in Europe must have extended, genus. Thus the Woolwich-and-Read- during the warm Miocene period, at ing beds of Suffolk have yielded an least fourteen degrees north of the presopossum (Didelphys). Marsupial mam- ent limit of the Old World apes. mals are known to have existed through- Whether we regard the apes or any out the secondary period, and it is there- other of the terrestrial mammals of the fore only probable that they should have Miocene fauna, it is a significant fact attained in Eocene times to a more ad- that we fail to find any trace of a single vanced stage of evolution than that existing species. Upon this fact Profesreached at the same period by the higher sor Dawkins bases a strong argument mammalia. But, so far as the placental against the probability of ever finding mammals are concerned, all the fossils any remains of man in strata of Miocene found in the Eocene rocks are referred age. Man, the most highly specialized to extinct genera, and consequently the of all creatures, had no place in a fauna Eocene fauna is not likely to have con- which is conspicuous by the absence of tained man. To seek for highly- all the mammalia now associated with specialized man in a fauna where no liv- him."

him." Yet it must be remembered that ing genus of placental mammal was pres- several eminent naturalists in France ent would be,” in Professor Dawkins's have confidently expressed their belief opinion, an idle and hopeless quest. in the existence of Miocene man. Some

In the Miocene, or middle stage of the of the evidence upon which this belief is Tertiary strata, the proportion of recent grounded has already been set forth in species of mollusca is larger than in the

It is true that Miocene Eocene beds, but still the extinct forms Europe, with its warm climate and with are dominant. Professor Dawkins would abundance of food in its luxuriant define the Miocene as that period in forests, appears to have offered all the which living genera of the placental needful surroundings for the developmammalia first make their appearance. ment of man. But Professor Dawkins, Although the Miocene mammalia are reasoning on the evolution of the higher represented in Britain only by the hog- mammalia, refuses to include man in the like Hyopotamus, yet on the continent, Miocene fauna, and expresses his opinion where the Miocene strata are strongly that “were any man-like animal living developed, there is a rich mammalian in the Miocene age, he might reasonably fauna of this period. The Miocene be expected to be not man, but intermefauna includes representatives of a large diate between man and something else." number of existing genera, and Profes- With regard to the chipped flints and sor Dawkins's studies lead him to the incised bones, to which the French an

these pages.

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