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each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have at all times over-mastered other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was young, budding twigs; and this connection of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear the other branches; so with the species which lived during long-past geological periods, very few have left living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these fallen branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only in a fossil state. As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give

rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications.

JOHN TYNDALL: THE GEYSERS OF ICELAND

From "Heat a Mode of Motion," New York, 1883, pp. 166-173.

This example combines the best qualities of popular science: it is based on exact knowledge of the subject, it is beautifully clear and exact in the exposition, it is written in the service of an intelligent curiosity in natural history, and in style it shows the results of cultivation and good breeding. After a short description of what would interest an intelligent and activeminded man in these devil's kitchens of Iceland it narrows down to the Great Geyser and to the curious problems which it presents. Then it works out the theoretical solution of these problems; and clinches this with a mechanical proof which was performed on the stage at the lecture. It closes with a return to larger and more general considerations. As a piece of exposition it could hardly be better rounded.

The diagrams and the illustration, it will be noticed, are invaluable in making the explanation clear and easy to grasp; and they have also their share in doing what the experiment must have done in the lecture, — i. e. in stimulating your interest.

LET me now direct your attention to a natural steam-engine, which long held a place among the wonders of the world—the Great Geyser of Iceland. The surface of that country gradually rises from the coast towards the centre, where the general level is about two thousand feet above the sea. On this, as on a pedestal, are planted the Jokull, or icy mountains of the island, which extend both ways in a north-easterly direction. Along this chain occur

the active volcanoes of Iceland, and the thermal springs follow the same general direction. From the ridges and chasms which diverge from the mountains, enormous masses of steam issue at intervals, and when the escape occurs at the mouth of a cavern, the resonance of the cave often raises the sound of the steam to the loudness of thunder. Lower down, in the more porous strata, are to be found smoking mud pools, where a blue-black aluminous paste is boiled, rising at times in huge bubbles, which, on bursting, scatter their slimy spray around. From the base of the hills upwards extend the glaciers, and above these are the snow-fields which crown the summits. From the arches and fissures of the glaciers, vast masses of water issue, falling at times in cascades over walls of ice, and spreading for miles over the country before they find definite outlet. Extensive morasses are thus formed. Intercepted by the cracks and fissures of the land, a portion of the water finds its way to the heated rocks beneath; and here, meeting with the volcanic gases which traverse these underground regions, both travel on together, to issue, at the first convenient opportunity, either as an eruption of steam or as a boiling spring.

The most famous of these springs is the Great Geyser. It consists of a tube, seventy-four feet deep and ten feet wide. The tube is surmounted by a basin, which measures from north to south fifty-two feet across and from east to west sixty feet. The interior of the tube and basin is coated with a beautiful smooth siliceous plaster, so hard as to resist the

blows of the hammer; and the first question is, How was this wonderful tube constructed how was this perfect plaster laid on? Chemical analysis shows that the water holds silica in solution, and it might therefore be conjectured that the water had deposited silica against the sides of the tube and basin. But such is not the case. The water deposits no sediment; no matter how long it may be kept, no solid substance is separated from it. I have here a specimen which has been bottled up and preserved for years, as clear as crystal, without showing the slightest tendency to form a precipitate. To answer the question in this way would moreover assume that the shaft was formed by some foreign agency, the mineral water merely lining it. The geyser-basin, however, rests upon the summit of a mound about forty feet high, and it is evident, from mere inspection, that the mound has been deposited by the geyser. But in building up this mound the spring must have formed the tube which perforates the mound; hence the suggestion that the geyser is the architect of its own tube.

If we place a quantity of the geyser water in an evaporating basin, the following takes place: In the centre of the basin the liquid deposits nothing, but at the sides, where it is drawn up by capillary attraction, and thus subjected to speedy evaporation, we find a ring of silica deposited. Not until the evaporation has continued a considerable time is the slightest turbidity found in the middle of the water. This experiment is the microscopic repre

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