SCIENTIFIC TALKS.

Otago Witness, Issue 1942, 7 February 1889, Page 37

SCIENTIFIC TALKS.

By F. G, S.

THE GEOLO&ICAL ACTION OF WATER.

No. 11. In the last article we discussed the visible geological action of water, dwelling shortly on the washing away of the land both by terrestrial and marine aqueous agencies. To-day it is proposed to descend into the interior of the earth, to trace the streams in their subterranean course, and to emerge with them possibly in the centre of some populous city, where they issue as artesian wells ; perhaps in the middle of some arid desert, where a lovely green oasis marks their presence, or it may be in some fernclad gully, where lies the source of some vast river ; or to peer down into the ; ocean depths, where a bubbling ' spring of fresh water issues from the ocean bed, and, rising to the surface, supplies the weary and wandering mariner with an unexpected and refreshing draught. If we examine the interior of a railway arch, or tunnel, or of any building of that nature, in the construction of which mortar has boen used, we' shall probably find little solidified drops pendant from the under surface of the masonry. Some are very minute, others have grown* as long as small icicles, which they much resemble, and from the end of each usually hangs a drop of water, which as it dries adds another fragment to the stalactite, as it is called, from which it 1 hung. Now these stalactites, which may be found wherever limestone exists and circumstances admit of their formation, are composed of carbonate of lime dissolved by the water in its course among the stones, and redeposited on evaporation in the atmosphere. In an exactly similar manner the water which percolates the crust of the earth dissolves the ingredients of the xocks, and frequently deposits them on the surface of the ground. Even pure water will do something in this way, but during its passage through the air oxygen and carbonic acid gas are absorbed, and from the soil organic acids are taken up ; by the aid of these The Most Durable flj«?ks ai*e Attacked and theif ingredients' carried away^ Carbonate of lime is one of ,the most easily dissolved rocks/ and as itexists'in- gfreat quantities as limestone, and 'covers' considerable areas, this salt ordinarily forms a large percentage of , '.the minerals carried 'away in solution I .' Trickling down 'the 'smallest crevices of the rock, the water makes for itself a gradually increasing passage, until large caverns and clefts are^hollowed out, some of which are among the natural wonders of the world.' The Peak' Cavern of , Derbyshire, which forms one of the show places of a locality rich in geological, wonders, is believed to be 2300 ft long; and J in places" 120 ft high, while "tb'e Mammoth .Qave .of, Kentucky is supposed to have in its accessible passages a length of' about' 150 "miles, and includes many caverns, the largest of which is two acres in extent, and 125 ft high. It is almost pertain that these enormous'excavations have been made by the solvent action of water on the limestone rocks. This is called the chemical, in, contradistinction to the mechanical action 'of springs, which' is.uihainly.apparent in the form of landslips. Where a porous mass of rock lies upon an impervious stratum the upper a one may'be'eome saturated, and, its cohesion being loosened, gravitate to a lower level. In this colony, where the strata lie frequently at high angles) and especially where the works of man, in the form of roacs and railways, have interfered with the stability, of the country,, landslips of a disastrous nature are of frequent occurrence. In Switzerland also, where the natural features somewhat resemble our own, great loss of life and destruction of property have taken place. T About 80 years a^o on the Rossberg, l a ' 'mountain r lying behind the Righi, a slip occurred which buried four villages and 457 inhabitants. The geological action of snow and ice, though not so /apparent as] the phenomena we . have mentioned, are of no less importance. ' Snow, it 'is true, as it falls in downy, lingering flakes, each as perfectly crystallised and as beautifully cut as the jewels ,in a monarch's diadem, would seem little able to mould trie face of Natnfe, or to leave -its' lasting impress on the hard, unyielding hills; and indeed, when it descends on level giound, its action is more distinctly preservative than otherwise. It protects the ground from the action of frost, and preserves in a like manner the winter vegetation. To quote Miss Procter :— Why do fche flowers die ? Prisoned they lie In the cold tomb, heedless of tears and rain. Oh, doubting heart t They only 6lpep below The sofb white ermine snow, While winter winds shall blow To breathe and smile upon you*soon*again. But when on the hills, where it may be stored for an indefinite time, snow becomes a threatening agent, to be hurled down with devastating" fury in the form -of avalanches, or to be melted by the warmth of summer and descend into the valleys as those terrible floods we know so well. These are, however, what may be "termed unusual or perhaps periodic effects. It is in the solid form of" ice that water has perhaps most markedly left its impress on the surface of the earth, and .shown how a substance'which has been ever proverbial for its instability, may conquer and subdue the everlasting hills. Before considering the phenomena of glaciers we may dwell a few moments en Tke Behaviour of Water at tlic.Moment of Freezing. By a very beautiful provision of Nature it attains its maximum density not, like most substances, when solid, but a few degrees before it assumes that form. Thus down to 39deg Fahrenheit water contracts-^-that is, it becomes more dense and heavier bulk for bulk. Below this it .gradually ex-, pands until reaching. 82deg, \ or, freezing; point, when it expands . suddenly,, and witS overwhelming force.' Long.ago~the Florentines -burst'by this means-a sphere made- ofj copper three-qtaarters of an inch in

thickness ; and in Canada an experiment was made by exposing to a temperature 50deg below freezing point a cannon filled , with water, and closed by a wooden plug. The cannon withstood the strain, but the plug was projected to a distance of 400 ft. The same force is utilised by the slate workers, who quarry large blocks of stone in the winter, and allow them to be exposed to the- frost and rain, by which means their fispility or power of splitting is greatly increased. In the same way frost opens up the.surface of the ground, and renders it fit for the roots of plants to penetrate ; everyone who has a garden or a farm knows the excellent effect of severe cold on a clayey or retentive soil. Now, every rock is traversed by tiny fissures, into which the water finds its way partly by the force of gravity and partly by capillary attraction. During severe weather these threads of liquid become frozen, and split off huge masses of rock, with a noise like artillery. If, instead of acting as has been described, the density of water increased with decreasing temperature, the effects would be disastrous. The ice would sink to the bottom of all our lakes and pools ; the screen of water between it and the sun would prevent its being melted ; gradually all inland sheets of water would become solid, varied only by a thin covering of liquid during the heat of summer. By a beautiful design in the great scheme of Nature, our admiration for the perfect harmony and excellence of which must ever increase as we study its working, this is prevented. The cold currents of. water descend until the whole body attains a temperature of 39deg ; then the change commences. Instead of sinking, the colder water remains on the surface, till at 32deg a protecting coat of ice is formed, which disappears when the balmy breath of spring blows over it.