130 



THE FORMS OI WAT.LR 



443. According to Professor James Thom- 

 son's theory, pressure is necessary to lique- 

 fy the ice. The heat necessary for liquefac- 

 tion must be drawn from the ice itself, and 

 the cold water must escape from the pressure 

 to be re-frozen. Now in the foregoing ex- 

 periments the cold water, instead of being 

 allowed to freeze, issues into the warm water, 

 still the floatiEg fragments regelate in a mo- 

 ment. The touching surfaces may, more- 

 over, be convex ; they may be reduced prac- 

 tically to points, clasped all round by the 

 warm water, which indeed rapidly dissolves 

 them as they approach each other ; still they 

 freeze immediately when they touch. 



444. You may learn from this discussion 

 that in scientific matters, as in all others, 

 there is room for differences of opinion. 

 The frame of mind to be cultivated here is a 

 suspension of judgment as long as the mean- 

 ing remains in doubt. It may be that Fara- 

 day's action and Thomson's action come both 

 into play. I cannot do better than finish 

 these remarks by quoting Faraday's own con- 

 cluding words, which show how in his mind 

 scientific conviction dwelt apart from dog- 

 matism : "No doubt," he says, "nice ex- 

 perknents will enable us hereafter to criticise 

 such results as these, and separating the true 

 from the untrue will establish the correct 

 theory of regelation. ' ' 



Gi. THE BLUE VEINS OF GLACIERS. 



445. We now approach the end, one im- 

 portant question only remaining to be dis- 

 cussed. Hitherto we have kept it back, for 

 a wide acquaintance with the glaciers was 

 necessary to its solution. We had also to 

 make ourselves familiar by actual experiment 

 with the power of ice, softened by thaw, to 

 yield to pressure, and to liquefy under such 

 pressure. 



446. Snow is white. But if you examine 

 Its individual particles you would call them 

 transparent, not white. The whiteness 

 arises from the mixture of the ice particles 

 with small spaces of air. In the case of all 

 transparent bodies, whiteness results from 

 such a mixture. The clearest glass or crys- 

 tal when crushed becomes a white powder. 

 The foam of champagne is w r hitc through 

 the intimate admixture of a transparent 

 liquid with transparent carbonic acid gas. 

 The whitest paper, moreover, is composed of 

 fibres which are individually transparent. 



447. It is not, however, the air or the gas, 

 but the optical severance of the particles, giv- 

 ing rise to a multitude of reflections of the 

 white solar light at their surfaces, that pro- 

 duces the whiteness. 



48. The whiteness of the surface of a clean 

 glacier (112), and of the icebergs of the 

 Margelm See (357), has been already referred 

 lo a similar cause. The surface is broken 

 into innumerable fissures by the solar heat, 

 the reflection of solar light from the sides of 

 the little fissures producing the observed ap- 

 pparance. 



449. In like manner if you freeze water in 

 a test-tube by plunging it into a freezing 



mixture, the ice produced is white. For the 

 most part also the ice formed in freezing ma- 

 chines is white. Examine such ice, and you 

 will find it filled with smaU air-bubbles. 

 When the freezing is extremely slow the 

 crystallizing force pushes the air effectually 

 aside, and the resulting ice is transparent 

 when the freezing is rapid, the air is entan- 

 gled before it can escape, and the ice is 

 translucent. But even in the case of quick 

 freezing Mr. Faraday obtained transparent 

 ice by skilfully removing the air-bubbles, as 

 fast as they appealed, with a feather. 



45U. In the case of lake ice the freezing is 

 not uniform, but intermittent. It is some- 

 times slow, sometimes rapid. When slow 

 the air dissolved in the water is effectually 

 squeezed out and forms a layer of bubbles on 

 the under surface of the ice! An act of sud 

 den freezing entangles this air, and hence we 

 find lake ice usually composed of layers alter- 

 nately clear, and filled with bubbles, buci? 

 layers render it easy to detect the planes of 

 freezing in lake ice. 



451. And now for the bearing of these facts. 

 Under the fall of the Geant, at the base of 

 the Talefre cascade, and lower down the Mer 

 de Glace ; in the higher regions of the Grin- 

 delwald, the Aar, the Aletsch and the Gor- 

 ner glaciers, the ice does not possess the 

 transparency which it exhibits near the ends 

 of the glaciers. It is white, or whitish. 

 Why ? Examination shows it to be filled 

 with small air-bubbles ; and these, as we 

 now learn, are the cause of its whiteness. 



452. They are the residue of the air orig- 

 nally entangled in the snow, and connected, 

 as before stated, with the whiteness of the 

 snow. During the descent of the glacier, 

 the bubbles are gradually expelled 'by the 

 enormous pressures brought to bear upon the 

 ice. Not on4y is the expulsion caused by the 

 mechanical yielding of the soft thawing ice, 

 but the liquefaction of the substance at 

 places of violent pressure, opening, as it 

 does, fissures for the escape of the air, must 

 play an important part in the consolidation 

 of the glacier. 



453. The expulsion of the bubbles is, how- 

 ever, not uniform ; for neither ice nor any 

 other substance offers an absolutely uniform 

 resistance to pressure. At the base of every 

 cascade that we have visited, and on the 

 walls of the crevasses there formed, we have 

 noticed innumerable blue streaks drawn 

 through the white translucent ice, and giv- 

 ing the whole mass the appearance of lamina, 

 tion. These blue veins turned out upon ex- 

 amination to be spaces from which the air- 

 bubbles had been almost wholly expelled, 

 translucency being thus converted into trans 

 parency. 



454. This is the veined or ribboned structure 

 of glaciers, regarding the origin of which 

 diverse opinions are now entertained. 



455. It is now our duty to take up the 

 problem, and to solve it if we can. On the 

 neves of the Col du Geant, and other gla- 

 ciers, we have found great cracks, and 

 faults, and Bergschrunds, exposing deep sec- 



