128 



THE FORMS OF WA^ER 



the branch glaciers. 



424. The gist of the Revelation Theory is 

 that the ice of glaciers changes its form and 

 preserves its continuity under pressure -which 

 keeps its particles together. But when sub- 

 jected to tension, sooner than stretch it breaks, 

 and behaves no longer as a viscous body. 



62. CAUSE OF REGELATION. 

 436. Here the fact of regelation is applied 

 to explain the plasticity of glacier ice, no 

 attempt being made to assign the cause of 

 regelation itself. They are two entirely dis- 

 tinct questions. But a little time will be 

 well spent in looking more closely into the 

 cause of regelation. You may feel some 

 surprise that eminent men shoukl devote their 

 attention to so small a point, but we must 

 not forget that in nature nothing is small. 

 Laws and principles interest the scientific 

 student most, and these may be as well illus- 

 trated by small things as by large ones. 



426. The question of regelation immediate- 

 ly connects itself with that of " latent heat," 

 already referred to (383), but which we must 

 now subject to further examination. To 

 melt ice, as already stated, a large amount 

 of heat is necessary, and in the case of the 

 glaciers this heat is furnished by the sun. 

 Neither the ice so melted nor the water which 

 results from its liquefaction can fall below 

 32 Fahrenheit. The freezing-point of water 

 and the melting-point of ice touch each 

 other, as it were, at this temperature. A 

 hair's-breadth lower water freezes ; a hair's- 

 breadth higher ice melts. 



427. But if the ice could be caused to melt 

 without this supply of solar heat, a tempera- 

 ture lower than that cf ordinary thawing ice 

 would result. When gnow and salt, or 

 pounded ice and salt, are mixed together, the 

 salt causes the ice to melt, and in this way a 

 cold of 20 or 30 degrees below the freezing- 

 point may be produced. Here, in fact, the 

 ice consumes its own warmth in the work of 

 liquefaction. Such a mixture of ice and salt 

 is called " a freezing mixture." 



428. And if by any other means ice at the 

 temperature of 32* Fahrenheit could be 

 liquefied without access of heat from with- 

 out, the water produced would be colder than 

 the ice. Now Professor James Thomson has 

 proved that ice may be liquefied by mere 

 pressure, and his brother. Sir William Thom- 

 son, has also shown tbat water under press- 

 ure requires a lower temperature to freeze it 

 than when the pressure is removed. Pro- 

 fessor Mousson subsequently liquefied large 

 masses of ice by a hydraulic press ; and by 

 a beautiful experiment Professor Helmholtz 

 has proved that water in a vessel from which 

 the air has been removed, and which is 

 therefore relieved from the pressure of the 

 atmosphere, freezes and forms ice-crystals 

 when surrounded by melting ice. All these 

 facts are summed up in the brief statement 

 that the freezing-point of water is lowered b$ 

 pressure. 



429. For our own instruction we may pro- 

 duce the liquefaction of ice by -pressure in 



the following way : You remember the 

 beautiful flowers obtained when a sunbeam 

 is sent through lake ice ( 11), and you have 

 not forgotten that the flowers always foirn 

 parallel to the surface of freezing. Let us 

 cut a prism, or small column of ice with <be 

 planes of freezing lunning across it at right 

 angles ; we place that prism between two 

 slabs of wood, and bring carefully to bear 

 upon it the squeezing force of a 'small hy- 

 draulic press. 



430. It is well to converge by means of a 

 concave mirror a good light upon the ice, 

 and to view it through a magnifying lens. 

 You already see Ihe result. Hazy surfaces 

 are formed in the very body ct the ice, 

 which gradually expand as ttie pressure is 

 slowly uugmenltu. Heie an-d there you 

 notice something resembling crystallization ; 

 fern-shaped figures run with consideiable 

 rapidity through the ice, and when you look 

 carefully at their points and edges you find 

 them in visible motion. These hazy sur- 

 faces are spaces of liquefaction, and the 

 motion you see is that of the ice falling to 

 water under the pressure. That water is 

 colder than the ice was before the pressure 

 was applied, and if the pressure be relieved 

 not only does the liquefaction cease, but the 

 water re-freezes. The cold produced by its 

 liquefaction under pressure is sufficient to 

 re-congeal it when the pressure is removed. 



431. If instead of diffusing the pressure 

 over surfaces of consideiable extent, we con- 

 centi ate it on a small surface, the liquefac- 

 tion will of course be more rapid, and this is 

 what Mr. Bottomley has recently done in an 

 experiment of singular beauty and interest. 

 Let us support on blocks of wood the two 

 ends of a bar of ice 10 inches long, 4 inches 

 deep, and 3 wide, and let us loop over its 

 middle a copper wire one twentieth, or even 

 one tenth, of an inch in thickness. Con- 

 necting the two ends of the wire together, 

 and suspending from it a weight of 12 or 14 

 pounds, the whole pressure of this weight is 

 concentrated on the ice which supports the 

 wire. What is the consequence ? The ice 

 underneath the wire liquefies ; .the water of 

 liquefaction escapes round the wire, but the 

 moment it is relieved from the pressure it 

 freezes, and round about the wire, even be- 

 fore it has entered the ice, you have a frozen 

 casing. The wire continues to sink in the 

 ico ; the water incessantly escapes, freezing 

 as it does so behind the wire. In half an 

 hour the weight falls ; the wire has gone 

 clean through the ice. You can plainly see 

 where it has passed, but the two severed 

 pieces of ice are so firmly frozen together that 

 they will break elsewhere as soon as along 

 the surface of regelation. 



432. Another beautiful experiment bear, 

 ing upon this point has recently been made 

 by M. Boussingault. He filled a hollow steel 

 cylinder with water and chilled it. In pass- 

 ing to ice, water, as you know, expands 

 ( 45) ; in fact, room for expansion is a nec- 

 essary condition of solidification. But in the 

 present case the strong steel resisted the ex- 



