August 20, 1908J 



NA TURE 



)79 



serve to fill up many a gap in the early history of the 

 country. The main hindrances to progress are at pre- 

 sent : — first, the exclusiveness of both Hindus and 

 Mohammedans, who object to the examination of buildings 

 which are still used for worship; secondly, the fart that 

 many important ancient buildings arc situated within the 

 territories of native States, where it has up to the present 

 been impossible to arrange satisfactory schemes for con- 

 servation and excavation. In spite of these drawbacljs, 

 archaeological research in India has now been placed upon 

 a sound footing, and in the immediate future a largo 

 amount of fresh material, in the shape of sculptures and 

 inscriptions, will be at the disposal of students. 



O.'V ICE AND ITS NATURAL HISTORY'. 



T~)URING the Antarctic cruise of the Challenger in the 

 early part of the year 1874 I carefully examined the 

 chemical and physical properties of sea ice. The melting 

 temperature of the ice varied with the samples, but it was 

 always below 0° C, and it was found impossible by any 

 means to produce pure water by melting it. These two 

 facts were for me convincing evidence, at that date, that 

 the salt was present in the ice in the solid state, and that, 

 consequently, the crystalline body, formed by freezing sea- 

 water and similar saline solutions, was not pure ice. 



About nine years later Dr. Otto Petterssen, having his 

 attention directed in a similar way to the same subject, 

 arrived at the same conclusion. His observations and their 

 discussion are embodied in a remarliable paper on the 

 properties of ice and water (Publications of the Vega 

 Expedition, 1883). In the careful study which I made of 

 this work the following passage arrested my attention : — 



A thermometer immersed in a mixture of snow and sea 

 water, which is constantly stirred, indicates — 1°.8 C." 



If this statement was exact, it was clear that the 

 evidence furnished by the melting temperature of the sea 

 ice was not entitled to the weight which I attached to it, 

 and that the conclusion at which we had independently 

 arrived was open to doubt. On repeating the experi- 

 ment, I was able to confirm Petterssen 's statement. I 

 tlien proceeded to investigate the subject in detail. The 

 principle which guided the investigation was the follow- 

 ing : — if the crystalline body, which is formed when a non- 

 saturated saline solution is partially frozen, is pure ice, 

 then pure ice of independent origin, such as snow, must, 

 when mixed with the same saline solution, and heat is 

 supplied, melt at the same temperature when the con- 

 centration is the same. 



This was found to be the case ; and the result of the 

 research was definitively to establish, on experimental 

 evidence, the validity of the principle that, when a non- 

 saturated saline solution is partially frozen, the crystals 

 which are formed are pure ice ; and, by consequence, that 

 the salt from which it is, in practice, impossible to free 

 them, belongs to the adhering brine. - 



It was not until after this had been established, in 

 1S87, that it became legitimate to say " the freezing 

 point of_ mater is lowered by the presence of salt dissolved 

 in it," instead of saying " the freezing point of a saline 

 solution is so much lower than that of pure water." The 

 former of these statements expresses the fundamental prin- 

 ciple of cryometric chemistry. 



Shortly, I define the freezing and melting temperature 

 of a substance to be the temperature at ■which it as a 

 solid passes into itself as a liquid, and as a liquid passes 

 into itself as a solid. In terms of this definition the 

 freezing and melting temperature of the substance H,0 

 is 0° C. In order to represent the temperature at which 

 ice melts, this definition requires a double qualification. 

 At constant pressure the temperature at which ice melts 



1 Abridged from a di'Jcourse delivered at the Royal Institulion on Friday 

 evening, May 8, by J. Y. Buchan.m, F R.S. 



" The results of the research which I began !n the year 1886 were con-- 

 municated 10 the Royaf Society of Edinburgh in a paper "On Ice and 

 Brines, which was read on March 21, 1887. and was published in the Pro- 

 ceedings of the Society, vol. xiv., pp. 129-149. A full account of it was also 

 pubhsheH in Nature, 1887. vol. xxxv., p. 608, and vol. xxxvi., p. 9. 

 J he whole subject of the influence of dissolved salt on the state of aggre- 

 gation of the subsiance HoO at temperatures below its normal freezing and 

 meltinp point and above its normal boiling and condensing pnint was passed 

 in review m my chemical and physical notes in the "Antarctic Manual," 

 1901, pp. 73-108. 



NO. 2025, VOL. 78] 



depends on the nature of the medium in which it njelts, 

 and if the nature of the mediuin be constant, the tempera- 

 ture at which ice melts depends on the pressure. Of the 

 two modifying agencies, the influence of the medium pre- 

 ponderates in nature. Indeed, inasmuch as perfectly pure 

 water is rarely, if ever, met with, it is probable that, 

 III nature, ice never melts and water never freezes exactly 

 at 0° C. 



The principle that the temperature at which ice melts 

 depends on the nature of the medium in which it melts 

 is the key to the natural history of ice, and it forms the 

 theme or te.xt of this discourse. It at once brings into 

 order the anomalies frequently observed in the experi- 

 mental determination of many of the physical constants 

 of ice, such as its coefficient of thermal expansion, its 

 specific and its latent heat. When the ice or the water 

 in which it is immersed contains any impurity, the tempera- 

 ture at which the ice begins to melt is below 0° C, and 

 the substance under examination, which is taken for pure 

 solid ice, is in reality a mixture of ice and impure water. 



In discussing this subject, chloride of sodium is taken 

 as the representative impurity, because it is the most widely 

 disseminated ingredient of natural waters. It has also 

 been more thoroughly studied than other salts in its 

 behaviour to ice, water, and steam. In discussing the 

 influence which this salt exercises over the apparent 

 physical properties of ice a constant quantity of it is con- 

 sidered, and the quantity of ice on which it acts is varied- 

 The constant quantity of the salt is 1-5105 grams, which 

 contain 0-9167 gram of chlorine. The specific gravity of 

 pure ice is taken as 0-9167 referred to that of water at 

 the same temperature as unity. 



It will be recognised that when 1000 c.c. of ice con- 

 taining 1-5105 grams NaCl are melted, they furnish a 

 water which contains chlorine in the proportion of i : 1000 

 by weight. The coefficient of cubic expansion by heat of 

 pure ice is taken as o-oooi6, and it is assumed to be 

 constant at the teinperatures under consideration. The 

 volume occupied by the 1-5 105 grams NaCl is disregarded. 

 The cryohydric temperature of NaCI solution is taken as 

 — 2i°-72 C., and its cryohiWric concentration as 29-97 

 grams salt to 100 grams water. 



Using these constants, we will apply the principle to the 

 calculation of the apparent variations of volume of a block 

 of ice the volume of which at 0° C. is 1000 c.c. It 

 contains diffused through it 1-5105 grams NaCI, which 

 we assume to be provisionally in the inert state, in which 

 it is deprived of the power to induce the melting of ice 

 at temperatures between 0° C. and — 2i°-72 C. Let the 

 temperature of the block containing the inert NaCl be 

 reduced to —23° C. ; its volume will be reduced to 

 996-320 c.c, and as the temperature is below the cryo- 

 hydric temperature, the salt is by nature inert ; at such 

 teinperatures ice and common salt are indifferent to each 

 other. Let the teinperature of the block of ice be now 

 raised to —22°; the salt remains inert, and the volume 

 of the ice increases to 996-48 c.c. If the temperature is 

 further increased to — 2i°-72i, the NaCl will still remain 

 inert, and the volume of the ice will become 996-525 c.c. 



If the heating is continued the temperature rises exactly 

 to the cryohydric point, — 2i°-72, at which temperature the 

 indifference of chloride of sodium to ice ceases, and induced 

 melting at that temperature takes place. It will then be 

 observed that the temperature remains constant for a time, 

 while the volume of the block diminishes. When the 

 temperature begins to rise, the volume of ice melted will 

 be 5-498 c.c. As this produces 5-040 c.c. water, the 

 diminution of volume is 0458 c.c, and the apparent volume 

 of the block is gg6-o67 c.c. 



Let us now go back to the initial state, in which we 

 have the block of 1000 c.c. ice, containing 1-5105 grams 

 inert NaCI diffused through it, at the temperature 0° C. 

 Let the temperature be reduced to —21° C., the ice re- 

 maining inert. The volume of the ice will then be 

 996-64 c.c. Let the NaCl recover its activity, it will 

 melt 5629 c.c. ice, producing 5-160 c.c. water under a 

 contraction of 0-469 c.c, so that the apparent volume of 

 the ice at —21° C. is 996-64 — 0-469 = 996-171 c.c. Proceed- 

 ing by steps in this way, we obtain, for different tempera- 

 tures t, the volume of the ice containing inert salt V, 

 the volume of ice melted by the salt when its activity is. 



