Prof. Tyndall on some Physical Properties of Ice. 355 



lower temperature than 32° for congelation; this it would 

 acquire from the cold ice above, and by that it would become 

 lighter and float, tending to remain uppermost; for it has 

 already been shown that the diminution of temperature below 

 32° in sea water and solution of salts, is accompanied by the 

 same enlargement of bulk as between 32° and 40° with pure 

 water. The stratum of water, therefore, below the ice, would 

 not of necessity sink because it contained a little more salt than 

 the stratum immediately below it ; and certainly would not if the 

 increase of gravity conferred by the salts was less than the de- 

 crease by lowering of temperature. An approximation of the 

 strata between the freezing- place and the layer at 40° would 

 occur, i. e. the distance between these temperatures would be 

 less, but the water particles would keep their respective places. 



When water freezes, it does not appear that this process is 

 continuous ; for many of the characters of the ice seem to show 

 that it is intermittent : i. e. either a film of ice is formed, and 

 then the process stops until the heat evolved by solidification 

 has been conducted away upwards, and the next stratum of water 

 has been sufficiently cooled to freeze in turn ; or else the freezing 

 being, so to speak, continuous, still is not continued at the same 

 constant rate, but, as it were, by intermittent pulsations. Now 

 it may well be, when a layer next the previously-formed ice, and 

 containing an undue proportion of salts, has been cooled down 

 to its required temperature for freezing (which would be below 

 32°), that, on freezing, the congelation will pervade at once a 

 certain thickness of the water, excluding the salts from the larger 

 portion of ice formed, but including them as a weak solution 

 within its interstices. The next increment of cold conducted 

 from the ice above would freeze up these salts in the ice con- 

 taining them, at the same time that a layer of pure ice was formed 

 beneath it. Thus a layer of ice fusible at a lower temperature 

 than the ice either above or below it might be produced ; and 

 by a repetition of the process many such layers might be formed. 



It does not follow necessarily that the layers would be perfectly 

 exact in their disposition. Very slight circumstances tending to 

 disturb the regularity of the water-molecules would be sufficient, 

 probably, to disturb the layers more or less. Ice contains no 

 air, and the exclusion of a minute bubble of air from the water 

 in the act of freezing might disturb the direction and progress 

 of the congelation, and cause accumulation of the extra saline 

 liquid in one spot rather than another : so might the tendency 

 to the formati(;n (jf little currents, cither arising from the sepa- 

 ration of the saline water from the forming ice, or from the 

 elevation of temperature in different degrees at those places where 

 the congelation was going on at different rates. 



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