18 Mr. F. Guthrie on Salt Solutions and Attached Water. 



solid matter in this also was determined. The following num- 

 bers show the result of this examination : — 



Per cent, at 100° of 

 solid residue. 



Sea-water 6*5786 



Frozen sea- water 5*4209 



Frozen and pressed sea-water . 0*4925 



It appears, then, that under these conditions the freezing of 

 sea-water is little more than the freezing of ice, and that the 

 almost undiminished saltness of the unpressed ice is due, as 

 suggested by Dr. Rae, to the entanglement amidst the ice- 

 crystals of a brine richer in solid constituents than the original 

 water itself. Such brine, which is here squeezed out in the 

 press, drains in nature down from the upper surface of the ice- 

 floe by gravitation, and also is replaced by osmic action by new 

 sea- water which again yields up fresh ice ; so that while new 

 floes are porous and salt, old ones are more compact and much 

 fresher, as the traveller observed. 



§ 33. But, bearing in mind the existence of the cryobydrates, 

 certainly of sulphate of magnesium and doubtless also of chlo- 

 ride of calcium at temperatures not far below 0° 0., a rapid fall 

 of temperature may be accompanied by more complex pheno- 

 mena of gelation • for if the ice be quickly removed from a 

 large mass of water by freezing, the resulting brine may easily 

 be so enriched as to throw out one or more cryobydrates, which 

 thus perpetuate in situ a definite amount of saline matter. How 

 far such cryohydrates are soluble in the chief cryohydrate, 

 namely that of NaCl, which by itself resists the cold the 

 longest, is an important matter for future research. But there 

 can even, viewed in the light of the experiments given above, 

 be little doubt that the degree of saltness of a floe depends not 

 only upon its age, but also upon the rapidity with which it was 

 at first formed, and upon the lowest temperature to which it 

 has subsequently been exposed. 



§ 34. Since sea-water has no maximum density below its 

 freezing-point, when a mass of sea-water is uniformly cooled 

 to — 2° C. ice will be formed at any point, whether at the 

 bottom or at the surface, which loses more heat. Even the 

 middle of a mass of sea-water may lose heat by radiation, 

 and crystals of ice be thus formed in the mass. Or the 

 bottom of the sea may radiate sufficient heat through the ice- 

 cold layers above to freeze the water in contact with it, and 

 thus generate large masses of ice which break off and carry with 

 them parts of the sea-bottom. But I suppose the ice of the sea 

 is mainly formed at or near the surface by radiation from the 

 surface into space and by contact with tbe colder air. To imitate 

 as nearly as I could the condition which I suppose to exist, I 



