PHYSICAL PROPERTIES OF ICE. 643 



tunities not always occurring during the winter of examining 

 the ice formed naturally, I endeavoured to fill up the blanks by 

 artificially freezing sea water. The results given in the table may 

 not seem as uniform as would be expected, consequent on the 

 amount of efflorescence not being always crystallized, and ad- 

 hering to the ice at the time of thawing and testing. Still, from 

 the results obtained, and the composition of the efflorescence and 

 the residue, the conclusion may with propriety be educed, that, 

 as the temperature decreases from +28°'o to the lowest amount 

 of natural cold, the ice formed will, in equal times, also decrease 

 in density and increase in thickness. Most likely there will be a 

 point at which this decrease in density will be in a degree limited, 

 due to the rapid increase of thickness under extreme cold (say 

 —60° or —70°) in a short space of time. 



I have before stated that we were frozen in on the 7tli of 

 September 1857 ; the new ice then forming around us was of 

 specific gravity 1'0235 (30°); on the 15th of January 1858 this 

 same surface ice was specific gravity 1 '0102 (30°) ; on the 22nd of 

 March 1858 specific gravity 1-0078 (30°) ; so that this surface 

 ice, which was exposed for 6\ months to a temperature varying 

 from +28 "2 to — 47°, had, by contraction, squeezed out so much 

 extra of its remaining saline constituents. 



Set No. 4 gives the results obtained from the testing of specific 

 gravity of efflorescence formed at different temperatures. If the 

 temperature at which the efflorescence was expressed were not 

 sufficiently low to freeze it, it remained for a time semi-fluid and 

 pasty, only producing crystals on its surface ; but as the cold 

 increased, all the quantity squeezed out became crystalline. At 

 first, the spots of emergence of those crystals were few and widely 

 separated, but as the ice increased in thickness, the efflorescence 

 also increased in quantity, and, after collecting in tufts, ultimately 

 spread over the whole ; except the temperature were low, these 

 crystals generally presented a moist appearance, but if below — 10° 

 they were always dry and hard. These crystals, from below 

 + 28° '5 to —2o°, presented almost the same form, the changes 

 being very slight and dependent on the amount of aggregation, 

 the original being in the form of a broad feather, with plumes 

 and secondary plumes branching off these ; length varying from 

 ;| to 2 inches, breadth from J to 1 inch. But at and below the 

 temperature of —25°, the crystals presented a very different form, 

 having the appearance of long acicular fibres, varying from ^ to 

 2 inches in length, of no definite structure, but always fibrous, 

 from J^ to ^-^ of an inch in diameter, sprouting up from a basic 

 tuft very like pure crystals of " cajffeine.^'' Whether this change 

 in form in the crystal is dependent on the cold, or greater amount 

 of saline ingredients present, I cannot say. 



When the sun-rays in spring become sufficiently intense, the 

 exposed surfaces of snow, ice, or efflorescence, begin to melt. 

 During the winter, as cracks and lanes of water appear in the 

 floe, these are covered with ice, efflorescence, and some little 

 snow. The melting point of this mixture of efflorescence and 



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