652 PHYSICAL PROPERTIES OF ICE. 



that its bottom just touched the surface of the water ; and a 

 freezing-mixture Avas placed in the tin vessel. When about yig- 

 of the whole had solidified, the solid was removed and divided into 

 two parts : one was allowed to melt, and its per-centage of solid 

 matter was determined as above ; the other was broken up and 

 frequently pressed between linen and flannel in a screw press, 

 being allowed to melt as little as possible. The per-centage of 

 solid matter in this also was determined. The following numbers 

 show the result of this examination : — 



Ter cent, at 100° of 

 solid residue. 

 Sea-water - - - - 6*5786 



Frozen sea-water - - . 5 • 4209 



Frozen and pressed sea- water - - • 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 gravita- 

 tion, 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. 



7. Observations on Sea-water Ice. By J. Y. Buchanan. 



These observations were made on board II. M.S. Challenger in 

 the broken pack ice in the Antarctic regions. An analysis of the 

 melted ice showed the presence of lime, magnesia, and sulphuric 

 acid, and on the average • 1723 gramme of chlorine per litre. 

 Another piece, when pounded and melted in a beaker and ex- 

 amined, gave • 0520 gramme of chlorine per litre, showing that 

 the lump of ice was not homogeneous, as might be expected from 

 the different ways in which it may be formed in the Polar regions. 



The melting point of tlie pack ice was determined. The fresh 

 ice began to melt at —1° C. ; after twenty minutes the thermo- 

 meter had risen to —0*9 C, then to —0* 4 C, where it remained 

 constant for an hour, and then to — 0*3 C. 



" These determinations show that the salt in sea-water ice is not 

 " contained in it only in the form of mechanically enclosed brine, 

 " but exists in the solid form either as a single crystalline sub- 

 " stance, or as a mixture of ice and salt crystals." The melting 

 point of ice crystals, formed by freezing salt water in a bucket, 

 was found to be — 1 ' 3° C. (Royal Societv Proceedings, vol. xxii., 

 p. ^^31.) ^ 



8. The Temperature of Ice and Snow. (Sir John Ross.) 



The thickness of ice was measured regularly every month, and 

 increased until the end of May, Avheu it was 10 feet in the sea, and 



