804 
MR J. M. WORDIE ON THE NATURAL HISTORY OF PACK-ICE 
David and Priestley saj the cause is similar to what occurs in lake-ice, where 
during pauses in crystallisation large quantities of gas rise from decaying organic 
matter on the lake-bottom to the under surface of the ice. Some further 
explanation, however, appears, necessary. 
Plasticity . — The most striking physical character of young-ice is its plasticity. 
Pancake-ice, however, and ice formed from slush were never so plastic as black-ice, 
and this presumably was the result of their not being so salt. Pettersson contends 
that plasticity is a peculiarity connected with the expanding properties of sea-ice 
down to about - 20° C. All varieties of ice are more or less plastic at their 
melting-point, but in the case of sea-ice, he says, the wide range of the eutectic 
points for the different salts in it virtually means that it has already partially 
begun to melt at —20° C., and is therefore plastic from there up to the melting- 
point of the ice proper (— 1'8° C.). 
Chlorine Percentage . — The saltness of young-ice must depend partly on the rate 
at which it freezes ; and possibly at this stage, when it is only 2 to 3 inches thick, 
the chlorine percentage may be an index to the total amount of included salt. The 
methods on the Endurance , however, allowed of the amount of chlorine only 
being determined. In young-ice about 2| inches thick, it was on different occasions 
found to be present in the proportion of 7'23, 7'6 9, and 8'5 grms. per thousand. 
On the one occasion on which sea-water, over which ice was forming, was tested, 
the former contained 19 '46 per thousand Cl, as against presumably 7 to 8 per 
thousand in the young-ice. # 
III. — The Ice in Motion. 
Along the Antarctic coasts the rule seems to be for the sea-ice to be at all times 
liable to break up and drift away to the north and west. To this, however, there 
are two exceptions : firstly, where the configuration of the land is such as to shelter 
the ice from the prevailing winds, thus preventing disruption in winter, and 
sometimes even in summer ; and secondly, where there are obstructions of the 
nature, for instance, of stranded bergs, which hold up the ice both in summer and 
in winter. The Weddell Sea fully bore out the general rule. At two places only 
is the ice at present known to be stationary ; one of these is off Leopold Coast, 
where a chain of stranded bergs a little distance out from the land effectively 
anchors a long strip of ice and prevents it being continually subject to the restless 
motion of the pack ; the other is on the coast of Graham Land between Eoss Island 
and Joinville Peninsula, where, to judge by Eoss’s narrative and Nordenskjold’s 
descriptions sixty years later, the ice very seldom breaks out. The former appears 
* These figures were determined by Mr James, physicist to the Expedition, using what under the circumstances 
is the most convenient method, namely titration with silver nitrate. Unfortunately, no experiments were made to 
determine the specific gravity of ice as young as the above. The figures for specific gravity discussed later on were 
all got from ice over 2 feet thick, where the chlorine content only averaged about 2 to 3 per thousand, and was 
sometimes even less. 
