© 52 THE OCEAN WORLD. 
prevent snow from melting as it falls. On the approach of congela- 
tion the surface solidifies, and seems as if covered with oil; small 
circles are formed, which press against each other, and are finally 
soldered together until they form a vast field of ice, the thickness of 
which increases from the lower surface. 
The water produced from melted ice is perfectly fresh—the result 
of a well-known physical cause. When a saline solution like sea- 
water is congealed by cold, pure water alone passes into the solid 
state, the saline solution becomes more coneentrated, increases in 
density, and sinks to the bottom. Blocks of ice, therefore, in the 
Polar Seas, are always available for use. There are, however, salt 
blocks of ice, which are distinguished from fresh-water ice by their 
opaqueness and their dazzling white colour—their saltness is due to 
the sea-water retained in their interstices. 
The ice-fields, which are formed in high latitudes, are driven 
towards the south both by winds and currents; but sooner or later 
the action of the waves breaks them up into fragments. The edges 
of the broken icebergs are thus often rising and continually changing. 
These asperities and protuberances are called Aummocks by English 
navigators ; they give to the polar ice an odd, irregular appearance. 
Hummocks form themselves of the stray, broken icebergs which come 
in contact with each other at their edges, and thus form vast rafts, 
the pieces of which may exceed roo yards in length. 
When these icebergs are separated by open spaces, through which 
vessels can be navigated, the pack ice is said to be open; but it often 
happens that mountains of ice occur partly submerged, where one 
edge is retained under the principal mass, while the other is above the 
water. Scoresby once passed over a ca/f, as English mariners call 
these icy mountains ; but he trembled while he did so, dreading lest 
it should throw his vessel, himself, and crew into the air before he 
could pass it. The aspect of the ice-fields varies in a thousand ways. 
Here one forms an incoherent chaos resembling some volcanic dis- 
trict, with crevices in all directions, bristling with unshapely blocks 
piled up at random; there it is a strongly-marked plain, an immense 
mosaic formed of vast blocks of ice of every age and thickness, the 
divisions of which are marked by long ridges of the most irregular 
forms ; sometimes resembling walls composed of great rectangular 
blocks, sometimes resembling chains of hills, with great rounded 
summits. 
In the spring, when a thaw sets in, and the fields begin to break 
up, the pieces of light ice which unite the great blocks into single 
amsses are the first to melt ; the several blocks then separate, and 
