Professor Faraday attributes the purity of Wenham-Lake Ice to its being
free from air-bubbles and from salts. The presence of the first makes
it extremely difficult to succeed in making a lens of English ice which
will concentrate the solar rays, and readily fire gunpowder; whereas
nothing is easier than to perform this singular feat of igniting
a combustible body by aid of a frozen mass if Wenham-Lake ice be
employed. The absence of salts conduces greatly to the permanence of
the ice; for where water is so frozen that the salts expelled are still
contained in air-cavities and cracks, or form thin films between the
layers of ice, these entangled salts cause the ice to melt at a lower
temperature than 32°, and the liquefied portions give rise to streams
and currents within the body of the ice which rapidly carry heat to the
interior. The mass then goes on thawing within as well as without, and
at temperatures below 32°; whereas pure, compact, Wenham-Lake ice can
only thaw at 32°, and only on the outside of the mass.--_Sir Charles
Lyell’s Second Visit to the United States._