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white precipitate with a soluble salt of silver and possessed, moreover, a decidedly saline taste. On
examining the clear and opaque ice bacteriologically, Abba found that while the germs in the opaque
core of the ice ranged between 100 and 1,000 per c.c. in the large majority of instances, the number
in the clear ice very rarely exceeded 100 per c.c., and was often less than 10 per c.c. In four
instances out of sixty examinations, the specimen from the clear ice was found to be sterile. Abba
concludes, therefore, that the biological self-purification of ice is effected in the same way as its
chemical purification, and that it is necessary in making artificial ice to have regard to the fact that
the central core contains the impurities of the water. Hence, if distilled water be not used, the
liquid remaining within the block of ice when solidification is nearly completed should be removed
and replaced by clean water.1
The Clearness and Sparkle of Crystal Ice.—" The beautiful clearness and sparkle of wellformed
ice," says Dr. Prudden, " tend to confirm the belief that the act of freezing is a very
perfect purifying agency." He goes on to explain, however, that bacteria may exist in water in
enormous numbers without in the slightest degree impairing its clearness and transparency.
Thus he examined water containing one and a half million organisms per " tablespoonful" in a
thin walled glass tube half an inch in diameter, and on passing a beam of light from a lens
through the tube only the faintest opalescence could be detected. " Snow ice " and " bubbly
ice" were found on examination to compare unfavourably with crystal ice, and the results of
precise bacteriological experiment strikingly confirmed the " popular prejudice against bubbly
ice." It must be borne in mind that ice made from the purest water may be quite opaque, for, as
Dr. Prudden says, " ordinary opaque ice . . . with comparatively few bubbles is . . .
as free from bacteria" as the most transparent ice. Further, with regard to the source of the
bacteria in snow and ice, he says " a part of the excess may be due to the washing down upon the
surface of the ice, from the shores, of bacterial impurities, or their collection from the air; a not
insignicant share is probably due, at least in the bubbly streaks, to the tendency which some
forms of bacteria exhibit to collect in the vicinity of air bubbles, and perhaps also to unknown
physical conditions in the irregularly crystallised ice about these bubbles." He points out,
moreover, that freezing must not be regarded as necessarily giving protection against impurity
similar in kind to that afforded by filtration; for, he says, freezing is a selective filtration, germs
may not all be eliminated with equal efficiency, and the dangerous disease-producing species may
possess special capacity for resisting low temperatures.
In a report presented to the State Board of Health of Massachusetts for the year 1899,
distinction is made between " snow ice," "bubbly ice," and " clear ice," and again between " top
ice," and " bottom ice." It is stated that " when ice first forms on the surface of a pond or river,
a considerable part of the impurity in the water near the surface is entangled in the first inch or less
in depth, and that the ice which forms below this first inch contains but a very small percentage of
the impurities of the water." Again : " If snow falls upon the thin ice, causing it to sink, so that
water from below saturates the snow, it will freeze without purification ; or if rain falls upon the
snow and freezes, the ice thus formed contains the impurities of the snow and of the rain water and
whatever else may have settled out of the air. The method often pursued of flooding the ice of a
pond or a river by cutting holes through it gives a layer of ice as impure as the water from which
it is formed."
The earlier notions concerning clear, bubbly, and opaque ice, require to be considerably
modified in the light of knowledge gained concerning artificial ice. As has already been seen, ice
made from distilled water may be quite opaque, owing to the entanglement within its substance of
minute air-bubbles. Hence such " tombstone ice," as it was called in America when it was first
placed on the market, was at a great disadvantage as compared with the relatively transparent ice
from ponds and rivers, although bacteriologically the former is far superior to the latter. Makers
of artificial ice gradually perfected devices for securing clearness as well as purity—notably the
water in the freezing well was kept in a state of agitation by the movement of a plunger, and when
this system was adopted the outer layers of the ice-block, those which froze first, were quite clear,
and it was only the internal core, containing the water which froze last of all, which was opaque.
Professor Abba suggests that this core should be removed, the impure water being replaced by pure
water.2 In American cities, in which ice is largely used, it is a well-recognised fact that the opaque
core has a pronounced salt taste, and servants are instructed to detach this opaque ice, and to
destroy it. If by an oversight a piece of this ice were inadvertently brought to table, it would
be removed, and the glass in which it was found would be carefully cleansed, the fact that this
ice contains the bulk of the foreign matter in the original water being fully appreciated.
The London Ice Trade.
Some 200,000 to 300,000 tons of Norway ice are brought to London each year, and in addition
some 30,000 to 40,000 tons (approximately) of artificial ice are manufactured in London. The
amount of artificial ice made has rapidly increased in the last few years.
The ships which bring ice from Norway are old sailing vessels. I am indebted to
Dr. Williams, medical officer of the Port of London, and to Dr. R. K. Brown, medical officer of
1 This device has also been recommended with a view to avoidance of the " cloud and colour which occur
frequently near the centre of ice-blocks." It has been suggested that this water, " which would otherwise form the
opaque core of a block of ice," should be replaced by clean water and freezing be then allowed to proceed, the water
being agitated during the early stage of coagulation."—Ice and Cold Storage, May, 1904.
2 In " Mechanical Refrigeration " (Hal Williams, Whittaker & Co., 1903) an account is given under the
heading " Defects in Ice due to the Use of Distilled Water," of " red core," which is due to rust, and " white core,"
due to the presence of air and sometimes of carbonate of lime and magnesia. Ice made from London water, soon
after heavy rainfall had caused the river Thames to be in flood has been found to exhibit " red core." (See p. 10).
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