property of great importance to aquatic animals and plants, which require air for their existence. A like absorption takes place whenever water, which from any cause is imperfectly supplied with air, comes in contact with it. On the other hand, when water freezes, the air thus absorbed is thrown out as it were, and forms distinct bubbles or globules, which, if the liquid remain at rest, often become embedded in the ice, and disturb its clearness and transparency. A gentle agitation of the water, whilst freezing, with a feather, will brush away the globules of air, and cause them to rise and escape as fast as they are formed.

Ice, being so pure and transparent a substance, is used as window glass in the Arctic Regions, and may even be manufactured into a magnifying glass or burning lens. If a block of transparent ice be placed in a hemispherical bowl of tin plate, and this be placed on hot sand, the ice will begin to melt and mould itself to the inner surface of the bowl. When this form has been attained, the ice is to be placed with its flat surface on a flat tin plate, resting on hot sand. The flat face will thus be made quite level, and a plano-convex lens be formed. If such a lens be presented with its flat face to the sun, its rays may be collected into a focus sufficient to ignite a piece of German tinder, and indeed gunpowder has been exploded in this way. As ice can be purchased in England at any season of the year, this experiment can be tried in summer when the sun is stronger. What is very

remarkable in this experiment is, that the sun's rays should pass through ice and lose scarcely any of their heating power; whereas, radiant heat from artificial sources is nearly all absorbed by ice, and is expended in liquefying it.

Ice expands and contracts by cold nearly twice as much as lead does,* and more than twice as much as any other solid.

According to the Rev. Canon Moseley, "If ice were as elastic as slate, and did not resist crushing more than hard brick, a block of it placed with its ends between two immovable obstacles, would crumble when its temperature was raised 1° Fahrenheit. It is its great dilatability which gives to ice this tendency to disintegrate when, not being free to dilate, its temperature is raised even so slightly as this."

Ice, like water, is colourless in small masses, but greenish or bluish in large ones. Professor Tyndall passed a powerful light from the voltaic lamp through a long tube of water closed by glass at both ends, and received the image on a screen. In this experiment, with no greater thickness than twenty feet, the colour of the water was found to be yellowish green.

Ice transmits sound probably with nearly the same facility as water. If a heavy blow be struck against the frozen surface of a large lake, a person at a considerable distance from the spot will feel

* The expansion for lead is one part in 351 in length, and one in 117 in bulk.

under his feet a sensible tremor some considerable time before the noise conveyed through the air has reached his ear. It is said that the tribes who rove on the icy steppes of Tartary can readily distinguish from afar the approach of cavalry, by applying the head close to the frozen surface of the ground.

We have already seen that ice is lighter than water. It is commonly stated that a cubic foot of water at 62° weighs 1000 ounces. A cubic foot of ice is generally held to weigh about 930 ounces; while a cubic foot of sea water weighs 1,028 ounces. This is what is called the specific gravity of these bodies, or the relation of their weight to their bulk; thus, if we call the density or specific gravity of water 1000, the specific gravity of ice will be 0.930, of sea water 1028, while the water of the Dead Sea, which contains a considerable quantity of the chlorides of sodium, magnesium, and calcium, is 1-211, and the specific gravity of brine is 1.220.

The determination of the precise specific gravity of ice is a point of considerable difficulty, on account of its expansible nature, and the difficulty of keeping the water and the apparatus used in determining it at a fixed temperature. Thus the density has been variously given by the following good authorities: Henrich, 0·905; Kopp, 0-909, Berzelius, 0·916; Brunner, 0·918; Plucker and Geissler, 0.920; Osann, 0-927; Thomson, 0.940; and Dumas, 0.950. These differences of density expressed in the different increase of volume

at the moment of freezing, correspond to values between 4th and th.

M. Dufour, to get rid of the difficulty, took the specific gravity by weighing the ice in a mixture of water and alcohol, in which the ice would remain at rest indifferently at any part, and then taking the specific gravity of the mixture. The ice was formed by freezing distilled water, which had previously been boiled for some time. Many specimens gave a density of 0.922; the highest was 0.923, and the lowest, 0.914. The mean of twenty-two experiments was 0.9175. M. Dufour thinks that this may safely be taken as the density of ice at 0° Centigrade, or 32° Fahrenheit. This corresponds to an increase of volume at the moment of congelation, of 18ths, or very nearly

th, so that water not only expands from its point of maximum density to its freezing point, but in the very act of freezing the ice occupies

th more space than water at 32° does. It is this expansion which produces such amazing mechanical effects in the loosening of masses of rock, the bursting of strong iron and other vessels to which we have already alluded.

In the year 1850, Professor Faraday, at one of the Friday evening meetings of the Royal Institution, announced a hitherto unnoticed property in ice, which has been singularly suggestive in the explanation of some great natural phenomena. He stated that if two surfaces of ice be brought together at temperatures above 32°, they will form

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a perfect weld, and become one piece. We saw, on that occasion, Dr. Faraday perform the remarkable experiment of placing two pieces of ice in water so hot that the hand could only just bear it, and on pressing them together they were taken out as one piece. In fact, it is the freezing of the film of water between the two surfaces that produces their union. When ice is below 32°, and consequently dry, no such union takes place. He referred to the experience of every school-boy in making a snowball. When the snow is very cold, and consequently dry, it will not ball together; but when from the warmth of the hands it becomes partially thawed, it can be squeezed into a firm compact mass.

It appears then that ice has a tendency to solidify the film of water in contact with its surface, which tendency is of course the greater when the film is inclosed between two surfaces of ice.

In illustration of the above, Professor Tyndall stated that on one of the warmest days of July, 1856, at a time when the thermometer marked 80° in the shade, and above 100° in the sun, he observed a pile of ice-blocks in the window of the Wenham Lake Company's ice shop in the Strand. He went into the shop, and asked permission to examine whether the pieces were united at their surfaces of contact. Laying hold of the topmost block, the whole heap, consisting of several large lumps, was lifted bodily out of the vessel, and even at this high temperature, the pieces were