All similar bodies contract equally during the process of cooling, from the highest to the lowest points to which the experiments have been carried. It has been thought that if this applied to water, the result would be the sudden consolidation of the whole mass. A modification of the law has been supposed to take place to suit the peculiar circumstances of water. Nature never modifies a law for a particular purpose; we must, therefore, seek to explain the action of the formation of ice, as we know it, by some more rational view.
Water expands by heat, and contracts by cold; consequently, the coldest portions of this body occupy the lower portions of the fluid; but it must be remembered that these parts are warmed by the earth. Ross, however, states that at the depth of 1,000 fathoms the sea has a constant temperature of 39°. Water is said to be at its point of greatest density at 40° of Fahrenheit’s thermometer; in cooling further, this fluid appears to expand, in the same way as if heated: and, consequently, water colder than this point, instead of being heavier, is lighter, and floats on the surface of the warmer fluid. It does not seem that any modification of the law is required to account for this phenomenon. Water cooled to 40° still retains its peculiar corpuscular arrangement; but immediately it passes below that temperature, it begins to dispose itself in such a manner that visible crystals may form the moment it reaches 32°. Now, if we conceive the particles of water, at 39°, to arrange themselves in the manner necessary for the assumption of the solid form, by the particular grouping of molecules in an angular instead of a spheroidal shape, it will be clear, from what we know of the arrangement of crystals of water—ice—that they must occupy a larger space than when the particles are disposed, side by side, in minute spheres. Even the escape of air from the water in which it is dissolved is sufficient to give an apparent lightness to the colder water. This expansion still goes on increasing, from the same cause, during the formation of ice, so that the specific gravity of a mass of frozen water is less than that of water at any temperature below 40°. It must not be forgotten that ice always contains a large quantity of air, by which it is rendered buoyant.
Water, at rest, may be cooled many degrees below the freezing point without becoming solid. This is easily effected in a thin glass flask; but the moment it is agitated, it becomes a firm mass. Here we have the indication of another cause aiding in producing crystals of ice on the surface of water, under the influence of the disturbance produced by the wind, which does not extend to any depth.
As oxygen and hydrogen gases enter largely into other chemical compounds besides water, it is important to consider some of the forms of matter into the composition of which these elements enter. To examine this thoroughly, a complete essay on chemical philosophy would be necessary; we must, therefore, be content with referring to a few of the more remarkable instances.
The waters of the ocean are salt: this arises from their holding, in solution chloride of sodium (muriate of soda—common culinary salt) and other saline bodies. Water being present, this becomes muriate of soda,—that is, a compound of muriatic acid and soda: muriatic acid is hydrogen, combined with a most remarkable gaseous body, called, from its yellow colour, chlorine; and soda, oxygen in union with the metal sodium,—therefore, when anhydrous, culinary salt is truly a chloride of sodium. Chlorine in some respects resembles oxygen; it attacks metallic bodies with great energy; and, in many cases, produces the most vivid incandescence, during the process of combination. It is a powerful bleaching agent, is destructive to animal life, and rapidly changes all organic tissues. There are two other bodies in many respects so similar to chlorine, although one is at the ordinary temperatures solid, and the other fluid, and which are also discovered in sea-water, or in the plants growing in it, that it is difficult to consider them otherwise than as different forms of the same principle. These are iodine and bromine, and they both unite with hydrogen to form acids. The part which chlorine performs in nature is a great and important one. Combined in muriate of soda, we may trace it in large quantities through the three kingdoms of nature, and the universal employment of salt as a condiment indicates the importance to the animal economy of the elements composing it. Iodine has been traced through the greater number of marine plants, existing, apparently as an essential element of their constitution; in some land plants it has also been found, particularly in the Armeria maritima, when this plant grows near the sea:[216] it has been detected in some mineral springs, and in small quantities in the mineral kingdom[217] combined as iodide of silver, and in the aluminous slate of Latorp in Sweden.[218] Bromine is found in sea-water, although in extremely minute quantities, in a few saline springs, and in combination with silver; but we have no evidence to show that its uses are important in nature.
Hydrogen, again, unites with carbon in various proportions, producing the most dissimilar compounds. The air evolved from stagnant water, and the fire-damp of the coal mine, are both carburetted hydrogen; and the gas which we employ so advantageously for illumination, is the same, holding an additional quantity of carbon in suspension. Naphtha, and a long list of organic bodies, are composed of these two chemical elements.
These combinations lead us, naturally, to the consideration of the great chemical phenomena of combustion, which involve, indeed, the influences of all the physical powers. By the application of heat, we produce an intense action in a body said to be combustible; it burns,—a chemical action of the most energetic character is in progress, the elements which constitute the combustible body are decomposed, they unite with some other elementary principles, and new compounds are formed. A body burns—it is entirely dissipated, or it leaves a very small quantity of ashes behind unconsumed, but nothing is lost. Its volatile parts have entered into new arrangements, the form of the body is changed, but its constituents are still playing an important purpose in creation.
The ancient notion that fire was an empyreal element, and the Stahlian hypothesis of a phlogistic principle on which all the effects of combustion depended,[219] have both given way to the philosophy of the unfortunate Lavoisier—which has, indeed, been modified in our own times—who showed that combustion is but the development of heat and light under the influence of chemical combination.
Combustion was, at one period, thought to be always due to the combination of oxygen with the body burning, but research has shown that vivid combustion may be produced where there is no oxygen. The oxidizable metals burn most energetically in chlorine, and some of them in the vapour of iodine and bromine, and many other unions take place with manifestations of incandescence. Supporters of combustion were, until lately, regarded as bodies distinct from those undergoing combustion. For example, hydrogen was regarded as a combustible body, and oxygen as a supporter of combustion. Such an arrangement is a most illogical one, since we may burn oxygen in an atmosphere of hydrogen, in the same manner as we burn hydrogen in one of oxygen; and so, in all the other cases, the supporter of combustion may be burnt in an atmosphere formed of the, so called, combustible. The ordinary phenomena of combustion are, however, due to the combination of oxygen with the body burning; therefore every instance of oxidization may be regarded as a condition of combustion, the difference being only one of degree.
Common iron, exposed to air and moisture, rusts; it combines with oxygen. Pure iron, in a state of fine division, unites with oxygen so eagerly, that it becomes incandescent, and in both cases oxide of iron is formed. This last instance is certainly a case of combustion; but in what does it differ from the first one, except in the intensity of the action? The cases of spontaneous combustion which are continually occurring are examples of an analogous character to the above. Oxygen is absorbed, it enters more or less quickly, according to atmospheric conditions, into chemical combination; heat is evolved, and eventually,—the action continually increasing,—true combustion takes place. In this way our cotton-ships, storehouses of flax, piles of oiled-cloth, sawdust, &c., frequently ignite; and to such an influence is to be attributed the destruction of two of our ships of war, a few years since, in Devonport naval arsenal.[220]