« ZurückWeiter »
nineteen ounces out of water, and lost one ounce by being weighed in water, the cubic inch of water it displaces must weigh that one ounce: consequently gold would be nineteen times as heavy as water.
The specific gravity of a body lighter than water cannot be ascertained in the same manner. If a body were absolutely light, it would float on the surface without displacing a drop of water ; but bodies have all some weight, and will, therefore, displace some quantity of water. A body lighter than water will not sink to a level with the surface of the water, and, therefore, will not displace so much water as is equal to its bulk, but a quantity equal to its weight. A ship sinks to some depth in the water, and the heavier it is laden the deeper it sinks, the quantity of water which it displaces being always equal to its weight. This quantity cannot, however, afford a convenient test of its specific gravity, from the difficulty of collecting the whole quantity of water displaced, and of measuring the exact bulk of the body immersed. In order practically to obtain the specific gravity of a body which is lighter than water, a heavy one, whose specific gravity is known, must be attached to it, and they must be iinmersed together : the specific gravity of the lighter body may then be easily calculated. Bodies which have exactly the same specific gravity as water, will remain at rest, in whatever situation they are placed in water. If a piece of wood, by being impregnated with a little sand, be rendered precisely of the weight of an equal bulk of water, it will remain stationary in whatever part of a vessel of water it be placed. If a few drops of water be poured into the vessel (so gently as not to increase their momentum by giving them velocity), they would mix with the water at the surface, and not sink lower.
The specific gravity of fluids is found by means of an instrument called an hydrometer, represented by the diagram in the following page. It consists of a thin glass ball, A, with a graduated tube, B; and the
specific gravity of the liquid is estimated by the depth
to which the instrument sinks in it; for the less the specific gravity of the
fluid, the further will the instrument B
sink in it. There is a smaller ball, c, attached to the instrument below, which contains a little mercury; but
this is merely for the purpose of equiA
poising the instrument, that it may remain upright in the liquid under trial.
The weight of a substance, when not compared to that of any other, is
perfectly arbitrary; and when water is adopted as a standard, we may denominate its weight by any number we please; but then the weight of all bodies tried by this standard must be signified by proportional numbers.
If we call the weight of water, for example, 1, then that of gold would be 19.; or, if we call the weight of water 1000, that of gold would be 19000. In short, the specific gravity indicates how much more or less a body weighs than an equal bulk of water.
ON THE MECHANICAL PROPERTIES OF FLUIDS.
The science of the mechanical properties of fluids is called Hydrostatics. A fluid is a substance which yields to the slightest pressure. Fluids are divided into two classes, distinguished by the names of liquids and elastic fluids, or gases; which latter comprehends the air of the atmosphere, and all the various kinds of air with which chemistry makes us acquainted. We shall confine our attention, at present, to the mechanical properties of liquids, or non-elastic fluids.
Water, and liquids in general, are little susceptible of
being compressed, or squeezed into a smaller space than that which they naturally occupy. This is supposed to be owing to the extreme minuteness of their particles, which, rather than submit to compression, force their way through the pores of the substance which confines them, as was shown by a celebrated experiment, made at Florence, many years ago. A hollow globe of gold was filled with water, and on its being submitted to great pressure, the water was seen to exude through the pores of the gold, which it covered with a fine dew. But more recent experiments, in which water has been confined in strong iron tubes, proves that it is susceptible of compression.
Liquids are porous, like solid bodies, but the pores are too minute to be discovered by the most powerful microscope. The existence of pores in liquids can be ascertained by dissolving solid bodies in them. melt some salt in a glass full of water, the water will not overflow, and the reason probably is, that the particles of salt will lodge themselves in the pores of the liquid, so that the salt and water together will not occupy more space than the water did alone. attempt to melt more salt than can find room within these pores, the remainder will subside at the bottom, and occupying a space which the water filled before, oblige the latter to overflow. A certain proportion of spirit of wine may also be poured into water without adding to the bulk, as the spirit will introduce itself into the pores of the water. Fluids show the effect of gravitation in a more perfect manner than solid bodies ; the strong cohesive attraction of the particles of the latter, in some measure counteracting the effect of gravity. In a table, for instance, the strong cohesion of the particles of wood enables four slender legs to support a considerable weight. Were the cohesion so far destroyed as to convert the wood into a fluid, no support could be afforded by the legs; for the particles no longer cohering together, each would press separately and
independently, and would be brought to a level with the surface of the earth.
This deficiency of cohesion is the reason why fluids can never be formed into figures, or maintained in heaps ; for though it is true the wind raises water into waves, they are immediately afterwards destroyed by gravity. Thus liquids always find their level. The definition of the equilibrium of a fluid is, that every part of the surface is equally distant from the point to which gravity tends; that is to say, from the centre of the earth. Hence the surface of all fluids must partake of the spherical form of the globe, and be bulging. This is evident in large bodies of water, such as the ocean; but the sphericity of small bodies of water is so trifling as to render their surface apparently flat.
EQUILIBRIUM OF FLUIDS. The equilibrium of fluids is the natural result of their particles gravitating independently of each other; for, when any particle of a fluid accidentally finds itself elevated above the rest, it is attracted down to the level of the surface of the fluid ; and the readiness with which fluids yield to the slightest pressure, will enable the particle, by its weight, to penetrate the surface of the fluid, and mix with it. But this is the case only with fluids of equal density; for a light fluid will float on the surface of a heavy one, as oil on water; and air will rise to the surface of any liquid whatever, being forced up by the superior gravity of the liquids.
The figure here represents an instrument called a water level, which is constructed upon the principle of
the equilibrium of fluids. It consists of a short tube, A B, closed at both ends, and containing water and a bubble of air : when the tube is not perfectly horizontal, the water runs to the lower end, which makes the bubble of air rise to the upper end, and it remains in the centre only when the tube does not incline on either side. It is by this means that the level of any situation, to which we apply the instrument, is ascertained.
Solid bodies, therefore, gravitate in masses, the strong cohesion of their particles making them weigh altogether; while every particle of a fluid may be considered as a separate mass, gravitating independently. Hence the resistance of a fluid is considerably less than that of a solid body. The particles of fluids acting thus independently, press against each other in every direction, not only downwards but upwards, and laterally or sideways; and, in consequence of this equality of pressure, every particle remains at rest in the fluid. If
you agitate the fluid, you disturb this equality; and the fluid will not rest till its equilibrium be restored.
Were there no lateral pressure, water would not flow from an opening on the side of a vessel ; sand will not run out of such an opening, because there is scarcely any lateral pressure among the particles.
Were the particles of fluids arranged in regular columns, there would be no lateral pressure; for when one particle is perpendicularly above the other, it can only press it downwards; but as it must continually happen that a particle passes between two particles beneath, these last suffer a lateral pressure; just as a wedge, driven into a piece of wood, separates the parts laterally. The lateral pressure is the result, therefore, of the pressure downwards, or the weight of the liquid above; and, consequently, the lower the orifice is made in the vessel, the greater will be the velocity of the water rushing out of