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Swimming.

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heavier than fresh water. The difficulty in swimming consists, not so much in floating, as in keeping the head above water, so as to breathe freely. In man the head is heavier than the lower parts, and consequently tends to sink, and hence swimming is not natural to him, but is an art which requires to be learned. With quadrupeds, on the contrary, the head being less heavy than the posterior part of the body, remains above water without any effort, and these animals therefore swim naturally.

If a person who cannot swim, and who falls into the water, retains

Fig. 77.

coolness enough to turn on his back, so that his face is out of water, he can breathe freely, and wait until help arrives. Instead of this, however, he generally attempts to raise his arms out of water, as if grasping at some fixed support. This is very dangerous, for as the arms no longer displace a quantity of liquid equal to their own bulk, their weight is not diminished to that extent, but concurs with that of the head in making them sink.

Weight for weight, fat persons swim more easily than lean ones, for they displace more water. It is for the same reason that air bladders, or cork girdles, are fastened to persons who are learning to swim (fig. 77), for then, without any considerable increase of weight, they displace far more water, which increases the buoyancy and keeps them up.

Several kinds of birds, such as ducks, geese, and swans, swim easily on water. They owe this property to a thick coating of a light impervious down which covers the lower part of the body, so that they displace, even with a small immersion, a weight equal to their own.

100. Specific gravities.-Daily experience shows us that different substances have very unequal weights for one and the same volume. For instance, we all know that gold weighs more than silver, lead than iron, stones than wood. In order to compare equal volumes of various substances as to their weights, the weight of water has been taken as a standard of comparison-as unity. For water is everywhere met with, and can always be had pure; this latter condition is necessary, for the weight of a given quantity of water differs with the substances it holds in solution. As, moreover, the weight varies with the temperature, a constant temperature must be adopted. Hence the unit of weight is distilled water at a temperature of 4 degrees, for at this point, as we shall afterwards see, water has its greatest density.

Fig. 78.

Thus having agreed to represent by 1 the weight of a certain volume of distilled water at 4 degrees, the specific gravity of a body

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Specific Gravity of Solids.

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is the weight of the same volume of it as compared with that of water, or what is the same, the number which expresses how much it weighs as compared with water. Thus, when we say that the specific gravity of gold is 19, and that of lead 11, we mean that the former metal is 19 times, and the latter II times as heavy as water. IOI. Determination of the specific gravity of solids.-Three methods are commonly used in determining the specific gravities of solids and liquids. These are the method of the hydrostatic balance, that of the hydrometer, and that of the specific gravity flask. All three, however, depend on the same principle, that of first ascertaining the weight of a body, and then that of an equal volume of water. We shall first apply these methods to determining the specific gravity of solids, and then to the specific gravity of liquids.

i. Hydrostatic balance. To obtain the specific gravity of a solid, a piece of iron for instance, by the hydrostatic balance (fig. 78), it is first weighed in air by suspending it to the hook of one of the plates. Let us suppose that its weight is 585 grains. It is then weighed while immersed in distilled water, as shown in fig. 78. It will now weigh less; suppose the weight to be 510 grains, this is in accordance with Archimedes' principle, for it now loses a weight equal to that of the water it displaces. Hence, subtracting 510 from 585, the difference 75 represents the weight of the displaced water, that is, the weight of a volume of water equal to that of the iron: we need now only investigate how often the weight 75, that of the water, is contained in 585, that of the iron, and the quotient 7.8 is the specific gravity of iron; it says that, for equal volumes, this substance weighs 7.8 times as much as water.

Nicholson's hydrometer. This apparatus consists of a hollow metallic cylinder (fig. 79), to which is fixed a cone, d, loaded with lead. The object of the latter is to depress the centre of gravity so that the cylinder does not upset when in the water. At the top is a stem, c, terminated by a pan, a, in which is placed the substance whose specific gravity is to be determined. On the stem a standard point, c, is marked.

The apparatus stands partly out of the water, and the first step is to ascertain the weight which must be placed in the pan in order to make the hydrometer sink to the standard point c (fig. 80). Let this weight be 125 grains, and let sulphur be the substance whose specific gravity is to be determined. The weights are then removed from the pan, and replaced by a piece of sulphur which weighs less

than 125 grains, and weights added until the hydrometer is again depressed to the standard, c. If, for instance, it has been necessary

to add 55 grains, the weight of the sulphur is evidently the difference between 125 and 55 grains, that is, 70 grains.

Having thus determined the weight of the sulphur in air, it is now only necessary to ascertain the weight of an equal volume in water. To do this, the piece of sulphur is placed in the lower pan at d, as represented in fig. 81. The whole weight is not changed, nevertheless the hydrometer no longer sinks to the standard; the sulphur, by immersion, has lost a part of its weight equal to that of the water displaced. Weights are added to the upper pan until the hydrometer sinks again to the standard. This weight, 34'4 grains for example, represents the weight of the volume of water displaced; that is, of the volume of water equal to the volume of the sulphur. It is only necessary, therefore, to divide 70 grains, the weight in air, by 34'4 grains, and the quotient 2.03 is the specific gravity.

Specific gravity flask. In this method, which is advantageously used for the determination of the specific gravity of bodies in a state of powder, a wide-necked flask is used which can be care. fully closed by a ground glass disc (fig. 82). Having filled it with water it is closed with the disc, great care being taken that not a bubble of air is left. After being carefully wiped dry, it is placed in the pan of a balance, and by its side is the substance, a, whose

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specific gravity is to be determined. The whole is then equipoised by placing weights in the other pan of the balance. The substance, a, is then removed, and weights

added in its place, until equilibrium is again established. The weight necessary for this purpose gives the weight of the substance in air.

To obtain its weight in water it is placed in the flask, the disc adjusted, and the whole again carefully wiped. In order now to equipoise the tare in the second pan, weights must be added on the side of the flask to make up for the water displaced. The weights necessary for this purpose represent then the weight of a volume of water equal to that of the body.

Fig. 82.

Dividing, then, the weight of the body in air by the weight of an equal volume of water, we have the specific gravity sought.

102. Specific gravity of liquids. These are determined by the same methods as those of solids.

Hydrostatic balance. In determining the specific gravity of a liquid by this means, a body is suspended to one of the pans of the balance, which is not dissolved by the liquid whose specific gravity is to be determined, nor in water; for instance, a ball of platinum, which is insoluble in all ordinary liquids. This ball is first weighed in air, then in water, and finally in the liquid in question, which we will suppose is alcohol. Let us assume that in air the ball weighs 510 grains, in water 486 grains, and in alcohol 489 grains. The loss of weight in water has thus been 510 less 486, or 24 grains, and in alcohol 510 less 499, or 21 grains; which tells us that if a volume of water equal to that of the ball weighs 24 grains, the same volume of alcohol weighs 21 grains. Hence, to obtain the specific weight of alcohol we must ascertain how many times the number 21 contains 24, which of course is obtained by division. The quotient thus obtained is 0.866, which represents the specific gravity of alcohol as compared with water.

ii. Fahrenheit's hydrometer. This instrument (fig. 83) resembles Nicholson's hydrometer, but is made of glass, so as to be used in all liquids. At its lower extremity, instead of a pan, it is loaded with a small bulb containing mercury. There is a standard mark on the stem, at the top of which is a pan.