Examples of Liquid Buoyancy.

195

element as to be able to compete with its most active inhabitants. A horse while swimming can carry his rider with half the body out of the water. Dogs commonly swim well on the first trial. Swans, geese, and water-fowl in general, owing to the thickness of their feather-coating, kept dry by the oil spread upon them, as well as the great volume of their lungs, and the hollowness of their bones, are so light for their bulk that they float like stately ships, oaring themselves about by their webbed feet.

Fishes can increase or diminish the amount of water they displace, by expanding or contracting a little air-bag or bladder contained in their body, and in this way can rise to the surface of the water, or sink, at will. It is because this bag is situated towards the under side of their body, that a dead fish floats with that side uppermost.

Animal substances, when undergoing putrefaction, generate or give out a gaseous matter. Hence the bodies of drowned persons remaining in the water, generally acquire buoyancy after a few days, and rise to the surface, but they sink again when the still increasing quantity of gas bursts the skin and escapes..

328. The buoyancy of a liquid increases with its weight or specific gravity.

A ship draws less water, or swims lighter, by about a thirty-fifth part, in the heavy salt water of the sea than in the fresh water of a river; and for the same reason, a man swimming supports himself more easily in the sea than in a river.

Some men can swim easily in salt water who do so only with difficulty in fresh, and who would be altogether unable to support themselves if they suddenly fell into a vat of spirits, or wine, or oil. Some kinds of wood that float in sea-water will sink in river-water, or which float in water will sink in oil.

water.

A man would float on mercury as the lightest cork floats on One might even walk or hobble along the surface of mercury, sinking little more than ankle deep; but it would be very difficult to keep one's balance. Iron and all common metals float on mercury. Gold and platinum sink in it.

Had sea-water been a little heavier than it is, men after shipwreck would have had to think of famine and cold as much as of drowning. The water of the Dead Sea, in Palestine, has a specific gravity of 1160. A man readily floats upon it. All parts of the dead body float upon it excepting bone.

195

Mixture of Liquids of Different Densities.

329. One liquid floats upon or is buoyed up by another heavier than itself, because a pint of the lighter liquid displacing a pint of the heavier suffers an upward pressure greater than its own weight.

Thus oil floats on water, but sinks in alcohol or ether. Cream, consisting chiefly of oil or butter, rises to the surface of milk. Mercury, water, oil, and air all shaken together in a vessel, will, when left at rest, arrange themselves in the order of their weights or densities. Any liquids will do the same, if there is no adhesive attraction between them to make them form a compound liquid ; and even in this case, by carefully pouring the lighter on the heavier, as wine upon water, the lighter will float for a considerable time without mixing. The following liquids may be thus placed upon each other, in the order of their respective densities: - Mercury, chloroform, water, olive oil, alcohol, and naphtha. On the surface of the last we may place the lightest metal known-iithium, which floats as readily on naphtha as iron or brass does upon mercury. Wine that is rich in alcohol, if carefully poured on water, will float

a

upon it. In a vessel shaped like a common hourglass, as in fig. 86, only with a larger opening at c, between the two chambers, if wine be put into the under chamber, and water into the upper, the two liquids will gradually, to a considerable extent, change places. The liquids are less mixed, and change places sooner, when there is a tube, b, to carry the water down to the bottom without touching the wine, and a tube, a, to carry the wine directly to the top, without touching the water.

Fig. 86. 330. When, in a mass of water, part of it is heated more than the rest, that part, by its expansion, becomes specifically lighter than the rest, and takes its place on the surface. Hence, when heat is applied to the bottom of a vessel containing water, as to a boiler placed on a fire, a circulation is established, which goes on from the first moment until the communication of heat ceases: water is always rising from the hotter parts of the vessel, and descending from the upper and colder parts.

So when a tall glass containing hot water is dipped into cold water, a downward current begins within the tall glass near the sides all round, and an upward current in the middle. The motion is readily shown by putting small bits of amber or thin paper into the water, for these being nearly of the specific gravity of water, rise and descend with it. On account of these currents, heat applied to the bottom

The "Heat-transferrer."

197

of a vessel of liquid is soon equally diffused through the whole; but if it is applied at the top the heated and lighter fluid cannot descend, and the heat passes but very slowly down through the liquid.

331. The currents in a fluid, produced by local differences of temperature, are important parts of various processes which the author suggested in the first edition of this work by the following paragraph :—

"Heat may be transferred from one liquid to another, without mixing them, by making the hot liquid descend in a very thin metallic tube, through the cold liquid rising around it in a larger tube. Boiling water from the vessel e, for instance, may descend slowly by the small tube e a bf, which is surrounded from a to b by cold water ascending through the tube c g. Then, as the temperature of two liquids brought so nearly into contact with each other as in these tubes will not, after a very short time, differ in any one place more than a few degrees, it follows that the water lately cold will, on leaving the part of the tube g, which is in contact with the boil

Fig. 87.

a

ing water descending directly from e, be nearly boiling, while the water lately hot will, on leaving the tube b, which is in contact with cold water just arrived from h, be itself nearly cold; and thus equal quantities of hot and cold water will not have become a double quantity of a medium temperature, but will have made nearly a total exchange of temperatures. The flow of the hot water is to be regulated by a cock b, and that of the cold water by a cock h. The water in the part of the tube cg d rises, because it is hotter and therefore lighter specifically than that in the part h c. The author believes that an apparatus made on this principle, with an arrangement of many thin flat tubes instead of a single large tube, for the

198

Expansion of Water before Freezing.

descending fluid, and a spacious case, g, to contain these and the rising fluid, would be an excellent refrigerator in a distilling apparatus, and for cooling the wort of brewers; or would serve as a means of diminishing the expense of warm baths, by transferring the heat from the water lately used to pure water. In distilling, the wash or low wines about to enter the still might be used as the cold condensing fluid to surround the worm or vapour tubes, and thus, without expense, would be heated in its progress to the still. Half the original expense of a great porter brewery is in the construction of the numerous water-tight floors on which the hot wort is thinly spread to cool."

Various practical applications of the principle explained in the preceding paragraph, which have been usefully made since the first publication of this work, are described in the chapter on Heat.

332. As a general rule, substances contract and become denser as they cool. Water, however, is a curious exception to this rule, for it contracts only down to the temperature of 40° Fahrenheit, below which, towards 32°, the freezing-point, it goes on dilating again, and in the form of ice is about one-fifteenth more bulky than water. Ice therefore floats on the surface of water, and being a very slow conductor of heat, defends the water underneath from the cold air, and preserves it liquid, and a fit dwelling for the finny tribes until the return of the mild season; just as very hot water in summer remains uppermost, preserving underneath an agreeable coolness. Thus nature has secured a winter garb or protection for the inhabitants of lakes and rivers, as effectual as it has for terrestrial animals, by the periodical thickening of their wool or fur. Had ice been denser than water, it must have fallen to the bottom, and have left the surface without protection; a deep lake, even in mild European winters, might have been frozen into a solid lifeless mass, which summer suns would no more have melted than they now do the glaciers of Switzerland.

"Application of Archimedes' principle to finding SPECIFIC GRAVITIES."

333. The specific gravity of a body (that is, its gravity or weight compared with that of an equal bulk of water), may be found by comparing its weight with the amount by which it is buoyed up when immersed in water, since this buoyancy is the weight of its bulk of water.

Different methods of finding the amount of this buoyancy must

Method of finding Specific Gravities.

199

be employed according as the substance whose specific gravity we wish to ascertain is in the form of a solid, of a powder, or of a liquid.

334. Specific gravity of solids

Fig. 88 reThe mass of gold,

Suppose that we require to find the specific gravity of a solid body heavier than water, such as a mass of gold. presents the method employed in such a case. <, is suspended by a thread or hair from the bottom of one scale of a weighing beam, and is balanced by weights put into the other scale a. If then a vessel of water be lifted up under the solid, so that the water shall surround it, the body is buoyed up by the water with force equal to the weight of the water displaced. Weights therefore are required in the scale b to overcome the buoyancy or restore the balance, and these show the weight of the water displaced, or of water equal in bulk to the body. Thus the weights in the two opposite scales show the comparative weights of the body and of its bulk of water.

Fig. 88.

We should find in the case of gold, that the weight in the scale ¿ would be about 1th part of that in a ; that is to say, gold loses th of its weight when immersed in water, and is therefore 19 times as heavy as water, or has a specific gravity of 19.

The following is another but not very accurate method of obtaining a similar result. Take a vessel quite full of water, and lower into it by a thread any convenient weight of the solid whose density is required. Collect and weigh carefully the water which overflows. The specific gravity is then represented by the relative weights of the solid and of liquid so displaced.

The two facts on which the rules for determining the specific gravity of solids depend (Arts. 317, 318), admit of easy demonstration. 1. A body immersed in water displaces as much water as is equal to its bulk or volume. Thus a cubic inch of brass immersed