CHAPTER VIII.

SPECIFIC GRAVITY.

134. =Nature of the Subject.=--We now come to a very interesting subject, which is at least intimately connected with Hydrostatics, if it may not be considered a part of it. The principles which have been developed in the chapter on Hydrostatics in relation to liquids are to be applied here to various kinds of substances. And as we proceed you will see that all the phenomena brought to view in this chapter are to be referred to the same cause with those of the previous chapter; viz., the attraction of gravitation.

135. =Specific Gravity Defined.=--Before proceeding with the investigation I will give you the definition of specific gravity. The specific gravity of any substance is its weight as compared with the same bulk or volume of other substances. Water is taken as a standard, and its specific gravity is for convenience called 1. Mercury, then, is said to have a specific gravity of 13.5, for it is thirteen and a half times as heavy as the same volume of water. It is easy to see how the specific gravities of different fluids may be ascertained. One mode, and the most obvious one, is to weigh in a vessel equal quantities of them. In what way the specific gravities of solids are ascertained will be explained in another part of this chapter.

136. =Action of Gravity on Solids in a Liquid.=--The reason that a very heavy substance, as a stone, sinks in water is simply that the earth attracts it more strongly than it does the water, and so drags the stone down through it. If the stone lay upon a bladder filled with water, it would press upon it with the force with which it is attracted by the earth. But where water is not thus confined, the stone thrusts its particles to the one side and the other till it gets to the bottom.

It is the attraction of gravity, also, that makes light substances, as wood and cork, rise in water. In this case the water is attracted by the earth more strongly than the wood or cork, and so gets below it, and in so doing pushes the lighter substance up above itself.

But you will observe that the wood, on rising in the water, does not come completely out of it and lie upon the surface, but a part of it remains immersed in the water. The explanation of this will furnish you with the key to the understanding of many very interesting facts. Suppose that half of a block of wood, A, Fig. 87, weighing a pound, is above the surface of water. As it is attracted to the earth with the force of a pound, it has pushed to the one side and the other just a pound of water, and taken its place. It is drawn down toward the earth with the same force with the pound of water on either side of it, _b_ or _c_. If it were attracted any more than with the force of a pound, that is, if it weighed more than a pound, it would displace more than a pound of water. If it were of just the same weight with the same volume of water, it would displace a volume of water equal to itself; it would be wholly immersed, and would stay any where in the water, wherever you placed it, because it is attracted by the earth with the same force that the same bulk of water is.

137. =Farther Explanation.=--Suppose water in a vessel divided into equal portions of a pound each, as represented in Fig. 88. Now suppose that the portion _a_ should at once change into solid ice without at all altering its bulk or weight. It would not move from its position, because it is attracted by the earth precisely as much as when it was water, and as much as is each of the equal portions of water around it. But as water on becoming ice does really increase in bulk, and therefore become lighter, this block of ice would rise so that a part of it would be above the surface.

The lighter a substance is that is immersed in water, the more there will be of it above the surface. Take two blocks of wood of different weights though of the same size. Suppose the heaviest one, A, Fig. 89, is one third lighter than the same bulk of water. One third of it will be above the surface. If the other, B, is half the weight of water, half of it will be above the surface. We should say, then, that the specific gravity of the wood in the first block is two thirds of that of water, and the specific gravity of the wood in the second is one half that of water.

138. =Illustrations.=--There are many interesting facts that illustrate the principles which I have developed. A stone is lifted much more easily in water than in air, because of the support afforded by the upward pressure of the water. A boy will often wonder why he can lift a very heavy stone to the surface, but can get it no farther. When a bucket of water is drawn up a well much less exertion is required to raise it through the water than through the air after it emerges from the water. While it is in the water you raise only the bucket itself, the water in it having no weight, being sustained by the water around it. But when it comes to the air you have the weight of the water added to that of the bucket. When a person lies in a bath for some time, on raising his arm from the water it seems to be very heavy. The reason is, that it has had for so long a time the support of the water that when it is lifted into the air the want of this support is sensibly felt, just as we perceive the difference between raising a bucket of water through water and raising it through air. It is said that Archimedes took in the full idea of the principles of specific gravity as his limbs felt the liquid support of a bath, and so overjoyed was he with the discovery, that he ran home crying out all the way, "Εὕρηκα! εὕρηκα!"--I have found it! I have found it! It was a rational joy, for he had found a principle of immense value to science and to the world.

139. =Boats and Life-Boats.=--A boat of iron will float with as much of it out of water as one of wood of the same size, provided that the iron be made so thin that the boat is not heavier than the wooden one. For what is it that floats? Not the iron or wood, but a wooden or iron boat filled with air. If it were filled with water instead of air it would sink, the specific gravity of the materials of which it is built being on the whole of greater specific gravity than water. Life-boats have in their structure either a large quantity of cork or air-tight vessels of tin or copper, and in this way they are made so light that they will float even when filled with water.

As the weight of a body can be estimated from the quantity of water which it displaces, we can very readily estimate the weight of the load of a canal-boat, as its form is so simple and regular. In order to do this we must first know how far the boat sinks in the water when empty, or, in other words, how much water it displaces.

140. =Specific Gravity of Animals.=--Birds have a much less specific gravity than animals that walk, in order that they may mount up easily in the air. Their light feathers increase greatly their bulk, as you may see whenever a bird is stripped of them. Besides this, the bones are hollow and communicate with the lungs. Birds that swim, as ducks, swans, etc., have so small a specific gravity--that is, are so large in proportion to their weight--that but a small part of the body is under water, and the motion of their feet is not required at all to sustain them, but only, like the action of oars, to propel them along. Insects are of small specific gravity, those that fly the most swiftly being the lightest. Fishes are very nearly of the same specific gravity with water, and hence require but little muscular effort to move about in their element. They are assisted much in rising and falling by a contrivance by which they can instantaneously alter their specific gravity. They have an air-bladder, which they can dilate or contract at pleasure. When dilated, the bulk of the fish is increased and his specific gravity lessened, and he rises easily and at once. By compressing it he as readily sinks.

141. =Specific Gravity of the Human Body.=--The human body, when the chest is filled with air, is so much lighter than water that it will float with about half the head above the surface. A knowledge of this fact, with proper presence of mind, might ordinarily save persons from drowning; for if the body be put in the proper position, the feet downward and the head thrown backward, the nose and mouth will be out of the water. So little is required in the way of support to keep the whole head out of water, that persons who can not swim are often saved from drowning by catching hold of very small pieces of wood. An oar would support half a dozen men, if they would be satisfied with keeping only the head out of water; but if each one struggle to get his whole body upon the oar, they may all be lost.[1] A life-preserver is a great aid in preservation from drowning, for it diminishes the specific gravity of the body. It is commonly an air-tight bag fastened round the upper part of the body, which can be filled by blowing into it through a pipe which has a valve in it. "On the great rivers of China," says Dr. Arnot, "where thousands of people find it more convenient to live in covered boats upon the water than in houses on the shore, the younger male children have a hollow ball of some light material attached constantly to their necks, so that in their frequent falls overboard they are not in danger."

When a person is drowned the body sinks because in the struggle much of the air in the lungs is lost, just as the fish sinks when his air-bladder is contracted. It is, however, so little heavier than water after this is done, that it very readily rises when any gas is produced in it by putrefaction. It is a common popular notion that firing cannon over the water will raise the drowned. But it can produce no effect, unless perhaps the agitation caused by the concussion may hasten a very little the rising of a body which from commencing putrefaction is about to rise.

In wading a river the feet press upon the bottom with only a force equal to the weight of half the person's head, this being the difference between the weight of the body and the weight of the same bulk of water. Now this pressure is not sufficient to give a sure footing against even a moderate current. Many persons have been drowned from ignorance of this fact. A man carrying a load may often ford a river safely where without a load to press him down, and thus give him a sure footing, he would be carried down the stream. So a man may walk in deep water upon broken glass with impunity.

142. =How to Ascertain the Specific Gravity of Solids.=--It results from the upward pressure of water that a body weighs less in water than in air. Take a piece of gold or any other substance, _a_, Fig. 90 (p. 107), and weigh it suspended as you see from one of the scales. Introduce the gold now into a cup of water, and you will find that a part of the weight must be taken from the opposite scale to preserve the balance. The weight which you take from the scale will be the weight of a quantity of water equal in bulk to the piece of gold; for the immersed body is supported with a force equal to the weight of the water it displaces (§ 137). By comparing, therefore, its weight in water with its weight in air we determine its specific gravity. Thus if a lump of gold weigh nineteen ounces, and on being weighed in water weighs eighteen, it will prove that gold is nineteen times as heavy as water. And if a lump of copper weigh nine ounces in air and eight in water, it is nine times as heavy as water. Calling water, therefore, 1, the specific gravity of gold is 19, and of copper 9. It is obvious that a body of the same specific gravity with water would weigh nothing when immersed in water, for it would be supported with an upward pressure precisely equal to its own weight, just as the same bulk of water is. A pound of water, therefore, will weigh nothing in water. The experiment can easily be tried. Weigh a glass bottle, suspended on one arm of the scale-beam, and then put a pound of water in it. On immersing it in water it will be balanced if you take out the pound weight in the opposite scale.

143. =Archimedes and the Crown.=--Hiero, King of Syracuse, stipulated for a crown of pure gold. But suspecting the maker of it of adulterating the gold, he called upon Archimedes to detect the imposture. He did it in this way: He procured two lumps of gold and silver of the same weight with the crown, and observed the quantity of water which each displaced. He then tried the crown, and found that it displaced less than the silver and more than the gold, and therefore concluded that it was an alloy of the two metals. All this was suggested to him by his experience in the bath, referred to in § 138.

144. =How to Ascertain the Specific Gravity of Liquids.=--There are several modes of ascertaining the specific gravities of different liquids. The instrument called a Hydrometer furnishes the most common mode. This is used chiefly in determining the quality of spirit. The more alcohol and the less water spirit contains the less is its specific gravity. The Hydrometer consists of two bulbs of glass, A B, Fig. 91, with a slender stem, C, which is graduated. In the lower bulb are a few shot or a little mercury, to give the instrument its proper weight, and to make its centre of gravity to be in the lower part. The lighter the fluid to be tested is the lower will the instrument sink in it. This is a very accurate instrument, detecting the slightest adulteration in spirits. Dr. Arnot tells an amusing story of the detection of a Chinese trader in liquors. He had sold a quantity of liquor to the purser of a ship, averring it to be of the same quality with a sample which he had given him. The purser tried it with his Hydrometer, and found it to be of greater specific gravity than the sample. The Chinese promptly denied the fraud; but on being told the exact quantity of water which he had added, he was so much confounded that he immediately confessed his guilt, and made ample amends. When the Hydrometer was shown to him he offered a large price for what appeared to him to be a magical instrument, foreseeing that it would be of great advantage to him in his business.

In Switzerland and in the north of Italy, where the peasants bring their milk to a common dairy, and are allowed a quantity of cheese at the end of the season in proportion to the amount of milk which they have brought, a Hydrometer is used to test the quality of the milk. There is a propriety in this, not only as a safe-guard against adulteration, but because there is a difference of quality in the milk of different cows, some giving a much more watery milk than others.

145. =Centre of Gravity in Floating Bodies.=--The same principles which apply to the centre of gravity in bodies standing on a firm basis apply also to floating bodies. That the centre of gravity may be low in a loaded vessel the heavy part of the cargo is put underneath, and generally ballast of stone or iron is necessary for the same purpose. In large flat-boats, the base of support being extensive, there is not the same need of taking care that the centre of gravity be low. If a ship be laden in part with an article which will dissolve in water, there is much danger, if the ship should leak, that this portion of the cargo shall be dissolved and be pumped out with the bilge-water, thus altering the trim of the vessel, or removing the centre of gravity from over the middle line, and bringing it too far forward, or carrying it too far back, making the ship wholly unmanageable. Four large English ships, in part loaded with saltpetre, were supposed to be lost from this cause in 1809 off the Isle of France. The immense ice-islands, or icebergs, which float about in summer in the polar regions, by melting irregularly often change the place of their centre of gravity, and in turning over present one of the most sublime spectacles in nature. A mountain of ice, extending high in the air and deep in the sea, suddenly turns over, and produces a rolling of the ocean which is often felt at the distance of many leagues.