The Boy's Playbook of Science Part 5

CHAPTER V.

SPECIFIC GRAVITY.

It is recorded of the great Dr. Wollaston, that when Sir Humphry Davy placed in his hand, what was then considered to be _the_ scientific wonder of the day--viz., a small bit of the metal pota.s.sium, he exclaimed at once, "How heavy it is," and was greatly surprised, when Sir Humphry threw the metal on water, to see it not only take fire, but actually _float_ upon the surface; here, then, was a philosopher possessing the deepest learning, unable, by the sense of touch and by ordinary handling, to state correctly whether the new substance (and that a metal), was heavy or light; hence it is apparent that the property of specific gravity is one of importance, and being derived from the Latin, means _species_, a particular sort or kind; and _gravis_, heavy or weight--i.e., the particular weight of every substance compared with a fixed standard of water.

[Ill.u.s.tration: Fig. 65. A. A large cylindrical vessel containing water, in which the egg sinks till it reaches the bottom of the gla.s.s. B. A similar gla.s.s vessel containing half brine and half water, in which the egg floats in the centre--viz., just at the point where the brine and water touch.]

We are so constantly in the habit of referring to a standard of perfection in music and the arts of painting and sculpture, that the youngest will comprehend the office of water when told that it is the philosopher's unit or starting-point for the estimation of the relative weights of solids and liquids. A good idea of the scope and meaning of the term specific gravity, is acquired by a few simple experiments, thus: if a cylindrical [Page 49] gla.s.s, say eighteen inches long, and two and a half wide, is filled with water, and another of the same size is also filled, one half with water and the other half with a saturated solution of common salt, or what is commonly termed brine, a most amusing comparison of the relative weights of equal bulks of water and brine, can be made with the help of two eggs; when one of the eggs is placed in the gla.s.s containing water, it immediately sinks to the bottom, showing that it has a greater specific gravity than water; but when the other egg is placed in the second gla.s.s containing the brine, it sinks through the water till it reaches the strong solution of salt, where it is suspended, and presents a most curious and pretty appearance; seeming to float like a balloon in air, and apparently suspended upon nothing, it provokes the inquiry, "whether magnetism has anything to do with it?" The answer, of course, is in the negative, it merely floats in the centre, in obedience to the common principle, that all bodies float in others which are heavier than themselves; the brine has, therefore, a greater weight than an equal bulk of water, and is also heavier than the egg. A pleasing sequel to this experiment may be shown by demonstrating how the brine is placed in the vessel without mixing with the water above it; this is done by using a gla.s.s tube and funnel, and after pouring away half the water contained in the vessel (Fig. 65.), the egg can be floated from the bottom to the centre of the gla.s.s, by pouring the brine down the funnel and tube. The saturated solution of salt remains in the lower part of the vessel and displaces the water, which floats upon its surface like oil on water, carrying the egg with it.

[Ill.u.s.tration: Fig. 66. A vessel half full of water, and as the brine is poured down the tube the egg gradually rises.]

The water of the Dead Sea is said to contain about twenty-six per cent.

of saline matter, which chiefly consists of common salt. It is perfectly clear and bright, and in consequence of the great density, a person may easily float on its surface, like the egg on the brine, so that if a s.h.i.+p could be heavily laden whilst floating on the water of the Dead Sea, it would most likely sink if transported to the Thames. This ill.u.s.tration of specific gravity is also shown by a model s.h.i.+p, which being first floated on the brine, will afterwards sink if conveyed to another vessel containing water. One of the tin model s.h.i.+ps sold as a magnetic [Page 50] toy answers nicely for this experiment, but it must be weighted or adjusted so that it just floats in the brine, A; then it will sink, when placed, in another vessel containing only water.

[Ill.u.s.tration: Fig. 67.

A. Vessel containing brine, upon which the little model floats.

B. Vessel containing water, in which the s.h.i.+p sinks.]

Another amusing ill.u.s.tration of the same kind is displayed with goldfish, which swim easily in water, floating on brine, but cannot dive to the bottom of the vessel, owing to the density of the saturated solution of salt. If the fish are taken out immediately after the experiment, and placed in fresh water, they will not be hurt by contact with the strong salt water.

These examples of the relative weights of equal bulks, enable the youthful mind to grasp the more difficult problem of ascertaining the specific gravity of any solid or liquid substance; and here the strict meaning of terms should not be pa.s.sed by. _Specific_ weight must not be confounded with _Absolute_ weight; the latter means the entire amount of ponderable matter in any body: thus, twenty-four cubic feet of sand weigh about one ton, whilst specific weight means the _relation_ that subsists between the _absolute weight_ and the _volume_ or _s.p.a.ce_ which that _weight_ occupies. Thus a cubic foot of water weighs sixty-two and a half pounds, or 1000 ounces avoirdupois, but changed to gold, the cubic foot weighs more than half a ton, and would be equal to about 19,300 ounces--hence the relation between the cubic foot of water and that of [Page 51] gold is nearly as 1 to 19.3; the latter is therefore called the specific gravity of gold.

Such a mode of taking the specific gravity of different substances--viz., by the weight of equal bulks, whether cubic feet or inches, could not be employed in consequence of the difficulty of procuring exact cubic inches or feet of the various substances which by their peculiar properties of brittleness or hardness would present insuperable obstacles to any attempt to fas.h.i.+on or shape them into exact volumes. It is therefore necessary to adopt the method first devised by Archimedes, 600 B.C., when he discovered the admixture of another metal with the gold of King Hiero's crown.

This amusing story, ending in the discovery of a philosophical truth, may be thus described:--King Hiero gave out from the royal treasury a certain quant.i.ty of gold, which he required to be fas.h.i.+oned into a crown; when, however, the emblem of power was produced by the goldsmith, it was not found deficient in weight, but had that appearance which indicated to the monarch that a surrept.i.tious addition of some other metal must have been made.

It may be a.s.sumed that King Hiero consulted his friend and philosopher Archimedes, and he might have said, "Tell me, Archimedes, without pulling my crown to pieces, if it has been adulterated with any other metal?" The philosopher asked for time to solve the problem, and going to take his accustomed bath, discovered then specially what he had never particularly remarked before--that, as he entered the vessel of water, the liquid rose on each side of him--that he, in fact, displaced a certain quant.i.ty of liquid. Thus, supposing the bath to have been full of water, directly Archimedes stepped in, it would overflow. Let it be a.s.sumed that the water displaced was collected, and weighed 90 pounds, whilst the philosopher had weighed, say 200 pounds. Now, the train of reasoning in his mind might be of this kind:--"My body displaces 90 pounds of water; if I had an exact cast of it in lead, the same _bulk_ and _weight_ of liquid would overflow; but the weight of my body was, say 200 pounds, the cast in lead 1000 pounds; these two sums divided by 90 would give very different results, and they would be the specific gravities, because the rule is thus stated:--'Divide the gross weight by the loss of weight in water, the water displaced, and the quotient gives the specific gravity.'" The rule is soon tested with the help of an ordinary pair of scales, and the experiment made more interesting by taking a model crown of some metal, which may be nicely gilt and burnished by Messrs. Elkington, the celebrated electro-platers of Birmingham. For convenience, the pan of one scale is suspended by shorter chains than the other, and should have a hook inserted in the middle; upon this is placed the crown, supported by very thin copper wire. For the sake of argument, let it be supposed that the crown weighs 17 ounces avoirdupois, which are duly placed in the other scale-pan, and without touching these weights, the crown is now placed in a vessel of water. It might be supposed that directly the crown enters the water, it would gain weight, in consequence of being wetted, [Page 52] but the contrary is the case, and by thrusting the crown into the water, it may be seen to rise with great buoyancy so long as the 17 ounces are retained in the other scale-pan; and it will be found necessary to place at least two ounces in the scale-pan to which the crown is attached before the latter sinks in the water; and thus it is distinctly shown that the crown weighs only about 15 ounces in the water, and has therefore _lost_ instead of _gaining_ weight whilst immersed in the liquid. The rule may now be worked out:

Ounces.

Weight of crown in air 17 Ditto in water 15 --- Less in water 2

17 / 2 = 8

The quotient 8 demonstrates that the crown is manufactured of copper, because it would have been about 19 if made of pure gold.

[Ill.u.s.tration: Fig. 68. A. Ordinary pair of scales. B. Scale-pan, containing 17 ounces, being the weight of the crown in air. C. Pan, with hook and crown attached, which is sunk in the water contained in the vessel D; this pan contains the two ounces, which must be placed there to make the crown sink and exactly balance B.]

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_Table of the Specific Gravities of the Metals in common use._

Platinum 20.98 Gold 19.26 to 19.3 and 19.64 Mercury 13.57 Lead 11.35 Silver 10.47 to 10.5 Bis.m.u.th 9.82 Copper 8.89 Iron 7.79 Tin 7.29 Zinc 6.5 to 7.4

The simple rule already explained may be applied to all metals of any size or weight, and when the ma.s.s is of an irregular shape, having various cavities on the surface, there may be some difficulty in taking the specific gravity, in consequence of the adhesion of _air-bubbles_; but this may be obviated either by brus.h.i.+ng them away with a feather, or, what is frequently much better, by dipping the metal or mineral first into alcohol, and then into water, before placing it in the vessel of water, by which the actual specific gravity is to be taken.

The mode of taking the specific gravity of liquids is very simple, and is usually performed in the laboratory by means of a thin globular bottle which holds exactly 1000 grains of pure distilled water at 60 Fahrenheit. A little counterpoise of lead is made of the exact weight of the dry globular bottle, and the liquid under examination is poured into the bottle and up to the graduated mark in the neck; the bottle is then placed in one scale-pan, the counterpoise and the 1000-grain weight in the other; if the liquid (such as oil of vitriol) is heavier than water, then more weight will be required--viz., 845 grains--and these figures added to the 1000 would indicate at once that the specific gravity of oil of vitriol was 1.845 as compared with water, which is 1.000. When the liquid, such as alcohol, is lighter than water, the 1000-grain weight will be found too much, and grain weights must be added to the same scale-pan in which the bottle is standing, until the two are exactly balanced. If ordinary alcohol is being examined, it will be found necessary to place 180 grains with the bottle, and these figures deducted from the 1000 grains in the other scale-pan, leave 820, which, marked with a dot before the first figure (_sic_ .820), indicates the specific gravity of alcohol to be less than that of water.

The difference in the gravities of various liquids is displayed in a very pleasing manner by an experiment devised by Professor Griffiths, to whom chemical lecturers are especially indebted for some of the most ingenious and beautiful ill.u.s.trations which have ever been devised. The experiment consists in the arrangement of five distinct liquids of various densities and colours, the one resting on the other, and distinguished not only by the optical line of demarcation, but by little b.a.l.l.s of wax, which are adjusted by leaden shot inside, so as to sink through [Page 54] the upper strata of liquids, and rest only upon the one that it is intended to indicate.

The manipulation for this experiment is somewhat troublesome, and is commenced by procuring some pure bright quicksilver, upon which an iron bullet (black-leaded, or painted of any colour) is placed, or one of those pretty gla.s.s b.a.l.l.s which are sold in such quant.i.ties at the Crystal Palace.

Secondly. Put as much white vitriol (sulphate of zinc) into a half pint of boiling water as it will dissolve, and, when cold, pour off the clear liquid, make up a ball of coloured wax (say red), and adjust it by placing little shot inside, until it sinks in a solution of sulphate of copper and floats on that of the white vitriol.

Thirdly. Make a solution of sulphate of copper in precisely the same manner, and adjust another wax ball to sink in water, and float on this solution.

Fourthly. Some clear distilled water must be provided.

Fifthly. A little cochineal is to be dissolved in some common spirits of wine (alcohol), and a ball of cork painted white provided.

Finally. A long cylindrical gla.s.s, at least eighteen inches high, and two and a half or three inches diameter, must be made to receive these five liquids, which are arranged in their proper order of specific gravity by means of a long tube and funnel.

The four b.a.l.l.s--viz., the iron, the two wax, and the cork b.a.l.l.s, are allowed to slide down the long gla.s.s, which is inclined at an angle; and then, by means of the tube and funnel, pour in the tincture of cochineal, and all the b.a.l.l.s will remain at the bottom of the gla.s.s. The water is poured down next, and now the cork ball floats up on the water, and marks the boundary line of the alcohol and water. Then the solution of blue vitriol, when a wax ball floats upon it. Thirdly, the solution of white vitriol, upon which the second wax ball takes its place; and lastly, the quicksilver is poured down the tube, and upon this heavy metallic fluid the iron or gla.s.s ball floats like a cork on water.

[Ill.u.s.tration: Fig. 69. Long cylindrical gla.s.s, 18 3 inches, containing the five liquids.]

The tube may now be carefully removed, pausing at each liquid, so that no mixture take place between them; and the result is the arrangement of five liquids, giving the appearance of a cylindrical gla.s.s painted with [Page 55] bands of crimson, blue, and silver; and the liquids will not mingle with each other for many days.

A more permanent arrangement can be devised by using liquids which have no affinity, or will not mix with each other--such as mercury, water, and turpentine.

The specific weight or weights of an equal measure of air and other gases is determined on the same principle as liquids, although a different apparatus is required. A light capped gla.s.s globe, with stop-c.o.c.k, from 50 to 100 cubic inches capacity, is weighed full of air, then exhausted by an air-pump, and weighed empty, the loss being taken as the weight of its volume of air; these figures are carefully noted, because _air_ instead of _water_ is the standard of comparison for all gases. When the specific gravity of any other gas is to be taken, the gla.s.s globe is again exhausted, and screwed on to a gas jar provided with a proper stop-c.o.c.k, in which the gas is contained; and when perfect accuracy is required, the gas must be dried by pa.s.sing it over some asbestos moistened with oil of vitriol, and contained in a gla.s.s tube, and the gas jar should stand in a mercurial trough. (Fig. 70.) The stop-c.o.c.ks are gradually turned, and the gas admitted to the exhausted globe from the gas jar; when full, the c.o.c.ks are turned off, the globe unscrewed, and again weighed, and by the common rule of proportion, as the weight of the air first found is to the weight of the gas, so is unity (1.000, the density of air) to a number which expresses the density of the gas required. If oxygen had been the gas tried, the number would be 1.111, being the specific gravity of that gaseous element. If chlorine, 2.470. Carbonic acid, 1.500. Hydrogen being much less than air, the number would only be 69, or decimally 0.069.

[Ill.u.s.tration: Fig. 70. A. Gla.s.s globe to contain the gas. B. Gas jar standing in the mercurial trough, D. C. Tube containing asbestos moistened with oil of vitriol.]

A very good approximation to the correct specific gravity (particularly where a number of trials have to be made with the same gas, such as [Page 56] ordinary coal gas) is obtained by suspending a light paper box, with holes at one end, on one arm of a balance, and a counterpoise on the other. The box can be made carefully, and should have a capacity equal to a half or quarter cubic foot; it is suspended with the holes downward, and is filled by blowing in the coal gas until it issues from the apertures, and can be recognised by the smell. The rule in this case would be equally simple: as the known weight of the half or quarter cubic foot of common air is to the weight of the coal gas, so is 1.000 to the number required. (Fig. 71.)

[Ill.u.s.tration: Fig. 71. A. The balance. B. The paper box, of a known capacity. C. Gas-pipe blowing in coal-gas, the arrows showing entrance of gas and exit of the air.]

As an ill.u.s.tration of the different specific weights of the gases, a small balloon, containing a mixture of hydrogen and air, may be so adjusted that it will just sink in a tall gla.s.s shade inverted and supported on a pad made of a piece of oilcloth shaped round and bound with list. On pa.s.sing in quickly a large quant.i.ty of carbonic acid, the little balloon will float on its surface; and if another balloon, containing only hydrogen, is held in the top part of the open shade, and a sheet of gla.s.s carefully slid over the open end, the density of the gases (although they are perfectly invisible) is perfectly indicated; and, as a climax to the experiment, a third balloon can be filled with laughing gas, and may be placed in the gla.s.s shade, taking care that the one full of pure hydrogen does not escape; the last balloon will sink to the bottom of the [Page 57] jar, because laughing gas is almost as heavy as carbonic acid, and the weight of the balloon will determine its descent. (Fig. 72.)

[Ill.u.s.tration: Fig. 72. Inverted large gla.s.s shade, containing half carbonic acid and half common air.]

[Ill.u.s.tration: Fig. 73. A. Inverted gla.s.s shade, containing the material, B, for generating carbonic acid gas. C. The soap-bubble. D D.

The gla.s.s tube for blowing the bubbles. E. Small lantern, to throw a bright beam of light from the oxy-hydrogen jet upon the thin soap-bubble, which then displays the most beautiful iridescent colours.]

A soap-bubble will rest most perfectly on a surface of carbonic acid gas, and the aerial and elastic cus.h.i.+on supports the bubble till it bursts. The experiment is best performed by taking a gla.s.s shade twelve inches broad and deep in proportion, and resting it on a pad; half a pound of sesquicarbonate of soda is then placed in the vessel, and upon this is poured a mixture of half a pint of oil of vitriol and half a pint of water, the latter being previously mixed and allowed to cool before use. An enormous quant.i.ty of carbonic acid gas is suddenly generated, and rising to the edge, overflows at the top of the gla.s.s shade. A well-formed soap-bubble, detached neatly from the end of a gla.s.s-tube, oscillates gently on the surface of the heavy gas, and presents a most curious and pleasing appearance. The soapy water is prepared by cutting a few pieces of yellow soap, and placing them in a two-ounce bottle containing distilled water. (Fig. 73.) The specific gravity of the gases, may therefore be either greater, or less than atmospheric air, [Page 58] which has been already mentioned as the standard of comparison, and examined by this test the vapours of some of the compounds of carbon and hydrogen are found to possess a remarkably high gravity; in proof of which, the vapour of ether may be adduced as an example, although it does not consist only of the two elements mentioned, but contains a certain quant.i.ty of oxygen. In a cylindrical tin vessel, two feet high and one foot in diameter, place an ordinary hot-water plate, of course full of boiling water; upon this warm surface pour about half an ounce of the best ether; and, after waiting a few minutes until the whole is converted into vapour, take a syphon made of half-inch pewter tube, and warm it by pouring through it a little hot water, taking care to allow the water to drain away from it before use.

After placing the syphon in the tin vessel, a light may be applied to the extremity of the long leg outside the tin vessel, to show that no ether is pa.s.sing over until the air is sucked out as with the water-syphon; and after this has been done, several warm gla.s.s vessels may be filled with this heavy vapour of ether, which burns on the application of flame. Finally, the remainder of the vapour may be burnt at the end of the syphon tube, demonstrating in the most satisfactory manner that the vapour is flowing through the syphon just as spirit is removed by the distillers from the casks into cellars of the public-houses. (Fig. 74.)

[Ill.u.s.tration: Fig. 74. A. Tin vessel containing the hot-water plate, B, upon which the ether is poured. C. The syphon. D. Gla.s.s to receive the vapour. E. Combustion of the ether vapour in another vessel.]

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