before. These weights will evidently be equal to the weight of the body, whether the arms of the balance be equal or not. This method of weighing is due to Borda.

In the use of the hydrostatical balance generally, it will be proper to observe the following general precautions. The water in which the solid is to be weighed, besides its being either distilled or rain water, must be quite clean. Its temperature, as well as that of the solid, must be as near as possible to 62° of Fahrenheit's thermometer; for which purpose the ball of the thermometer must be placed in the water, and the temperature is adjusted by the addition of hot or cold water. If the solid body be soluble in water, or if it be porous enough to absorb any water, then it must be varnished, or coated with some oily or greasy substance; but in that case some allowance is to be made on account of the varnish, &c. When the solid is weighed in water, its upper part ought to be a little way below the surface of the water; for instance, about an inch; and it must by no means be suffered to touch the sides or bottom of the jar. Care must be taken that no bubbles of air adhere to the solid under water; for they would partly buoy it up. These may be easily removed by means of a feather. The solid must be of a compact form, and free from accidental

or artificial vacuities, so as not to harbour any air; for otherwise its specific gravity cannot be ascertained by weighing in water, &c. Thus a piece of silver, which is much heavier than water, may be formed into a hollow sphere, which will appear to be much lighter than water; for, if this sphere were immersed in water, it would displace a quantity of water which is equal not only to the silver, but also to the space which is contained in the sphere. It is for this reason that a ship might be made of iron, or of copper, or, in short, of any substance whose specific gravity far exceeds that of water, and yet it would float as well as a ship which is made of wood in the usual way.

In order to determine the specific gravity of living men, Mr. Robertson prepared a cistern seventy-eight inches long, thirty inches wide, and thirty inches deep; and, having procured ter men for his purpose, the height of each was taken, and his weight, and afterwards they plunged successively into the cistern. A ruler, graduated to inches and decimal parts of an inch, was fixed to one end of the cistern, and the height of the water noted before each man went in, and to what height it rose when he immersed himself under its surface.

The following Table contains the several results of his experiments:

One of the reasons, Mr. Robertson says, that induced him to make these experiments, was a desire to know what quantity of fir or oak timber would be sufficient to keep a man afloat in river or sea water, thinking that most men were specifically heavier than river or common fresh water; but the contrary appears from the trials before recited; for, except the first and last, every man was lighter than his equal bulk of fresh water, and much more so than his equal bulk of sea water: consequently, if persons who fall into the water could preserve their presence of mind, many might be preserved from drowning, and a piece of wood, not larger than an oar, would buoy a man partly above water as long as he could adhere to it.

It is evident also that a reference to the same general principle will enable us to ascertain the specific gravities of different fluids. For, if the same substance be weighed in two fluids, the weight which it loses in each is as the specific gravity of that fluid. Thus a cubic inch of lead

loses 253 grains when weighed in water, and only 209 grains when weighed in rectified spirits; therefore a cubic inch of rectified spirit weighs 209 grains, an equal bulk of water weighing 253; and so the specific gravity of the water is about a fourth greater than that of the spirit.

Upon this principle the hydrometer is constructed. There are various kinds of hydrometers. One is a glass or copper ball with a stem, on which is marked a scale of equal parts or degrees. The point to which the stem sinks in any liquid being ascertained, and marked on this scale, we can tell how many degrees any other liquid is heavier or lighter, by observing the point to which the stem sinks in it. Another and a very simple hydrometer is formed by preparing a number of hollow glass beads of different weights, but the proportions of which are known, and the beads marked accordingly; they are then successively dropped in the fluid to be examined, until one is found which neither sinks nor comes up to the surface, but remains at rest,

wherever it is placed in the liquid. You thus ascertain that the liquid is of the same specific gravity with this bead. If the same bead be dropped into another liquid, and sink, that liquid must be lighter than the first; if the bead come to the top, the second liquid is heavier than the first; and by trying the liquid with the other beads, until one is found which neither sinks nor floats, you ascertain the relative weight of the liquid by the number of the bead.

A hydrometer of great delicacy and peculiarly useful for measuring the specific gravity of different waters, and thereby ascertaining their degrees of purity, consists of a ball of glass three inches diameter, with another joining it, and opening into it, of one inch diameter, B and C, plate I. fig. 10, and a brass neck d, into which is screwed a wire a o, about ten inches long, and one-fortieth of an inch diameter, divided into inches, and tenths of an inch. The whole weight of this instrument is 4000 grs. when loaded with shot in the lower ball. It is found that, when plunged into water in the jar, a grain laid upon the top a makes it sink one inch; therefore a tenth of a grain sinks it a tenth of an inch. Now it will stand in one kind of water a tenth of an inch lower than in another, which shows that a bulk of one kind of water equal to the bulk of the instrument weighs one-tenth of a grain less than an equal bulk of the other kind of water; so that a difference in specific gravity of one part in 40,000 is thus detected. This weight of 4000 grs. is convenient for comparing water; but the quantity of shot in the lower ball may be varied, so as to make it lighter or heavier, and so adapt it to measure the specific gravities of lighter or heavier liquids. It will always be an accurate and very delicate measure for liquids of nearly the same weight. Indeed its delicacy is so great, that an impurity too slight to be detected by the taste, will be discovered by this in

strument.

The areometer invented by M. De Parcieux, of Paris, is more simple, and affords a very accurate comparison of different liquids. It is only a different form of the instrument just described. A glass phial, about two inches or two inches and a half in diameter, and seven or eight long, with a plane or round bottom, is corked tight, and into the cork is fixed a perfectly straight wire of about one-twelfth of an inch diaineter, and thirty inches long. The phial is loaded with shot, so as to make it sink in the heaviest liquid to be examined, leaving the wire just below the surface. There is a cylinder of glass, about three or three and a half inches diameter, and three or four feet long, with a scale of equal parts on the side. The liquor to be tried is put into this; and the scale marks the point to which the top of the wire sinks. This instrument is so sensible, that if it stands at any point in water of the common temperature, and the sun's rays fall upon the water, the wire will sink several inches, from the slight increase of heat causing an increase of bulk, and consequently a diminution of relative weight in the water; and it will rise again when carried into the shade. A pinch of salt or sugar throwr. in makes it rise some inches, and a little spirits poured in make it sink. With one of these instruments, weighing somewhat less than twenty

four ounces, and plunged in water, there is a falk. or rise of above half an inch for every th part of the water displaced; so that the difference of 1000 th part is easily perceived.

Fahrenheit's hydrometer, like the common one, consists of a hollow ball, with a counterpoise c, fig. 11, but the stem B is very slender, and terminates in a small dish A. Round the middle of the stem is drawn a fine line; and there are no divisions on the stem, which is always immersed in the fluid to be tried, up to the mark, by placing as much weight as may be required in the small dish A. Hence, as the part immersed is constantly of the same magnitude, and the whole weight of the hydrometer is known, this last weight, added to the weight in the dish, will be equal to the weight of fluid displaced by the instrument.

Mr. Clarke's hydrometer is made of copper, because ivory imbibes spirituous liquors, and glass is apt to break. It consists of a brass wire about one-fourth of an inch thick, passing through and soldered into the copper ball Bb, fig 12, plate 1. The upper part of the wire is filed flat on one side of the stem of the hydrometer, and marked at m, to which division it exactly sinks in proof spirits. There are two other marks, A and B; the one showing that the liquor is one-tenth above proof, when this instrument sinks to A, and the other indicating one-tenth under proof when it emerges to B; a brass weight, as C, having been previously screwed on to the bottom at c. There is a great variety of weights of different sizes, as K, &c., adapted to liquors that differ more than one-tenth from proof, and for determining the specific gravities of all such liquors as occur in trade, as well as for showing the specific gravities of all fluids down to common water. The round part of the wire above the ball may be marked across, so that with the weight C, which fits the instrument for the trial of river water, in which it sinks to RW, it may serve for wines or other waters: thus in spring water it will sink to SP; in mineral water to M I; in sea-water to SE; and in the water of salt springs to SA: and the marks br, ra, po, me, denote the divisions to which the instrument descends in Bristol water, rain water, port wine, and mountain wine, respectively.

This hydrometer is inferior to Fahrenheit's in two respects. In the first place, either a bubble of air, or a portion of the fluid, will be hid in that part of the cavity of the ballast weight which is not filled by the screw; and it is of very different consequence which of the two is there. And secondly, the weights acting on the instrument, by their residual gravity, will not be constant; or, in other words, an additional weight will be accompanied by an addition to the bulk of the immersed part of the instrument; and in the case where the specific gravity of the liquid is not given, but required, it will not be easy to determine how much the operation of the one is counteracted by the other However, though this last consideration evinces that the instrument is not fit for general use, yet it is accurate for the trial of ardent spirits, or any other particula liquid, when the weights are adjusted by experiment to the intended use.

Sikes's hydrometer is now generally employed, especially since its adoption by the commissioners of his majesty's customs. This instrument has but nine shifting weights, applicable upon the upper part of the stem, and is used with a set of tables, or a sliding rule sold with it, for computing compensation for different temperatures. The scale is divided into ten principal divisions, each of which is subdivided into five parts, and by the separate application of the weights in succession completes the range of strength from pure alcohol to water, each weight being equivalent to ten principal divisions. This hydrometer, with the weight marked 60, screwed on to the lower stem, is so adjusted as to sink to the line mark P on the scale of the instrument when placed in proof spirit, of the temperature of 51° Fahrenheit, and, by the addition of the square weight on the top of the stem, it sinks to the same point in distilled water of the same temperature. This weight being just one-twelfth part of the entire weight of the whole hydroineter, together with its bottom weight No. 60, causes the scale to show the difference between water and proof spirit, which, the act states, shall weigh exactly twelve-thirteenth parts of an equal bulk of distilled water.

Mr. Meikle's hydrometer consists of a glass tube, open at both ends, and bent into a kind of double syphon, having four parallel legs; so that the open ends are pointed in the same direction or upwards, as shown in fig. 13, plate I. The manner of using it is very simple. Let one of the ends be stopped with a finger or cork, and water be poured into the other. This fluid will only rise a small way into the second leg, because of the included air. Next stop the other orifice, and open the one first closed; and, having poured into the latter the liquid whose specific gravity is to be tried, open the top of the water tube; then, the instrument being held up right, the two liquids will arrange themselves so as to press equally on the included air. This pressure will be measured by the difference in the heights of the two columns of either liquid, multiplied by its specific gravity, so that, by dividing the difference of the two columns of water by the difference of those of the other liquid, we obtain the specific gravity of the latter; that of water being unity. The difference between the columns may be measured by applying any scale of small equal parts, or the glass may be attached to a graduated plate furnished with verniers, &c. The longer the columns of liquids employed, the more accurate the process. The expansion of the glass, or its capillary action, cannot affect the result, nor is it influenced by the expansion of the scale; the only correction required will be to reduce the observations to one temperature.

There are other methods of judging of the strength of spirituous liquors, which though useful are not accurate, such as the taste, the size and appearance of the bubbles when shaken, the sinking or floating of olive oil in it, and the appearances that it exhibits when burned; if it burns away perfectly to dryness, and inflames gunpowder, or a piece of cotton immersed in it, it is considered as alcohol; the different spirituous

liquors leave variable proportions of water, when thus burned in a graduated vessel.

There is the greatest difficulty in ascertaining what is meant by the terms proof spirit. Dr. Thomson, quoting the act of parliament of 1762, states, that at the temperature of 60°, the specific gravity of proof spirit should be 0.916; and he also observes, that proof spirit usually means a mixture of equal bulks of alcohol and water; but the specific gravity of such a mixture will, of course, depend upon that of the standard alcohol, which is not specified. It appears from Gilpin's Tables that spirit of the specific gravity 916, at 60°, consists, by weight, of 100 parts of alcohol, specific gravity 825, at 60°, and 75 of water; and, by measure, of 100 parts of the same alcohol, and 61.87 of water.

One of the most accurate and convenient methods of obtaining the specific gravity of fluids is by what is called a thousand grain bottle. This is sold by most of the philosophical instrument makers, together with a weight, which is an exact counterpoise for the bottle when filled with distilled water; its magnitude being adjusted by grinding down the length of its neck, until it holds exactly 1000 grains of water at 60° of Fahrenheit. This instrument consequently requires no computation, but is simply to be filled with fluid, and placed in one scale of a balance, while its counterpoise is placed in the other. If the fluid put into it is lighter than water, it will appear deficient in weight, and as many grains must be added to the scale that contains it as will restore the balance. This at once shows that the specific gravity of the fluid under examination is negative, or less than the standard, and consequently must be a fractional number; but, should the fluid be heavier than water, the bottle will preponderate, and weights must be put into the opposite scale, when their amount will be positive, and must be added to the amount of the standard. For example: if the bottle were filled with sulphuric ether, it would require 739 grains to be placed in the same scale to restore the balance, consequently its specific gravity would be thus expressed 0.739. Had it been filled with seawater, which is rather denser than that which is distilled, twenty-six-hundredths, or rather better than one-fourth of a grain must have been added in the opposite scale, and these, as already explained, must be added to the standard 1.000 to express the specific gravity of such water, which would be thus written 1.026. Sulphuric acid again, being still heavier, would, in like manner require 875 grains, and would accordingly be expresscd 1.875.

A bottle, however, holding 1000 grains is often inconveniently large, and a small and thin globular phial, with a piece of thermometer tube ground into it by way of stopper, will be found more useful such a phial should not weigh more than from fifty to sixty grains, and may contain between 400 and 500 grains of water. To use it it should be accurately counterbalanced in a delicate pair of scales, and then filled with distilled water, and the stopper thrust in, the capillary opening in which allows a little to ooze out, and prevents the likelihood of bursting the

phial; it is then to be wiped clean and dry, and again carefully weighed, by which the quantity of water it contains is ascertained; the water being poured out it is next filled with the liquid whose specific gravity is required, taking care that it is of the same temperature as the water; we then weigh as before, and divide the weight by the former weight of water, the product gives the specific gravity required. Thus, suppose the phial to contain 425 grains of water at the temperature of 45°, it will be found to hold 5737.5 grains of pure mercury of the same temperature; and 5737·5 ÷ 425 13.5 the specific gravity of mercury. Or, supposing the liquid lighter than water, such as alcohol, of which we may assume the pnial to contain 350-5; then 350-5425 0.824, the specific gravity of the alcohol under trial.

The following table is given by Mr. Gilpin, in the eighty-fourth volume of the Philosophical Transactions, and is of essential use for taking the specific gravities both of solids and fluids, by enabling the operator to reduce the weight or bulk of the distilled water, employed in any case, to that which it would have at any other common temperature, and particularly to 60°, which is the usual standard.

Thus, for example, since the specific gravity of water at 47° is 1.0008 grains, and at 60° is 100000, (and consequently 1.0008 grains, at 47o, are equal in bulk to 100000 grains at 60°), it follows that it would require 252-708 grains at 47°, to equal the space of a cubic inch; for 100000 10008:: 252-506 (the weight of a cubic inch at 60°), : 252·708.

Do. fused

Citric

Formic

1-200

Muriatic Nitric

Do. highly concentrated

Phosphoric, liquid

2.750 Do. serum of 3.391 Borax

1.030

1.714

0.942

0.988

2.850 rubber

2.900 Caoutchouc, or Indian

0.933

Ammoniacal

0.590

0.770

Carbonic oxide Carbureted hydro

0.972 of

Do.

0.972 from 2.220 to 2.612 2.500 Malachite, compact, from

Anise-seed 0.986
Carraway seed 0.904
Cinnamon 1.043
Cloves
1.036
Fennel , 0-929
Lavender 0.894
Mint, common 0.898
Turpentine 0.870
Wormwood 0.907

Expressed Sweet al-
monds

0.932

0.923

gen

Chlorine

Chlorocarbonous

3.572 to 3.994

acid

3.472 Marble, Carrara

2.716

Chloroprussic acid 2·152

Cyanogen

1.805

Euchlorine

2.440

white, Italian 2-707 black veined Parian

2.704

2.560