suspending it from a shortened balance-pan by a fine thread or hair and immersing in a vessel of water. The buoyant properties of the water will cause the solid to apparently lose weight: this loss in weight is the exact weight of an equal bulk of water. The weight of the substance and the weight of an equal bulk of water being thus ascertained, a rule-of-three sum shows the proportional weight of the substance to 1.000 of water. Το express the same thing by rule, divide the weight in air by the loss of weight in water, the resulting number is the specific gravity in relation to 1 part of water, the conventional standard of comparison.

Verify some of the following specific gravities:

Specific gravities of solid substances should be taken in water having a temperature of about 60° F. The body should be immersed about half an inch below the surface of the water; adhering air-bubbles must be carefully removed; the body must be quite insoluble in water. The true weight of a body is its weight in air plus the weight of an equal bulk of air, or, in other words, its weight in vacuo, uninfluenced by the buoyancy of the air; but such a correction of the ordinary weight of a body is seldom necessary, or, indeed, desirable.

SPECIFIC GRAVITY OF SOLIDS IN POWDER OR SMALL FRAGMENTS.

Weigh the particles; place them in a counterpoised specificgravity bottle of known capacity, and fill up with water, taking care that the substance is thoroughly wetted; again weigh. From the combined weights of water and substance subtract the amount due to the substance; the residue is the weight of the water. Subtract this weight of water from the quantity which the bottle normally contains; the residue is the amount of water displaced by the substance. Having thus obtained the weights of equal bulks of water and substance, a rule-of-three sum shows the relation of the weight of the substance to 1 part of water— the specific gravity.

Or, suspend a cup, short glass tube, or bucket from a shortened balance-pan; immerse in water; counterpoise; place the weighed powder in the cup, and proceed as directed for taking the specific gravity of a solid in mass.

This operation may be conducted on fragments of any of the metals &c. the specific gravities of which are given in the foregoing Table, or on the powdered piece of marble the specific gravity of which has been taken in mass. The specific gravity of one piece of glass, first in mass then in powder, may be ascertained; the result should be identical. The specific gravity of shot is about 11·350; of sand, 2·600.

SPECIFIC GRAVITY OF SOLIDS SOLUBLE IN WATER.

Weigh a piece of sugar or other substance soluble in water; suspend it from a balance in the usual manner, and weigh it in turpentine, benzol, or petroleum, the specific gravity of which is known or has been previously determined; the loss in weight is the weight of an equal bulk of the turpentine. Ascertain the weight of an equal bulk of water by calculation :

The weights of equal bulks of sugar and water being thus obtained, the weight of a bulk of sugar corresponding to one of water is shown by a rule-of-three sum; in other words, divide the weight of sugar by that of the equal bulk of water, the product is the specific gravity of sugar.

SPECIFIC GRAVITY OF SOLIDS LIGHTER THAN WATER.

This is obtained in a manner similar to that for solids heavier than water; but the light body is sunk by help of a piece of heavy metal, the bulk of water which the latter displaces being deducted from the bulk displaced by both; the product is the weight of a bulk of water equal to the bulk of the light body. For instance, a piece of wood weighing 12 grammes (or grains) is tied to a piece of metal weighing 22 grammes, the loss of weight of the metal in water having been previously found to be 3 grammes. The two, weighing 34 grammes, are now immersed, and the loss in weight found to be 26 grammes. But of this loss 3 grammes have been proved to be due to the buoyant action of the water on the lead; the remaining 23, therefore, represent the same effect on the wood; 23 and 12, therefore, represent the weights of equal bulks of water and wood. As 23 are to 12 so is 1 to 5217. Or, shortly, as before, divide the weight in air by the weight of an equal bulk of water; 5217 is the specific gravity of the wood. Another specimen of wood may be found to be three-fourths (750) the weight of water, and others still heavier. Cork varies from ⚫100 to 300.

VOLUMETRIC ANALYSIS.

APPARATUS.

The only special vessels necessary in volumetric quantitative operations are:-1. A litre flask, which, when filled to a mark on the neck, contains one litre (1000 cubic centimetres, i. e. 1000 grammes of water *); it serves for preparing solutions in quantities of one litre. 2. A tall cylindrical graduated litre jar divided into 100 equal parts; it serves for the measurement and admixture of decimal or centesimal parts of a litre. 3. A graduated tube or burette, which, when filled to 0, holds 100 cubic centimetres (a decilitre), and is divided into 100 equal parts; it is used for accurately measuring small volumes of liquids.

The best form of burette is Mohr's (with Erdmann's float). It consists of a glass tube about the width of a little finger and the length of an arm from the elbow, contracted at the lower extremity and graduated. To the contracted portion is fitted a small piece of vulcanized caoutchouc tubing, into the other end of which a small spout made of narrow glass tube is tightly inserted. A strong wire clamp effectually prevents any liquid from passing out of the burette unless the knobs of the clamp are pressed by the finger and thumb of the operator, when a stream or drops flow at will. The accurate reading of the height of a solution in the burette is a matter of great importance. For this purpose a hollow glass float or bulb is used, of such a width that it can move freely in the tube without undue friction, and so adjusted in weight that it shall sink to not less than half its length in any ordinary liquid. A fine line is scratched round the centre of the float; this line must be always regarded as marking the height of the fluid in the burette. In charging the burette, a solution is poured in, not until its surface is coincident with 0, but until the mark on the float is coincident with 0.

* A cubic centimetre is, strictly speaking, the volume occupied by one gramme of distilled water at its point of greatest density, namely, 4° C.; metrical measurements, however, are uniformly taken at 15°-55 C. (60° F.).

ESTIMATION OF ALKALIES ETC.

An equation represents much more than the formation of certain substances from others. Thus

2AmHO+H,C,O,, 2H,0 = Am,C,O,, H20 + 3H,0

2

not only shows that oxalate of ammonium is produced when ammonia and crystallized oxalic acid are mixed together, but, among other facts, that 70 parts of ammonia and 126 of oxalic acid yield 142 of crystallized oxalate of ammonium and 54 of water. For formulæ represent molecules; the weight of a molecule is the sum of the weights of its atoms; and atomic weights are represented by definite invariable numbers (see the Table of atomic weights at the end of the volume).

=

3

3

=

As 126 parts (=1 molecule) of oxalic acid combine with 70 parts (2 molecules) of ammonia, 63 of oxalic acid will unite with 35 (=1 molecule) of ammonia; 63 parts of oxalic acid will also unite with 56 of caustic potash (KHO = 56), 40 of caustic soda (NaHO=40), 100 of acid carbonate of potassium (KHCO,, 100), 69 of anhydrous carbonate of potassium (K,CO, 138), 84 of acid carbonate of sodium (NaHCO, = 84), 53 of anhydrous carbonate of sodium (Na,CO, 106), 143 of crystallized carbonate of sodium (Na,CO,, 10H ̧0 = 286), &c. And if 63 parts of oxalic acid be dissolved in 100 volumes of water, the stated weights of these various salts should be exactly neutralized by such a solution. 143 parts of crystallized carbonate of sodium, for instance, should, if pure, be exactly neutralized by the 100 volumes of oxalic acid solution; and if a less number of volumes is required, the salt is so much per cent. impure. 143 parts by weight of a commercial sample of carbonate of sodium (common washingsoda") requiring only 97 of the standard oxalic acid solution is thus shown to contain 97 per cent. of pure carbonate of sodium, the remainder being impurities. Further, the strength of solutions of ammonia, soda, potash, and lime may be accurately determined by adding them gradually, from a burette, to a solution containing oxalic acid in known quantity, until exact neutralization is effected. If the quantity of oxalic acid be 53 parts by weight, then the parts by volume of the solutions added contain, of potash (KHO) 56 parts by weight, of soda (NaHO) 40, of ammonia (AmHO) 35 parts, of slaked lime (Ca2HO) 37, anhydrous lime (CaO) 28, &c. The strength of an alkaline solution (or, in other words, the proportion required to effect neutralization of 100 volumes of the acid solution) having once been determined

66