Experiment. Mercury is so much used in physical experiments that every student should know how to clean it. The impurities may be divided into three classes: first, mixture with metals, especially lead, zinc and tin; secondly, common dust and dirt; and thirdly, water or other liquids.

Redistillation is almost the only way to remove the metals, and even this is not perfectly effectual, especially in the case of zinc. Moreover, by long boiling a small amount of oxide is formed, which is dissolved by the metal. The mercury used for amalgamating battery plates should therefore be kept separate from the rest and used for this purpose only. If but little of the metal is present it may be removed by agitating with dilute nitric acid. The best way to do this is to fill a long vertical tube with the acid and allow the mercury to flow into it from a funnel, in which is a paper filter with a fine hole in the bottom. The mercury falls through the long column of liquid in minute globules, and is thus readily and thoroughly cleaned. It may be drawn out below by a glass stopcock, or by a bent tube in which a short column of mercury shall balance a long column of acid. As the mercury collects it flows out of the end of the tube into a vessel placed to receive it. Instead of nitric acid a solution of nitrate of mercury may be used, if preferred. Another method is to fill a bottle about a quarter full of mercury, add a quantity of finely powdered loaf sugar, and shake violently. The metallic impurities are oxidized at the expense of the air, which must be renewed by a pair of bellows.

A great variety of devices are used to remove the mechanical impurities of mercury. For example, pouring it into a bag of chamois leather and squeezing the latter until the mercury comes through in fine globules. Or, making a needle hole in the point of a paper filter, placing it in a funnel and letting the mercury run through. The mercury may be washed directly with water, by shaking them together in a bottle, or better, filling a jar half full of mercury and letting the water from the hydrant bubble up through it. This is an excellent way to remove most liquids.

Next, to remove the water, pour the mixture into a small bottle, when the mercury will settle to the bottom, and the water overflow from the top. When the mercury fills the bottle transfer it

to another vessel and repeat. If there is only mercury enough to half fill the bottle the second time, pour back some of the mercury already dried to displace the remaining water. Another way is to close the end of a funnel with the finger and pour in the mixture, drawing off the mercury below and leaving the water above. Care must be taken that the mercury does not spurt out on one side and escape. An inverted bottle, or better, a vessel with a tube and stopcock below, is more convenient for this purpose.

When only a few drops of water are present they may be removed by blotting paper, or a camel's hair brush. Also by applying heat; but in this case a stain will be left when the water evaporates, unless it has been previously distilled.

To see if the mercury is pure pour it into a porcelain evaporating dish. If lead is present it will tarnish the sides. A thin film will also, after a short time, form on, its surface, due to oxidation; zinc and tin produce a similar effect. The surface when at rest should be very bright and almost invisible, and small globules, if detached, should be perfectly spherical, and not adhere to the glass but roll over it when the surface is inclined.

10. CALIBRATION BY MERCURY.

Apparatus. The best way to perform this experiment is that given by Bunsen in his Gasometry, p. 27. This method is substantially as follows: Select a glass tube, about 2 cm. in diameter, and 40 cm. long, closed at one end. Fasten to it a paper millimetre scale. This is placed upright in a stand, at a short distance from a small telescope, by which the scale may be read with accuracy. On another stand is placed a vessel containing about two kilogrammes of pure mercury, covered with a layer of concentrated sulphuric acid, with a stopcock below, by which it may be drawn off. A small glass tube, also closed at one end, is used to receive it, which should contain, when filled, about 10 cm.3 Its open end is ground flat, and it may be closed with a plate of ground glass, which is fastened to the thumb by a piece of rubber.

3

Experiment. Both mercury and tube should be perfectly clean, but if not, a few drops of water may be placed in the longer tube, provided great accuracy is not required. Fill the small tube with mercury, holding it with the fingers of the left hand, and remove the surplus by pressing the glass plate, which should be attached to the left thumb, down on to it. Take care that no air bubbles

are imprisoned. Empty the mercury into the large tube, and read its height on the scale by the telescope, measuring from the top of the curved surface of the liquid. A clean wooden rod may be used to remove any bubbles of air or globules of mercury which adhere to the sides of the tube. Repeat this operation until the large tube is full of mercury. We now wish to know the volume of the small tube, as this is the unit in terms of which the larger one has been calibrated. The most accurate way to do this is to weigh the whole amount of mercury transferred, and divide by the number of times the smaller tube has been filled. But as it is generally difficult to weigh so heavy a body accurately, the contents of the smaller tube had better be weighed alone, repeating two or three times to see how much the quantity used will vary in consecutive fillings. The volume is then obtained by dividing the weight by 13.6, the specific gravity of mercury. Multiplying the quotient by 1, 2, 3, 4, &c., we obtain the volumes corresponding to our observed readings of the mercury column in the long tube.

Represent the results by a residual curve, as follows: Let s be the scale reading when the small tube has been emptied once into the long tube, and s' when the latter is full, or has received n times this volume of mercury, which we will call v. Then (n - 1)v of mercury will fill the space s' s, and the average of length will equal (n - 1)v ÷ (s′ — s) = a.

volume per unit If the tube was V for any scale

perfectly cylindrical we could find the volume reading S by the formula, V = a ( S − s) + v. In reality the (S tube is probably a little larger in some places than in others, it is therefore better to retain only two significant figures in a, and then compute by the formula the volumes corresponding to the various scale readings that have been observed. Subtract each of these from the corresponding volumes 1, 2, 3, &c., times v, and construct a residual curve in which ordinates equal these differences on an enlarged scale, and abscissas the scale readings. We can now obtain the volume with the greatest accuracy for any scale reading by adding to the value of V given by the formula, the ordinate of the corresponding point of the curve. A table may thus be constructed, giving the volume corresponding to each millimetre mark of the scale. But it is generally sufficiently accurate to make a simple interpolation from the original measurements,

using only the first differences, as when employing logarithmic tables.

11. CALIBRATION BY WATER.

Apparatus. A Mohr's burette B, Fig. 10, on a stand, and the vessel to be graduated A, which should be about six inches high, and an inch and a half in diameter. A paper scale divided into tenths of an inch should be attached to A with gum tragacanth, although shellac, or even mucilage, answers tolerably. A long string wound spirally around the vessel will keep the scale in place until the gum is dry.

B

Experiment. Fill the burette B to the zero mark. This is done by adding a little too much water, and drawing it off by the stopcock C into another vessel, until it stands at precisely the right level. Next, let the water flow into A until it reaches the one tenth of an inch mark, and read B. Do the same for each tenth of an inch, until the one inch mark is reached, and then for every half inch to the top. Do not let the water level in B fall below the 100 cm.3 mark, but when it reaches this point refill as before, and add 100 to the volume measured. Care should be taken not to get too much water into A; should this happen, a little may be drawn out with a pipette and replaced in B, but a slight error is thus introduced.

D

Fig. 10.

A

We have now a series of volumes corresponding to various scale readings. Construct a curve with these two quantities as coordinates. Find the point of the curve for which the volume is in turn 10, 20, 30, &c., cm.3, and record the corresponding scalereading. If the vessel is to be used for the measurement of volumes cover it with wax and draw horizontal lines on the latter, having the scale readings just found. Subject it to the fumes of fluorhydric acid, formed by mixing powdered fluor spar and concentrated sulphuric acid. The lines will thus be permanently etched on the glass.

12. CATHETOMETER.

Apparatus. A Cathetometer may be made by using as a base the tripod of a music stand or photographer's head-rest, and screw

ing into it a tube or solid rod of brass. To this is attached a small telescope with a clamp and set screw, and some form of slow motion. The latter may be obtained by placing the telescope on a hinge and raising and lowering one end by a screw. The slight deviation from a horizontal position will not affect the results, as the instrument is here used.

At a distance of five or ten feet is placed a U tube, open at both ends, with one arm about ten inches long, the other forty. The bend in the tube is filled with mercury, and water is poured into the long arm. We then have a long column of water sustaining a short column of mercury, the heights being inversely as the densities. By the side of this tube is a barometer, made by closing a common glass tube at one end, filling with mercury, and inverting over a cistern containing the same liquid. The precautions and details will be found under Experiment No. 55. By the side of this tube is placed a rod about ten inches long, sharply pointed at both ends, and capable of moving up and down so as to touch the surface of the mercury in the barometer cistern. A steel scale divided into millimetres is adjacent to both tubes, so that it can be read at the same time as the mercury columns.

Experiment. Focus the telescope so that both scale and mer

cury are distinctly visible. Then raise it until it is nearly on a level with A, the top of the column of water, and bring its horizontal cross-hair exactly to coincide by the slow motion. Read the scale, dividing the millimetres into tenths by the eye. Do the same at B and C; then the difference in height of A and C, divided by that of C and B, will equal the specific gravity of the mer

cury, which should be compared with its true value. As the surface of mercury is curved upwards, that of water downwards, the cross-hairs should be brought to the top of the former, and to the bottom of the latter. If great accuracy is required in this experiment, allow for the meniscus, or curved portion at the top of the