Chapter C: C C C, fig. 60., is a horizontal section of the furnace over the (1)

grate, showing the width of the mouth-piece, or plate of wrought iron, which is 6 inches, and the opening which receives the muffle-plate.

_Fig._ 61. represents the muffle or pot, which is 12 inches long, 6 inches broad inside; in the clear 6-3/4: in height 4-1/2 inside measure, and nearly 5-1/2 in the clear.

_Fig._ 62., the muffle-plate, which is of the same size as the bottom of the muffle.

_Fig._ 63. is a representation of the sliding-door of the mouth-plate, as shewn at _d_, in _fig._ 58.

_Fig._ 64., a front view of the mouth-plate or piece, _d_, _fig._ 58.

_Fig._ 65., a representation of the mode of making, or shutting up with pieces of charcoal, the mouth of the furnace.

_Fig._ 66., the teaser for cleaning the grate.

_Fig._ 67., a larger teaser, which is introduced at the top of the furnace, for keeping a complete supply of charcoal around the muffle.

_Fig._ 68., the tongs used for charging the assays into the cups.

_Fig._ 69. represents a board of wood used as a register, and is divided into 45 equal compartments, upon which the assays are placed previously to their being introduced into the furnace. When the operation is performed, the cupels are placed in the furnace in situations corresponding to these assays on the board. By these means all confusion is avoided, and without this regularity it would be impossible to preserve the accuracy which the delicate operations of the assayer require.

I shall now proceed to a description of a small assay furnace, invented by Messrs. Anfrye and d’Arcet, of Paris. They term it, _Le Petit Fourneau à Coupelle_. _Fig._ 70. represents this furnace, and it is composed of a chimney or pipe of wrought iron _a_, and of the furnace B. It is 17-1/2 inches high, and 7-1/4 inches wide. The furnace is formed of three pieces; of a dome A; the body of the furnace B; and the ash-pit C, which is used as the base of the furnace, _fig._ 70. and 71. The principal piece, or body of the furnace, B, has the form of a hollow tower, or of a hollow cylinder, flattened equally at the two opposite sides parallel to the axis, in such a manner that the horizontal section is elliptical. The foot which supports it is a hollow truncated cone, flattened in like manner upon the two opposite sides, and having consequently for its basis two ellipses of different diameters; the smallest ought to be equal to that of the furnace, so that the bottom of the latter may exactly fit it. The dome, which forms an arch above the furnace, has also its base elliptical, whilst that of the superior orifice by which the smoke goes out preserves the cylindrical form. The tube of wrought iron is 18 inches long and 2-1/2 inches diameter, having one of its ends a little enlarged, and slightly conical, that it may be exactly fitted or jointed upon the upper part of the furnace dome _d_, _fig._ 70. At the union of the conical and cylindrical parts of the tube, there is placed a small gallery of iron, _e_, _fig._ 70, 71. See also a plan of it, _fig._ 72. This gallery is both ingenious and useful. Upon it are placed the cupels, which are thus annealed during the ordinary work of the furnace, that they may be introduced into the muffle, when it is brought into its proper degree of heat. A little above this gallery is a door _f_, by which, if thought proper, the charcoal could be introduced into the furnace; above that there is placed at _g_ a throttle valve, which is used for regulating the draught of the furnace at pleasure. Messrs. Anfrye and d’Arcet say, that, to give the furnace the necessary degree of heat so as to work the assays of gold, the tube must be about 18 inches above the gallery, for annealing or heating the cupels. The circular opening _h_, in the dome, _fig._ 70., and as seen in the section, _fig._ 71., is used to introduce the charcoal into the furnace: it is also used to inspect the interior of the furnace, and to arrange the charcoal round the muffle. This opening is kept shut during the working of the furnace, with the mouth-piece, of which the face is seen at _n_, _fig._ 71.

The section of the furnace, _fig._ 71., presents several openings, the principal of which is that of the muffle; it is placed at _i_; it is shut with the semicircular door _m_, _fig._ 70., and seen in the section _m_, _fig._ 71. In front of this opening, is the table or shelf, upon which the door of the muffle is made to advance or recede; the letter _q_, _fig._ 71., shows the face, side, and cross section of the shelf, which makes part of the furnace. Immediately under the shelf, is a horizontal slit, _l_, which is pierced at the level of the upper part of the grate, and used for the introduction of a slender rod of iron, that the grate may be easily kept clean. This opening is shut at pleasure, by the wedge represented at _k_, _fig._ 70. and 71.

Upon the back of the furnace is a horizontal slit _p_, _fig._ 71, which supports the fire-brick, _s_, and upon which the end of the muffle, if necessary, may rest; _u_, _fig._ 71., is the opening in the furnace where the muffle is placed.

The plan of the grate of the furnace is an ellipse: _fig._ 73. is a horizontal view of it. The dimensions of that ellipsis determine the general form of the furnace, and thickness of the grate. To give strength and solidity to the grate, it is encircled by a bar or hoop of iron. There is a groove in which the hoop of iron is fixed. The holes of the grate are truncated cones, having the greater base below, that the ashes may more easily fall into the ash-pit. The letter _v_, _fig._ 71., shows the form of these holes. The grate is supported by a small bank or shelf, making part of the furnace, as seen at _a_, _fig._ 71.

The ash-pit, C, has an opening _y_ in front, _fig._ 71.; and is shut when necessary by the mouth-piece _r_, _fig._ 70. and 71.

To give strength and solidity to the furnace, it is bound with hoops of iron, at _b_, _b_, _b_, _b_, _fig._ 70.

_Figs._ 74. 75. 76. are views of the muffle.

_Fig._ 77. is a view of a crucible for annealing gold.

_Figs._ 78. 79. 80. are cupels of various sizes, to be used in the furnace. They are the same as those used by assayers in their ordinary furnaces.

_Figs._ 81. and 82. are views of the hand-shovels, used for filling the furnace with charcoal; they should be made of such size and form as to fit the opening _h_, in _figs._ 70. and 71.

The smaller pincers or tongs, by which the assays are charged into the cupels, and by which the latter are withdrawn from the furnace, as well as the teaser for cleaning the grate of the furnace, are similar to those used in the British Mint.

In the furnace of the Mint above described, the number of assays that can be made at one time, is 45. The same number of cupels are put into the muffle. The furnace is then filled with charcoal to the top, and upon this are laid a few pieces already ignited. In the course of three hours, a little more or less, according to circumstances, the whole is ignited; during which period, the muffle, which is made of fire-clay, is gradually heated to redness, and is prevented from cracking; which a less regular or more sudden increase of temperature would not fail to do: the cupels, also, become properly annealed. All moisture being dispelled, they are in a fit state to receive the piece of silver or gold to be assayed.

The greater care that is exercised in this operation, the less liable is the assayer to accidents from the breaking of the muffle; which it is both expensive and troublesome to fit properly into the furnace.

The cupels used in the assay process, are made of the ashes of burnt bones (phosphate of lime). In the Royal Mint, the cores of ox-horn are selected for this purpose; and the ashes produced are about four times the expense of the bone-ash, used in the process of cupellation upon the large scale. So much depends upon the accuracy of an assay of gold or silver, where a mass of 15lbs. troy in the first, and 60lbs. troy in the second instance, is determined by the analysis of a portion not exceeding 20 troy grains, that every precaution which the longest experience has suggested, is used to obtain an accurate result. Hence the attention paid to the selection of the most proper materials for making the cupels.

The cupels are formed in a circular mould made of cast steel, very nicely turned, by which means they are easily freed from the mould when struck. The bone-ash is used moistened with a quantity of water, sufficient to make the particles adhere firmly together. The circular mould is filled, and pressed level with its surface; after which, a pestle or rammer, having its end nicely turned, of a globular or convex shape, and of a size equal to the degree of concavity wished to be made in the cupel for the reception of the assay, is placed upon the ashes in the mould, and struck with a hammer until the cupel is properly formed. These cupels are allowed to dry in the air for some time before they are used. If the weather is fine, a fortnight will be sufficient.

An assay may prove defective for several reasons. Sometimes the button or bead sends forth crystalline vegetations on its surface with such force, as to make one suppose a portion of the silver may be thrown out of the cupel. When the surface of the bead is dull and flat, the assay is considered to have been too hot, and it indicates a loss of silver in fumes. When the tint of the bead is not uniform, when its inferior surface is bubbly, when yellow scales of oxide of lead remain on the bottom of the cupel, and the bead adheres strongly to it, by these signs it is judged that the assay has been too cold, and that the silver retains some lead.

Lastly, the assay is thought to be good if the bead is of a round form, if its upper surface is brilliant, if its lower surface is granular and of a dead white, and if it separates readily from the cupel.

After the lead is put into the cupel, it gets immediately covered with a coat of oxide, which resists the admission of the silver to be assayed into the melted metal; so that the alloy cannot form. When a bit of silver is laid on a lead bath in this predicament, we see it swim about for a long time without dissolving. In order to avoid this result, the silver is wrapped up in a bit of paper; and the carburetted hydrogen generated by its combustion, reduces the film of the lead oxide, gives the bath immediately a bright metallic lustre, and enables the two metals readily to combine.

As the heat rises, the oxide of lead flows round about over the surface, till it is absorbed by the cupel. When the lead is wasted to a certain degree, a very thin film of it only remains on the silver, which causes the iridescent appearance, like the colours of soap-bubbles; a phenomenon, called by the old chemists, fulguration.

When the cupel cools in the progress of the assay, the oxygenation of the lead ceases; and, instead of a very liquid vitreous oxide, an imperfectly melted oxide is formed, which the cupel cannot absorb. To correct a cold assay, the temperature of the furnace ought to be raised, and pieces of paper ought to be put into the cupel, till the oxide of lead which adheres to it, be reduced. On keeping up the heat, the assay will resume its ordinary train.

Pure silver almost always vegetates. Some traces of copper destroy this property, which is obviously due to the oxygen which the silver can absorb while it is in fusion, and which is disengaged the moment it solidifies. An excess of lead, by removing all the copper at an early stage, tends to cause the vegetation.

The brightening is caused by the heat evolved, when the button passes from the liquid to the solid state. Many other substances present the same phenomenon.

In the above operation it is necessary to employ lead which is very pure, or at least free from silver. That kind is called _poor lead_.

It has been observed at all times, that the oxide of lead carries off with it, into the cupel, a little silver in the state of an oxide. This effect becomes less, or even disappears, when there is some copper remaining; and the more copper, the less chance there is of any silver being lost. The loss of silver increases, on the other hand, with the dose of lead. Hence the reason why it is so important to proportion the lead with a precision which, at first sight, would appear to be superfluous. Hence, also, the reason of the attempts which have, of late years, been made to change the whole system of silver assays, and to have recourse to a method exempt from the above causes of error.

M. d’Arcet, charged by the Commission of the Mint in Paris, to examine into the justice of the reclamations made by the French silversmiths against the public assays, ascertained that they were well founded; and that the results of cupellation gave for the alloys between 897 and 903 thousandths (the limits of their standard coin) an inferior standard, by from 4 to 5 thousandth parts, from the standard or title which should result from the absolute or actual alloy.

The mode of assay shows, in fact, that an ingot, experimentally composed of 900 thousandths of fine silver, and 100 thousandths of copper, appears, by cupellation, to be only, at the utmost, 896 or 897 thousandths; whereas fine silver, of 1000 thousandths, comes out nearly of its real standard. Consequently a director of the Mint, who should compound his alloy with fine silver, would be obliged to employ 903 or 904 thousandths, in order that, by the assay in the laboratory of the Mint, it should appear to have the standard of 900 thousandths. These 3 or 4 thousandths would be lost to him, since they would be disguised by the mode of assay, the definitive criterion of the quantity of silver, of which the government keeps count from the coiner of the money.

From experiments subsequently made by M. d’Arcet, it appears that silver assays always suffer a loss of the precious metal, which varies, however, with the standard of the alloy. It is 1 thousandth for fine silver,

4·3 thousandths for silver of 900 thousandths,
4·9 -- for -- of 800 --
4·2 -- for -- of 500 --

and diminishes thereafter, progressively, till the alloy contains only 100 thousandths of silver, at which point the loss is only 0·4.

Assays requested by the Commission of the Paris Mint, from the assayers of the principal Royal Mints in Europe, to which the same alloys, synthetically compounded, were sent, afforded the results inscribed in the following table.

+----------------------+----------+-----------------------------+
| | | Standards found for the |
| | Cities | Mathematical Alloys. |
|Names of the Assayers.|where they+---------+---------+---------+
| | reside. |950 mill.|900 mill.|800 mill.|
+----------------------+----------+---------+---------+---------+
|F. de Castenhole, Mint| | | | |
|Assayer |Vienna |946·20 |898·40 |795·10 |
|A. R. Vervaëz, Ditto |Madrid |944·40 |893·70 |789·20 |
|D. M. Cabrera, Assayer| | | | |
|in Spain |Ditto |944·40 |893·70 |788·60 |
|Assayer |Amsterdam |947·00 |895·00 |795·00 |
|Mr. Bingley, Assay | | | | |
|Master |London |946·25 |896·25 |794·25 |
|Mr. Johnson, Assayer |Ditto |933·33 |883·50 |783·33 |
|Inspector of the Mint |Utrecht |945·00 |896·50 |799·00 |
|Assayer of the Mint |Naples |945·00 |891·00 |787·00 |
|Assayer of Trade |Ditto |945·00 |891·00 |787·00 |
|Assayer of the Mint |Hamburgh |946·13/72|897·41/72|798·44/72|
|Ditto |Altona |942·1/4 |894·00 |790 |
+----------------------+----------+---------+---------+---------+

These results, as well as those in still greater numbers, obtained from the ablest Parisian assayers, upon identical alloys of silver and copper, prove that the mode of assay applied to them brings out the standard too low; and further, that the quantity of silver masked or disguised, is not uniform for these different eminent assay masters. An alloy, for example, at the standard of 900 thousandths is judged at

M.
the Mint of Paris to have a standard of 895·6
At that of Vienna -- 898·4
-- Madrid -- 893·7
-- Naples -- 891·0

The fact thus so clearly made out of a loss in the standard of silver bullion and coin, merits the most serious attention; and it will appear astonishing, perhaps, that a thing recurring every day, should have remained for so long a time in the dark. In reality, however, the fact is not new; as the very numerous and well-made experiments of Tillet from 1760 to 1763, which are related in the memoirs of the Academy of Sciences, show, in the silver assays, a loss still greater than that which was experienced lately in the laboratory of the Commission of the French Mint. But he thought that, as the error was common to the nations in general, it was not worth while or prudent to introduce any innovation.

A mode of assaying, to give, with certainty, the standard of silver bullion, should be entirely independent of the variable circumstances of temperature, and the unknown proportions of copper, so difficult to regulate by the mere judgment of the senses. The process by the humid way, recommended by me to the Royal Mint in 1829, and exhibited as to its principles before the Right Honourable John Herries, then Master, in 1830, has all the precision and certainty we could wish. It is founded on the well-known property which silver has, when dissolved in nitric acid, to be precipitated in a chloride of silver quite insoluble, by a solution of sea salt, or by muriatic acid; but, instead of determining the weight of the chloride of silver, which would be somewhat uncertain and rather tedious, on account of the difficulty of drying it, we take the quantity of the solution of sea salt which has been necessary for the precipitation of the silver. To put the process in execution, a liquor is prepared, composed of water and sea salt in such proportions that 1000 measures of this liquor may precipitate, completely, 12 grains of silver, perfectly pure, or of the standard 1000, previously dissolved in nitric acid. The liquor thus prepared, gives, immediately, the true standard of any alloy whatever, of silver and copper, by the weight of it which may be necessary to precipitate 12 grains of this alloy. If, for example 905 measures have been required to precipitate the 12 grains of alloy, its standard would be 905 thousandths.

The process by the humid way is, so to speak, independent of the operator. The manipulations are so easy; and the term of the operation is very distinctly announced by the absence of any sensible nebulosities on the affusion of sea salt into the silver solution, while there remains in it 1/2 thousandth of metal. The process is not tedious, and in experienced hands it may rival the cupel in rapidity; it has the advantage over the cupel of being more within the reach of ordinary operators, and of not requiring a long apprenticeship. It is particularly useful to such assayers as have only a few assays to make daily, as it will cost them very little time and expense.

By agitating briskly during two minutes, or thereby, the liquid rendered milky by the precipitation of the chloride of silver, it may be sufficiently clarified to enable us to appreciate, after a few moments of repose, the disturbance that can be produced in it by the addition of 1000 of a grain of silver. Filtration is more efficacious than agitation, especially when it is employed afterwards; it may be sometimes used; but agitation, which is much more prompt, is generally sufficient. The presence of lead and copper, or any other metal, except mercury, has no perceptible influence on the quantity of sea salt necessary to precipitate the silver; that is to say, the same quantity of silver, pure or alloyed, requires for its precipitation a constant quantity of the solution of sea salt.

Supposing that we operate upon a gramme of pure silver, the solution of sea salt ought to be such that 100 centimetres cube may precipitate exactly the whole silver. The standard of an alloy is given by the number of thousandths of solution of sea salt necessary to precipitate the silver contained in a gramme of the alloy.

When any mercury is accidentally present, which is, however, a rare occurrence, it is made obvious by the precipitated chloride remaining white when exposed to daylight, whereas when there is no mercury present, it becomes speedily first grey and then purple. Silver so contaminated must be strongly ignited in fusion before being assayed, and its loss of weight noted. In this case, a cupel assay must be had recourse to.

_Preparation of the Normal Solution of Sea Salt, when it is measured by Weight._--Supposing the sea salt pure as well as the water, we have only to take these two bodies in the proportion of 0·5427 k. of salt to 99·4573 k. of water, to have 100 k. of solution, of which 100 grammes will precipitate exactly one gramme of silver. But instead of pure salt, which is to be procured with difficulty, and which besides may be altered readily by absorbing the humidity of the air, a concentrated solution of the sea salt of commerce is to be preferred, of which a large quantity may be prepared at a time, to be kept in reserve for use, as it is wanted. _Instruction de Gay Lussac._

_Preparation of the Normal Solution of Sea Salt, when measured by Volume._--The measure by weight has the advantage of being independent of temperature, of having the same degree of precision as the balance, and of standing in need of no correction. The measure by volume has not all these advantages; but, by giving it sufficient precision, it is more rapid, and is quite sufficient for the numerous daily assays of the mint. This normal solution is so made, that a volume equal to that of 100 grammes of water, or 100 centimetres cube, at a determinate temperature, may precipitate exactly one gramme of silver. The solution may be kept at a constant temperature, and in this case the assay stands in want of no correction; or if its temperature be variable, the assay must be corrected according to its influence. These two circumstances make no change in the principle of the process, but they are sufficiently important to occasion some modifications in the apparatus. Experience has decided the preference in favour of applying a correction to a variable temperature.

We readily obtain a volume of 100 cubic centimetres by means of a _pipette_, _fig._ 83., so gauged that when filled with water up to the mark _a_, _b_, and well dried at its point, it will run out, at a continuous efflux, 100 grammes of water at the temperature of 15 C. (59 Fah.). We say purposely at one efflux, because after the cessation of the jet, the pipette may still furnish two or three drops of liquid, which must not be counted or reckoned upon. The weight of the volume of the normal solution, taken in this manner with suitable precautions, will be uniform from one extreme to another, upon two centimetres and a half, at most, or to a quarter of a thousandth, and the difference from the mean will be obviously twice less, or one half. Let us indicate the most simple manner of taking a measure of the normal solution of sea salt.

After having immersed the beak _c_ of the pipette in the solution, we apply suction by the mouth, to the upper orifice, and thereby raise the liquid to _d_ above the circular line _a b_. We next apply neatly the forefinger of one hand to this orifice, remove the pipette from the liquid, and seize it as represented in _fig._ 84. The mark _a b_ being placed at the level of the eye, we make the surface of the solution become exactly a tangent to the plane _a b_. At the instant it becomes a tangent, we leave the beak _c_ of the pipette open, by taking away the finger that had been applied to it, and without changing any thing else in the position of the hands, we empty it into the bottle which should receive the solution, taking care to remove it whenever the efflux has run out.

If after filling the pipette by suction, any one should find a difficulty in applying the forefinger fast enough to the upper orifice, without letting the liquid run down below the mark _a b_, he should remove the pipette from the solution with its top still closed with his tongue, then apply the middle finger of one of his hands to the lower orifice; after which he may withdraw his tongue, and apply the forefinger of the other hand to the orifice previously wiped. This mode of obtaining a measure of normal solution of sea salt is very simple, and requires no complex apparatus; but we shall indicate another manipulation still easier, and also more exact.

In this new process the pipette is filled from the top like a bottle, instead of being filled by suction, and it is moreover fixed. _Fig._ 85. represents the apparatus. D and D´ are two sockets separated by a stop cock R. The upper one, tapped interiorly, receives, by means of a cork stopper L, the tube T, which admits the solution of sea salt. The lower socket is cemented on to the _pipette_; it bears a small air-cock R´, and a screw plug V, which regulates a minute opening intended to let the air enter very slowly into the pipette. Below the stop-cock R´, a silver tube N, of narrow diameter, soldered to the socket, leads the solution into the _pipette_, by allowing the air, which it displaces, to escape by the stop-cock R´. The screw plug, with the milled head V´, replaces the ordinary screw by which the key of the stop-cock may be made to press, with more or less force, upon its conical seat.

_Fig._ 86. represents, in a side view, the apparatus just described. We here remark an air-cock R, and an opening _m_. At the extremity Q of the same figure, the conical pipe T enters, with friction. It is by this pipe that the air is sucked into the pipette, when it is to be filled from its beak.

The _pipette_ is supported by two horizontal arms H K (_fig._ 87.) moveable about a common axis A A, and capable of being drawn out or shortened by the aid of two longitudinal slits. They are fixed steadily by two screw nuts _e e´_, and their distance may be varied by means of round bits of wood or cork interposed, or even by opposite screw nuts _o o´_. The upper arm H is pierced with a hole, in which is fixed, by the pressure of a wooden screw _v_, the socket of the _pipette_. The corresponding hole of the lower arm is larger; and the beak of the _pipette_ is supported in it by a cork stopper L. The apparatus is fixed by its tail-piece P, by means of a screw to the corner of a wall, or any other prop.

The manner of filling the pipette is very simple. We begin by applying the fore-finger of the left hand to the lower aperture _c_; we then open the two stop-cocks R and R´. Whenever the liquor approaches the neck of the _pipette_, we must temper its influx, and when it has arrived at some millimetres above the mark _a b_, we close the two stop-cocks, and remove our forefinger. We have now nothing more to do than to regulate the _pipette_; for which purpose the liquid must touch the line _a b_, and must simply adhere externally to the beak of the _pipette_.

This last circumstance is easily adjusted. After taking away the finger which closed the aperture _c_ of the _pipette_, we apply to this orifice a moist sponge _m_, _fig._ 88., wrapped up in a linen rag, to absorb the superfluous liquor as it drops out. This sponge is called the handkerchief (_mouchoir_), by M. Gay Lussac. The _pipette_ is said to be wiped when there is no liquor adhering to its point exteriorly.

For the convenience of operating, the handkerchief is fixed by friction in a tube of tin plate, terminated by a cup, open at bottom to let the droppings flow off into the cistern C, to which the tube is soldered. It may be easily removed for the purpose of washing it; and, if necessary, a little wedge of wood, _o_, can raise it towards the _pipette_.

To complete the adjustment of the _pipette_, the liquid must be made merely to descend to the mark _a_, _b_. With this view, and whilst the handkerchief is applied to the beak of the _pipette_, the air must be allowed to enter very slowly by unscrewing the plug V, _fig._ 85.; and at the moment of the contact the handkerchief must be removed, and the bottle F, destined to receive the solution, must be placed below the orifice of the _pipette_, _fig._ 88. As the motion must be made rapidly, and without hesitation, the bottle is placed in a cylinder of tin-plate, of a diameter somewhat greater, and forming one body with the cistern and the handkerchief. The whole of this apparatus has for a basis a plate of tinned iron, moveable between two wooden rulers R R, one of which bears a groove, under which the edge of the plate slips. Its traverses are fixed by two abutments _b b_, placed so that when it is stopped by one of them, the beak of the _pipette_ corresponds to the centre of the neck of the bottle, or is a tangent to the handkerchief. This arrangement, very convenient for wiping the _pipette_ and emptying it, gives the apparatus sufficient solidity, and allows of its being taken away, and replaced without deranging any thing. It is obvious that it is of advantage, when once the entry of the air into the _pipette_ has been regulated by the screw V, to leave it constantly open, because the motion from the handkerchief to the bottle is performed with sufficient rapidity to prevent a drop of the solution from collecting and falling down.

_Temperature of the Solution._--After having described the manner of measuring by volume the normal solution of the sea salt, we shall indicate the most convenient means of taking the temperature. The thermometer is placed in a tube of glass T, _fig._ 89., which the solution traverses to arrive at the _pipette_. It is suspended in it by a piece of cork, grooved on the four sides to afford passage to the liquid. The scale is engraved upon the tube itself, and is repeated at the opposite side, to fix the eye by the coincidence of this double division at the level of the thermometric column. The tube is joined below to another narrower one, through which it is attached by means of a cork stopper B, in the socket of the stop-cock of the _pipette_. At its upper part it is cemented into a brass socket, screw-tapped in the inside, which is connected in its turn by a cock, with the extremity, also tapped, of the tube above T, belonging to the reservoir of the normal solution. The corks employed here as connecting links between the parts of the apparatus, give them a certain flexibility, and allow of their being dismounted and remounted in a very short time; but it is indispensable to make them be traversed by a hollow tube of glass or metal, which will hinder them from being crushed by the pressure they are exposed to. If the precaution be taken to grease them with a little suet and to fill their pores, they will suffer no leakage.

_Preservation of the Normal Solution of Sea Salt in metallic Vessels._--M. Gay Lussac uses for this purpose a cylindrical vessel or drum of copper, of a capacity of about 110 litres, having its inside covered with a rosin and wax cement.

_Preparation of the Normal Solution of Sea Salt, measuring it by Volume._--If the drum contains 110 litres, we should put only 105 into it, in order that sufficient space may be left for agitating the liquor without throwing it out. According to the principle that 100 centimetres cube, or 1/10 of a litre of the solution should contain enough of sea salt to precipitate a gramme of pure silver; and, admitting moreover, 13·516 for the prime equivalent of silver, and 7·335 for that of sea salt, we shall find the quantity of pure salt that should be dissolved in the 105 litres of water, and which corresponds to 105 × 10 = 1050 grammes of silver, to be by the following proportion:--

13·516 : 7·335 ∷ 1050 gramm. : x = 569·83 gr.

And as the solution of the sea salt of commerce, formerly mentioned, contains approximately 250 grammes per kilogramme, we must take 2279·3 grammes of this solution to have 569·83 gram. of salt. The mixture being perfectly made, the tubes and the _pipette_ must be several times washed by running the solution through them, and putting it into the drum. The standard of the solution must be determined after it has been well agitated, supposing the temperature to remain uniform.

To arrive more conveniently at this result, we begin by preparing two _decimes_ solutions; one of silver, and another of sea salt.

The decime solution of silver is obtained by dissolving 1 gramme of silver in nitric acid, and diluting the solution with water till its volume become a litre.

The decime solution of sea salt may be obtained by dissolving 0·543 grammes of pure sea salt in water, so that the solution shall occupy a litre; but we shall prepare it even with the normal solution which we wish to test, by mixing a measure of it with 9 measures of water; it being understood that this solution is not rigorously equivalent to that of silver, and that it will become so, only when the normal solution employed for its preparation shall be finally of the true standard. Lastly, we prepare beforehand several stoppered phials, in each of which we dissolve 1 gramme of silver in 8 or 10 grammes of nitric acid. For brevity’s sake we shall call these tests.

Now to investigate the standard of the normal solution, we must transfer a _pipette_ of it into one of these test phials; and we must agitate the liquors briskly to clarify them. After some instants of repose, we must pour in 2 thousandths of the _decime_ solution of sea salt, which, we suppose, will produce a precipitate. The normal liquor is consequently too feeble; and we should expect this, since the sea salt employed was not perfectly pure. We agitate and add 2 fresh thousandths, which will also produce a precipitate. We continue thus by successive additions of 2 thousandths, till the last produces no precipitation. Suppose that we have added 16 thousandths: the last two should not be reckoned, as they produced no precipitate; the preceding two were necessary, but only in part; that is to say, the useful thousandths added are above 12 and below 14, or otherwise they are on an average equal to 13.

Thus, in the condition of the normal solution, we require 1013 parts of it to precipitate one gramme of silver, while we should require only 1000. We shall find the quantity of concentrated solution of sea salt that we should add, by noting that the quantity of solution of sea salt, at first employed, viz. 2279·3 grammes, produced a standard of only 987 thousandths = 1000 - 13; and by using the following proportion:

987 : 2279·3 ∷ 13 : x = 30·02 grammes.

This quantity of the strong solution of salt, mixed with the normal solution in the drum, will correct its standard, and we shall now see by how much.

After having washed the tubes and the _pipette_, with the new solution, we must repeat the experiment upon a fresh gramme of silver. We shall find, for example, in proceeding only by a thousandth at a time, that the first causes a precipitate, but not the second. The standard of the solution is still too weak, and is comprised between 1000 and 1001; that is to say, it may be equal to 1000-1/2, but we must make a closer approximation.

We pour into the test bottle 2 thousandths of the _decime_ solution of silver, which will destroy, perceptibly, two thousandths of sea salt, and the operation will have retrograded by two thousandths; that is to say, it will be brought back to the point at which it was first of all. If, after having cleared up the liquor, we add half a thousandth of the decime solution, there will necessarily be a precipitate, as we knew beforehand, but a second will cause no turbidity. The standard of the normal liquor will be consequently comprehended between 1000 and 1000-1/2, or equal to 1000-1/4.

We should rest content with this standard, but if we wish to correct it, we may remark that the two quantities of solution of salt added, viz. 2279·3 gr. + 30·02 gr. = 2309·32 gr. have produced only 999·75 thousandths, and that we must add a new quantity of it corresponding to 1/4 of a thousandth. We make, therefore, the proportion

999·75 : 2309·32 ∷ 0·25 : x.

But since the first term differs very little from 1000, we may content ourselves to have x by taking the 0·25/1000 of 2309·32, and we shall find 0·577 gr. for the quantity of solution of sea salt to be added to the normal solution.

It is not convenient to take exactly so small a quantity of solution of sea salt by the balance, but we shall succeed easily by the following process. We weigh 50 grammes of this solution, and we dilute it with water; so that it occupies exactly half a litre, or 500 centimetres cube. A _pipette_ of this solution, one centimetre cube in volume, will give a decigramme of the primitive solution, and as such a small _pipette_ is divided into twenty drops, each drop, for example, will represent 5 milligrammes of the solution. We should arrive at quantities smaller still by diluting the solution with a proper quantity of water; but greater precision would be entirely needless.

The testing of the normal liquor just described, is, in reality, less tedious than might be supposed. It deserves also to be remarked, that liquor has been prepared for more than 1000 assays; and that, in preparing a fresh quantity, we shall obtain directly its true standard, or nearly so, if we bear in mind the quantities of water and solution of salt which had been employed.

_Correction of the Standard of the Normal Solution of Sea Salt, when the Temperature changes._--We have supposed, in determining the standard of the normal solution of sea salt, that the temperature remained uniform. The assays made in such circumstances, have no need of correction; but if the temperature should change, the same measure of the solution will not contain the same quantity of sea salt. Supposing that we have tested the solution of the salt at the temperature of 15° C.; if, at the time of making the experiment, the temperature is 18° C., for example, the solution will be too weak on account of its expansion, and the _pipette_ will contain less of it by weight; if, on the contrary, the temperature has fallen to 12°, the solution will be thereby concentrated and will prove too strong. It is therefore proper to determine the correction necessary to be made, for any variation of temperature.

To ascertain this point, the temperature of the solution of sea salt was made successively to be 0°, 5°, 10°, 15°, 20°, 25°, and 30° C.; and three _pipettes_ of the solution were weighed exactly at each of these temperatures. The third of these weighings gave the mean weight of a _pipette_. The corresponding weights of a _pipette_ of the solution, were afterwards graphically interpolated from degree to degree. These weights form the second column of the following table, intitled, _Table of Correction for the Variations in the Temperature of the Normal Solution of the Sea Salt_. They enable us to correct any temperature between 0 and 30 degrees centigrade (32° and 86° Fahr.) when the solution of sea salt has been prepared in the same limits.

Let us suppose, for example, that the solution has been made standard at 15°, and that at the time of using it, the temperature has become 18°. We see by the second column of the table, that the weight of a measure of the solution is 100·099 gr. at 15°, and 100·065 at 18°; the difference 0·034 gr., is the quantity of solution less which has been really taken; and of course we must add it to the normal measure, in order to make it equal to one thousand _millièmes_. If the temperature of the solution had fallen to 10 degrees, the difference of the weight of a measure from 10 to 15 degrees would be 0·019 gr. which we must on the contrary deduct from the measure, since it had been taken too large. These differences of weight of a measure of solution at 15°, from that of a measure at any other temperature, form the column 15° of the table, where they are expressed in thousandths; they are inscribed on the same horizontal lines as the temperatures to which each of them relates with the sign + _plus_, when they must be added, and with the sign - _minus_, when they must be subtracted. The columns 5°, 10°, 20°, 25°, 35°, have been calculated in the same manner for the cases in which the normal solution may have been graduated to each of these temperatures. Thus, to calculate the column 10, the number 100·118 has been taken of the column of weights for a term of departure, and its difference from all the numbers of the same column has been sought.

Table of Correction for the Variations in the Temperature of the Normal Solution of the Sea Salt.

+-------+-------+-----+-----+-----+-----+-----+-----+
|Temper-|Weight.| 5° | 10° | 15° | 20° | 25° | 30° |
|ature. | | | | | | | |
+-------+-------+-----+-----+-----+-----+-----+-----+
| | gram. |mill.|mill.|mill.|mill.|mill.|mill.|
| 4 |100,109| 0·0|- 0·1|+ 0·1|+ 0·7|+ 1·7|+ 2·7|
| 5 |100,113| 0·0|- 0·1|+ 0·1|+ 0·7|+ 1·7|+ 2·8|
| 6 |100,115| 0·0| 0·0|+ 0·2|+ 0·8|+ 1·7|+ 2·8|
| 7 |110,118|+ 0·1| 0·0|+ 0·2|+ 0·8|+ 1·7|+ 2·8|
| 8 |100,120|+ 0·1| 0·0|+ 0·2|+ 0·8|+ 1·8|+ 2·8|
| 9 |100,120|+ 0·1| 0·0|+ 0·2|+ 0·8|+ 1·8|+ 2·8|
| 10 |100,118|+ 0·1| 0·0|+ 0·2|+ 0·8|+ 1·7|+ 2·8|
| 11 |100,116| 0·0| 0·0|+ 0·2|+ 0·8|+ 1·7|+ 2·8|
| 12 |100,114| 0·0| 0·0|+ 0·2|+ 0·8|+ 1·7|+ 2·8|
| 13 |100,110| 0·0|- 0·1|+ 0·1|+ 0·7|+ 1·7|+ 2·7|
| 14 |100,106|- 0·1|- 0·1|+ 0·1|+ 0·7|+ 1·6|+ 2·7|
| 15 |100,099|- 0·1|- 0·2|- 0·0|+ 0·6|+ 1·6|+ 2·6|
| 16 |100,090|- 0·2|- 0·3|- 0·1|+ 0·5|+ 1·5|+ 2·5|
| 17 |100,078|- 0·4|- 0·4|- 0·2|+ 0·4|+ 1·3|+ 2·4|
| 18 |100,065|- 0·5|- 0·5|- 0·3|+ 0·3|+ 1·2|+ 2·3|
| 19 |100,053|- 0·6|- 0·7|- 0·5|+ 0·1|+ 1·1|+ 2·2|
| 20 |100,039|- 0·7|- 0·8|- 0·6| 0·0|+ 1·0|+ 2·0|
| 21 |100,021|- 0·9|- 1·0|- 0·8|- 0·2|+ 0·8|+ 1·9|
| 22 |100,001|- 1·1|- 1·2|- 1·0|- 0·4|+ 0·6|+ 1·7|
| 23 | 99,983|- 1·3|- 1·4|- 1·2|- 0·6|+ 0·4|+ 1·5|
| 24 | 99,964|- 1·5|- 1·5|- 1·4|- 0·8|+ 0·2|+ 1·3|
| 25 | 99,944|- 1·7|- 1·7|- 1·6|- 1·0| 0·0|+ 1·1|
| 26 | 99,924|- 1·9|- 1·9|- 1·8|- 1·2|- 0·2|+ 0·9|
| 27 | 99,902|- 2·1|- 2·2|- 2·0|- 1·4|- 0·4|+ 0·7|
| 28 | 99,879|- 2·3|- 2·4|- 2·2|- 1·6|- 0·7|+ 0·4|
| 29 | 99,858|- 2·6|- 2·6|- 2·4|- 1·8|- 0·9|+ 0·2|
| 30 | 99,836|- 2·8|- 2·8|- 2·6|- 2·0|- 1·1| 0·0|
+-------+-------+-----+-----+-----+-----+-----+-----+

Several expedients have been employed to facilitate and abridge the manipulations. In the first place, the phials for testing or assaying the specimens of silver should all be of the same height and of the same diameter. They should be numbered at their top, as well as on their stoppers, in the order 1, 2, 3, &c. They may be ranged successively in tens; the stoppers of the same series being placed on a support in their proper order. Each two phials should, in their turn, be placed in a japanned tin case (_fig._ 90.) with ten compartments duly numbered. These compartments are cut out anteriorly to about half their height, to allow the bottoms of the bottles to be seen. When each phial has received its portion of alloy, through a wide-beaked funnel, there must be poured into it about 10 grammes of nitric acid, of specific gravity 1·28, with a _pipette_, containing that quantity; it is then exposed to the heat of a water bath, in order to facilitate the solution of the alloy. The water bath is an oblong vessel made of tin plate, intended to receive the phials. It has a moveable double bottom, pierced with small holes, for the purpose of preventing the phials being broken, as it insulates them from the bottom to which the heat is applied. The solution is rapid; and, since it emits nitrous vapours in abundance, it ought to be carried on under a chimney.

_The agitator._--_Fig._ 91. gives a sufficiently exact idea of it, and may dispense with a lengthened description. It has ten cylindrical compartments, numbered from 1 to 10. The phials, after the solution of the alloy, are arranged in it in the order of their numbers. The agitator is then placed within reach of the _pipette_, intended to measure out the normal solution of sea salt, and a _pipette_ full of this solution is put into each phial. Each is then closed with its glass stopper, previously dipped in pure water. They are fixed in the cells of the agitator by wooden wedges. The agitator is then suspended to a spring R, and, seizing it with the two hands, the operator gives an alternating rapid movement, which agitates the solution, and makes it, in less than a minute, as limpid as water. This movement is promoted by a spiral spring, B, fixed to the agitator and the ground; but this is seldom made use of, because it is convenient to be able to transport the agitator from one place to another. When the agitation is finished, the wedges are to be taken out, and the phials are placed in order upon a table furnished with round cells destined to receive them, and to screen them from too free a light.

When we place the phials upon this table, we must give them a brisk circular motion, to collect the chloride of silver scattered round their sides; we must lift out their stoppers, and suspend them in wire rings, or pincers. We next pour a thousandth of the decime solution into each phial; and before this operation is terminated, there is formed in the first phials, when there _should be_ a precipitate, a nebulous stratum, very well marked, of about a centimetre in thickness.

At the back of the table there is a black board divided into compartments numbered from 1 to 10, upon each of which we mark, with chalk, the thousandths of the decime liquor put into the correspondent phial. The thousandths of sea salt, which indicate an augmentation of standard, are preceded by the sign +, and the thousandths of nitrate of silver by the sign -.

When the assays are finished, the liquor of each phial is to be poured into a large vessel, in which a slight excess of sea salt is kept; and when it is full, the supernatant clear liquid must be run off with a syphon.

The chloride of silver may be reduced without any perceptible loss. After having washed it well, we immerse pieces of iron or zinc into it, and add sulphuric acid in sufficient quantity to keep up a feeble disengagement of hydrogen gas. The mass must not be touched. In a few days the silver is completely reduced. This is easily recognised by the colour and nature of the product; or by treating a small quantity of it with water of ammonia, we shall see whether there be any chloride unreduced; for it will be dissolved by the ammonia, and will afterwards appear upon saturating the ammonia with an acid. The chlorine remains associated with the iron or the zinc in a state of solution. The first washings of the reduced silver must be made with an acidulous water, to dissolve the oxide of iron which may have been formed, and the other washings with common water. After decanting the water of the last washing, we dry the mass, and add a little powdered borax to it. It must be now fused. The silver being in a bulky powder is to be put in successive portions into a crucible as it sinks down. The heat should be at first moderate; but towards the end of the operation it must be pretty strong to bring into complete fusion the silver and the scoriæ, and to effect their complete separation. In case it should be supposed that the whole of the silver had not been reduced by the iron or zinc, a little carbonate of potash should be added to the borax. The silver may also be reduced by exposing the chloride to a strong heat, in contact with chalk and charcoal.

The following remarks by M. Gay Lussac, the author of the above method, upon the effect of a little mercury in the humid assay, are important:--

It is well known that chloride of silver blackens the more readily as it is exposed to an intense light, and that even in the diffused light of a room, it becomes soon sensibly coloured. If it contains four to five thousandths of mercury, it does not blacken; it remains of a dead white: with three thousandths of mercury, there is no marked discolouring in diffused light; with two thousandths it is slight; with one it is much more marked, but still it is much less intense than with pure chloride. With half a thousandth of mercury the difference of colour is not remarkable, and is perceived only in a very moderate light.

But when the quantity of mercury is so small that it cannot be detected by the difference of colour in the chloride of silver, it may be rendered quite evident by a very simple process of concentration. Dissolve one gramme of the silver supposed to contain 1/4 of a thousandth of mercury, and let only 1/4 of it be precipitated, by adding only 1/4 of the common salt necessary to precipitate it entirely. In thus operating, the 1/4 thousandth of mercury is concentrated in a quantity of chloride of silver four times smaller: it is as if the silver having been entirely precipitated, four times as much mercury, equal to two thousandths, had been precipitated with it.

In taking two grammes of silver, and precipitating only 1/4 by common salt, the precipitate would be, with respect to the chloride of silver, as if it amounted to four thousandths. By this process, which occupies only five minutes, because exact weighing is not necessary, 1/10 of a thousandth of mercury may be detected in silver.

It is not useless to observe, that in making those experiments the most exact manner of introducing small quantities of mercury into a solution of silver, is to weigh a minute globule of mercury, and to dissolve it in nitric acid, diluting the solution so that it may contain as many cubic centimetres as the globule weighs of centigrammes. Each cubic centimetre, taken by means of a _pipette_, will contain one milligramme of mercury.

If the ingot of silver to be assayed is found to contain a greater quantity of mercury, one thousandth for example, the humid process ought either to be given up in this case, or to be compared with cupellation.

When the silver contains mercury, the solution from which the mixed chlorides are precipitated, does not readily become clear.

Silver containing mercury, put into a small crucible and mixed with lamp black, to prevent the volatilization of the silver, was heated for three quarters of an hour in a muffle, but the silver increased sensibly in weight. This process for separating the mercury, therefore, failed. It is to be observed, that mercury is the only metal which has thus the power of disturbing the analysis by the humid way.