Substances may be crystallized from weak solutions (or mother-liquors) either by spontaneous evaporation, or by concentrating the solution with the aid of heat, until a small quantity placed upon a cold surface (a glass plate or watch-glass) deposits crystals upon cooling: it may then be set aside for crystallization.

The method generally resorted to for obtaining perfect and large crystals of the substance, is that of growing or feeding a crystal, as it is termed. A small and perfect crystal is selected from a crop obtained by one of the above methods, and placed in a vessel of moderate depth, containing a concentrated solution of the same substance, in the cold. The vessel is then covered with filtering paper and set aside, where it is not likely to be disturbed. After the lapse of about twelve hours, the crystal is gently turned in the solution so as to rest upon another surface; and this operation is repeated regularly twice a day until the crystal has attained the desired size. As the solution becomes weaker it must be poured off, and replaced by a fresh quantity of the original strength. Great care must be taken that the solution be not too concentrated, since it most probably will then deposit fresh crystals, and very frequently upon the surfaces of the crystal to be fed; from which it is generally found difficult to detach them without injuring the surfaces of the large crystal.

MEANS OF PROMOTING CRYSTALLIZATION.-Two or three methods may be resorted to for promoting crystallization when it does not take place readily. The solution may be placed in a stoppered or corked bottle, or in a vessel the mouth of which may be closed by the thumb or the palm of the hand, and briskly agitated for two or three minutes; or the solution may be well stirred, and the sides of the vessel rubbed, with a smooth glass rod. The crystals obtained in both cases will be small, and generally speaking irregular; many substances, however, which would not be deposited from their solutions until after the lapse of some considerable time, may be made to crystallize out immediately by the above methods. The latter are, therefore, particularly useful in analysis, when it is wished to test for substances of this nature expeditiously.

Solutions which have remained for a considerable period without depositing crystals, may frequently be made to crystallize by the introduction into the solution of some angular fragment (a small crystal of some substance, or a grain of sand).

§ 56. CRYSTALLIZATION BY FUSION.-Several substances may be crystallized very beautifully by fusion. This is particularly the case with several metals, especially bismuth, and also sulphur, spermaceti, and other substances fusing at a comparatively low temperature, and capable of assuming crystalline forms. To effect crystallization by this method, the substance is melted in a flat ladle, and then placed upon a warm sand bath, so that it may cool very gradually; or the body may be first melted in a crucible, and then poured into a warmed flat vessel, and allowed to cool. When a tolerably solid crust has been formed on the surface, two holes should be pierced, at opposite sides of the edge, by means of a hot rod of iron; the substance that has not yet solidified in the interior is then poured out as rapidly as possible. When the mass in the flat vessel is cool, it is removed (which involves the fracture of the vessel, unless it be of iron), and very carefully cut open. The interior will be found crystallized. Fine masses of crystals can only be obtained by this method when large quantities of the substance are operated upon.

CRYSTALLIZATION BY VAPORIZATION.-The principal directions for obtaining crystals by vaporization have already been given under the head of sublima

1 With such substances as sulphur or spermaceti, this is readily effected by means of a hot knife. Sulphur must not be heated too strongly, as otherwise it will become thick, and adhere to the vessel.

tion (§44). We have but to add that the more gradual the application of heat, the finer are the crystals obtained.

IGNITION.

§ 57. By this term is understood the exposure of a solid substance to a high temperature, for the purpose of altering to a certain extent its chemical or physical constitution. Some substances, ordinarily acted upon by solvents, are rendered insoluble by ignition; others are reduced to bodies of a more simple nature, by the expulsion of certain substances which are either volatile, or are 'converted by auxiliary means into volatile substances at elevated temperatures.

Some substances require, in their ignition, to be distributed over a considerable surface, in order that they may be exposed, as far as possible, to the action of the air. Such operations are generally conducted over the flame of a gasburner, or powerful spirit-lamp, in thin flat dishes or capsules of porcelain, platinum, or silver. The heat should at first be applied very gradually (the dish or capsule being supported by a triangle or sand-bath), and care should be taken that the substances treated in this manner be thoroughly dried, and in many cases finely powdered, previously to ignition, in order to avoid decrepitation as much as possible. When it is desired to expose the substance thoroughly to the action of the air during ignition, it should be stirred from time to time with a piece of stout platinum wire, a platinum or steel spatula, or a glass rod (provided the temperature be not too high). Substances which require ignition in contact with air, and are likely to decrepitate or suffer loss from portions being carried away by the vapors evolved during the operation, should be heated in closed shallow crucibles (generally of porcelain or platinum), the lid being opened very slightly, to admit of the egress of vapor. At the close of the ignition, the lid may be partly or entirely withdrawn, and the access of air to the crucible facilitated by slightly tilting the latter, and holding the blade of a spatula, or the lid of a crucible, edgewise across the opening. It is also well, towards the close of the operation, to raise the temperature considerably by means of the blowpipe. Substances that require ignition out of contact of air, are heated in deep covered crucibles (of graphite or clay, according to the temperature applied). These are generally heated in furnaces; some substances of a peculiar nature are ignited over lamps, or by the blowpipe, in closed platinum or silver crucibles. Before placing a porcelain crucible in a fire, or exposing it to the full flame of a lamp, it should be first gradually heated to some extent, to avoid the risk of its fracture by the sudden change of temperature. It is always advisable to commence the ignition with a gentle fire, and gradually to increase the temperature. A flat piece of fire-brick or tile (or an inverted crucible), should be placed between the bars of the furnace and the bottom of the crucible; the latter is then surrounded with fuel, it being generally advisable not to place any above the cover. In removing a crucible from the fire, it should always be first placed on some warm spot (e. g. the top of the furnace), that it may undergo no very sudden change of temperature. Iron tongs, of various forms and sizes, are used for handling crucibles. It is always advisable slightly to incline the crucible in seizing and lifting it with the tongs, especially when its weight is considerable.

In qualitative analysis, small quantities of substance may be ignited upon a

1 These may be exposed over a lamp to a very high temperature, by placing them in an iron or copper jacket, consisting of a cone open at both ends, provided with projecting slips to support the crucible, and a second similar cone, the wide opening of which fits into that of the jacket. A silver crucible should only be heated over a spirit-lamp.

platinum spatula or scrap of platinum foil; when it is necessary to examine the matters given off during ignition, small portions of the substance may be heated in hard glass tubes, of moderate bore and about four inches in length, open at both ends, or closed at one extremity. The substance is placed at one end, about one inch from the opening (if an open tube is used), and then heated in the flame of a spirit or gas lamp (the tube being held more or less obliquely, according as a rapid or slow current of air is required to pass through it).

FUSION.

§ 58. The property common to a great number of solid bodies, of passing over into the liquid state at more or less exalted temperatures (. e. their fusibility), is applied by the chemist for effecting certain physical or chemical changes which cannot well be brought about at ordinary temperatures.

Many compounds which obstinately retain water of crystallization, or constitutional water, at any temperature below their freezing-point, may be rendered anhydrous by maintaining them in a state of fusion for some time. (It should, however, be first ascertained that such bodies undergo no further decomposition at their fusing temperature.) When substances are operated upon which first fuse in their water of crystallization (undergo an aqueous fusion), the application of heat must be persisted in until the substance has first returned to the solid, and then again to the liquid state. Some substances are rendered more dense and compact in structure by fusion, which is at times a matter of great importance.

The mechanical division of other substances (particularly metals), is indirectly effected by fusion (granulation, see § 46). Fusion is likewise very frequently resorted to for decomposing or altering, to a certain extent, the chemical constitution of substances.

Frequently, other agents besides heat are called into action to effect chemical changes by fusion. Such agents are substances having an affinity for some portion of the body operated upon, converting it into a volatile substance, or combining with it to form some fusible compound. These are generally employed either when it is wished to reduce a metallic oxide or its compound to the state of metal, or to decompose insoluble compounds in such a manner as to effect their subsequent solution. Some substances are employed to convert certain metallic oxides into oxides of a higher class. These reagents (in what is termed "the dry way") have received the name of fluxes. They may be divided into four classes, according to their peculiar action: reducing, oxidizing, doubledecomposing, and simple fluxes.

The most important reducing fluxes are, carbonate of soda or potassa, used together with charcoal, and in some cases alone; cyanide of potassium, and black flux.

The last is prepared by introducing gradually, in small quantities, into a crucible heated to a very dull redness, a mixture of two parts of cream of tartar and one of nitre. The resulting flux consists of a very intimate mixture of carbonate of potassa and charcoal, the latter resulting from the carbonization of tartaric acid.

Charcoal alone, although not a flux, is a powerful reducing agent; some oxides, or their compounds, fused upon or together with charcoal, are reduced to lower oxides or entirely deprived of oxygen.

Black flux is particularly useful in bringing the charcoal it contains into intimate contact with the substance to be operated upon.

The most important double-decomposition fluxes are, a mixture of three parts. of carbonate of soda with four of carbonate of potassa; and hydrate of baryta.

The nitrates of potassa and soda are the oxidizing-fluxes.

The simple fluxes act sometimes merely as purifiers or protectors, removing any mechanical impurity contained in the substance operated upon; or, by being placed upon its upper surface, preserving it from contamination by any foreign matter during the fusion.

They also dissolve, in a singular manner, a number of metallic oxides, yielding with them slags or glasses of various and beautiful colors. The principal of these simple fluxes are, biborate of soda, or borax, powdered flint or green bottle glass, and ammonio-phosphate of soda, or microcosmic salt.1

It is in most cases essential or advantageous that these fluxes be perfectly dry and finely powdered before use. The simple fluxes may, perhaps, be excepted from this rule.

Fluxes should all be preserved in well-closed bottles, in order to prevent their absorbing moisture, or becoming impure.

§ 59. CRUCIBLES. -Fusions are performed in crucibles of clay, iron, graphite, porcelain, silver, or platinum.

Those of hard sandy clay (for example, the Hessian crucibles) are most frequently employed. If properly made, they will stand a high red heat, and are sufficiently dense to retain any liquid mass for some time. The great disadvantage of their use is their liability to be attacked by the fluxes employed, the substance operated upon becoming thus contaminated with silica or alkalies; the dissolving action of the flux upon the crucible is sometimes so considerable as to destroy its power of retaining the substance.

Iron crucibles may be sometimes substituted with advantage; these, however, cannot be used in a great number of cases, since iron is also very easily acted upon by most fluxes. When the fusion has to be effected at a very high temperature, graphite crucibles are employed; these act at the same time as auxiliary reducing agents (in consequence of the carbon which they contain).

Crucibles lined with charcoal are sometimes used in effecting reductions. Fine charcoal-powder is mixed up with thin gum-water, sufficient only to moisten it, without causing it to adhere. The crucible is lined with a coating of about a quarter of an inch in thickness, and the central cavity afterwards made as smooth as possible, by means of the end of a pestle. The reduction and fusion

of iron is effected in crucibles prepared in this manner.

Fusions, and especially reductions on the small scale, are effected in crucibles of Berlin or Meissen porcelain, over gas or spirit lamps, by the aid of the blowpipe. Platinum crucibles are substituted for these when a high temperature is required for the decomposition of a substance by fusion, or when the glazing of the crucible is likely to be attacked by the flux. They must, however, on no account be employed when the reduction of a metal is likely to be effected by the fusion, as in such cases the crucible would be inevitably destroyed, or very much injured. Caustic alkalies, or phosphates, and silicates, fused in the presence of carbon, likewise attack platinum; they may, however, be safely fused in silver crucibles.2

PRECAUTIONS TO BE ATTENDED TO IN FUSIONS.-A fusion is an operation requiring great care and constant attention: there is, indeed, no other process in chemistry so liable to casualties. The following is an enumeration of the principal precautions that should always be strictly attended to.

The special directions given with regard to the proportions of flux and sub

1 Bisulphate of potassa is a powerfully cleansing flux, on account of the free acid it contains, and it is particularly useful for cleansing platinum vessels. It is also sometimes employed for dissolving minerals; e. g. chrome iron-ore.

2 Gold crucibles, alloyed with about five per cent. of platinum, are said to be much more convenient than these latter, since they stand a far higher temperature.

stance to be employed, their state of division, &c, must be always implicitly followed.

The charge of a crucible, and its size, must be regulated according to the nature of the fusion. Thus, if the operation is accompanied by the disengagement of a gas, or the swelling up of the mass, a capacious crucible must be used, and not more than half filled with the mixture. Substances diminishing in bulk upon fusion, may of course be heated in smaller crucibles, or in larger quantities.

The crucible must, in almost every case, be kept covered, but in such a manner that the lid may be easily removed at any point of the process, as it is necessary in most operations to examine the contents of the crucible from time to time.

The heating of a crucible must be conducted very gradually at the commencement, and care should be taken to have the source of heat as much as possible under control, so that the temperature may be lowered at any time, if the crucible becomes too hot, or the contents evince symptoms of boiling over.

If the operation is conducted in a furnace, it is frequently necessary to withdraw the crucible rapidly from the fire, placing it upon the top of the furnace, until it has somewhat cooled down. The heat should always be raised considerably towards the close of the operation. In many cases, particularly when reductions have been effected by fusion, it is advisable to sacrifice the crucible rather than to disturb the mass after it has been removed from the fire. In that case, the crucible, when sufficiently cooled down, is laid upon an anvil or stone slab, and smart blows are then applied to it, by means of a hammer, at a distance of about half an inch from the bottom. The fracture of the crucible is generally effected by this means in such a manner that the bottom separates from the sides, and any button of metal that may have collected there is then readily removed from the slag. When it is wished to preserve the crucible (those of porcelain, clay, &c. being referred to), the fused mass must not be allowed to solidify in it, even though capable of being afterwards removed by a solvent. Many fused substances, in the act of solidifying, endanger the safety of the crucibles; it is necessary, therefore, when the fusion is completed, to seize the latter firmly with the tongs, and to pour out the fused mass upon a cold, clean, and perfectly dry stone or iron surface; the crucible may then be placed in some warm spot, to cool gradually. In pouring out the mass, it is advisable to move the crucible about, so as to alter the direction of the stream issuing from it, thus spreading the mass over a larger surface. It then cools down more rapidly, and may be afterwards operated upon more conveniently than if it were in a thick mass.

In fusing any substance with nitre, when the oxidizing action of the latter. is so violent as to be accompanied by deflagration, as in the oxidation of any organic substance, the nitre should be first heated alone until perfectly fused, and the substance then gradually introduced by means of a spatula, in very small quantities at a time. It is advisable to hold the crucible cover in the one hand, by means of the tongs, while the substance is being introduced, and to replace it immediately after every addition, in order to avoid loss by spirting. By employing a mixture of nitre and chloride of sodium, the former is rendered far more manageable-its action being thus greatly moderated.

Cyanide of potassium must on no account be employed as a flux when the substance operated upon contains any nitrate or chlorate, since a violent explosion would invariably result. In addition to the above directions, those given with regard to ignition should also be applied to the operation of fusion.