on the prepuce, without pain; and though caused by coition, have nothing of infection attending them. The cause is supposed to be a contusion of the lymphatic vessels in the part affected. Dr Cockburn, who hath described this case, recommends for the cure a mixture of three parts of lime-water and two of rectified spirit of wine, to be used warm, as a lotion three times a day.CRYSTALLIZATION.

CRYSTALLIZATION is the symmetrical arrangement of the particles of a body when it passes from the liquid to the solid form. This arrangement is determined by the mutual action of the small solids of which the body is composed; and these solids are separated from the liquid by their force of cohesion. Crystallization is one of the most remarkable effects of cohesion. The qualities of a solid in which the force of cohesion is more easily overcome in one direction than another, its brittleness, elasticity, and ductility, depend on this arrangement of its particles.

Solid bodies are found either in irregular masses, or exhibit certain determinate forms by the process of crystallization. Those substances which are capable of assuming regular figures, uniformly affect the same form; subject, however, to certain deviations, from the operation of particular circumstances. Those bodies only can assume the form of crystals which are susceptible of being reduced to the fluid state. This is the usual method of crystallizing saline substances. The substance to be crystallized is dissolved in a sufficient quantity of water to retain it in solution. This is slowly evaporated; and as the bulk of the fluid is diminished, the particles are brought nearer to each other; they combine together by the force of cohesion, and form crystals. Some saline bodies, which dissolve but in small proportion in cold water, are found to be very soluble in hot water. But when this water cools, it is no longer capable of holding them in solution. The particles then gradually approach each other, and arrange themselves into certain determinate forms; or they crystallize. Many of the saline bodies which crystallize in this manner, combine with a considerable portion of water. This is called the water of crystallization. Other saline substances are equally soluble in hot and cold water. These substances do not crystallize by cooling the fluid; they assume regular forms only by diminishing its quantity. This is effected by means of evaporation by the application of heat. In salts which are crystallized in these circumstances, the proportion of water which enters into combination is small.

There are some classes of bodies which assume regular forms, but are not soluble in any liquid. Such, for instance, are metallic substances, glass, and some other bodies. Substances of this nature are crystallized, by being previously subjected to fusion; and thus having combined with caloric, they are reduced to the liquid state, and the particles being separated from each other are left at liberty to arrange themselves into regular forms, or to crystallize, as the body cools.

But what is the cause which operates in determining the regular arrangement of the particles of bodies in these circumstances? or what is the cause of the same bodies in the same circumstances assuming regular figures? The ancient philosophers supposed that the elements of bodies consisted of certain regular geometrical figures; but it does not appear that they applied this theory to explain crystallization. The schoolmen ascribed the regular figure of crystals to their substantial forms; and others supposed that it depended merely on the aggregation of the particles, but without explaining to what this aggregation was owing, or the reason of the regular figures thus produced. According to Sir Isaac Newton and the theory of Boscovich, Newton's particles of bodies held in solution in a fluid, are arranged at regular distances, and in regular order; and when the force of cohesion between the particles and the fluid is diminished, it is increased between the particles themselves. Thus they separate from the fluid, and combine together in groups which are composed of the particles nearest to each other. If we suppose that the particles composing the same body have the same figure, the aggregation of any determinate number of such particles will produce similar figures. Bergman is of opinion that the particles of saline substances possess a double tendency: by the one they arrange themselves in the form of spicules; and by the other, these spicules arrange themselves at certain angles of inclination, and according to the difference of these angles, different forms of crystals are produced. These effects are ascribed by the ingenious author to the mutual attraction which exists between the particles, which, according to the peculiar figures of the atoms, at one time, arranges them in the form of spicules, and then combines the spicules thus formed under different angles of inclination. But this arrangement of the particles, or tendency to arrangement, assigned by Bergman as a Bergman's cause, is only explaining the phenomenon by itself; while the cause of the tendency is yet unexplained. Nor will Newton's hypothesis be more satisfactory; for if the particles of a body, after being equally diffused in a fluid, are brought together by a general attraction, it will follow that every saline body should crystallize in the same manner.

According to the ingenious theory which has been proposed by Hauy, the integrant particles always combine in the same body in the same way; the same faces and the same edges are always attracted towards each other. But these faces and edges are different in different crystals; and hence originates that variety of forms which different bodies assuming regular figures by crystallization exhibit. But why are the same edges and the same faces attracted in the same way? This still wants explanation. If it be ascribed, as some have supposed, to a certain degree of polarity existing among the particles, it might enable us to account for the regular figures of bodies produced by the process of crystallization. For by the effects of this agent we might suppose that different parts of the particles of bodies are endowed with different forces; one an attractive, and another a repulsive force; and by the action of these two forces, the same arrangement of the particles will uniformly take place; for when one part of a particle is attracted, the other will be invariably repelled; and thus the same faces and edges will always be disposed in the same way. But it ought to be observed that the existence of this power, however satisfactorily factorily it might account for the phenomena, has by no means been proved; and even if its existence were completely established, the difficulty still remains how this polarity is to be explained.

Without entering farther into these speculations, we propose, in the two following sections, to present our readers with a comprehensive view of the formation and structure of crystallized bodies. In the first section we shall treat of the phenomena of crystallization, the means of conducting this process to obtain the most perfect crystals, and the modifications of which each of the forms is susceptible. In the second we shall give a short view of the theory of the structure of crystals.

Sect. I. Of the Phenomena of Crystallization, and the modifications to which it is subject.

The most complete set of observations which has yet appeared on this branch of practical chemistry has been made by M. Leblanc; and to his ingenious memoir† we must acknowledge ourselves indebted for what we now lay before our readers that is new or interesting on this subject. This art, he observes, of managing or conducting the crystallization of salts, is in a great measure new; for it has hitherto attracted little attention. To insure success in obtaining perfect crystals, the process must be conducted in flat-bottomed vessels; and vessels of glass or porcelain are found preferable to those of any other materials for this purpose. The salt employed should be in a state of purity; and to favour the increase and regular form of the crystals, they are to be placed at a distance from each other in the vessels containing the solution. To these necessary precautions, it may be added, that the vessels in which the evaporation goes on should be at perfect rest, and that it is requisite to observe the density, or specific gravity, at which the solution begins to yield crystals.

The particles of any saline body cannot come into contact and form crystals, as long as the force of affinity between these particles and the fluid in which they are held in solution is greater than the mutual affinity of the particles among themselves. A salt, for instance, which begins to crystallize at a certain specific gravity of its solution in water, will afford no crystals when that specific gravity is diminished; for then the particles of the salt are removed to a greater distance from each other; and while, by this distance, the force of their mutual attraction is diminished, the attraction between these particles and the water in which they are dissolved is increased by the increase of the quantity of the solvent. But, on the other hand, if a solution which begins to crystallize at a certain specific gravity, is more concentrated, the crystals which are thus obtained are greatly multiplied; but they are heaped together in confused masses, exhibiting no regular forms. Thus, a solution which has been scarcely reduced to that degree of concentration at which it begins to crystallize, being poured while it is hot into the proper vessel for carrying on the process, or left at rest in the same vessel in which the solution is made, to cool slowly, will yield a small number of crystals, which will have no other defects than what are occasioned by their contact with the vessel. Even perfect crystals will be sometimes found among the smaller ones. When the concentration of the solution has not been carried too far, or not farther than what is effected by slow cooling, not only have the embryo crystals less bulk, but the particles having come into contact slowly and without confusion, they possess a greater degree of transparency. After a certain period, which varies according to the species of salt which is subjected to the operation, small crystals may be distinctly observed. These are to be carefully detached from each other, and placed in a different position. Being placed by this management on a different side, the defects occasioned by their contact with the vessel are soon repaired. From the crystals treated in this way, the finest and most perfect are to be obtained. This operation of changing the position of the crystal from one side to the other, ought to be repeated at least once every day, if we wish to obtain the completest crystals.

At the end of a certain period, the small crystals are to be removed, that the fluid may be more concentrated, either by a new evaporation, or by dissolving a new portion of the same salt. After the new solution has cooled, and the crystals which have formed in it are separated, if it has been too much concentrated, or too great a portion of salt has been added; the crystals of the first solution are then to be introduced and treated in the same way as formerly.

When the crystals have acquired a sufficient volume to handle them, and to choose such as we wish shouldment of the increase to the largest size, either as simple or complete crystals, or as exhibiting varieties from position or particular circumstances, the individual crystals are then to be separated, and solutions are to be prepared for them, and brought to such a degree of concentration as to afford crystals in a mass; which latter being removed, the single crystals are introduced into these solutions, which are now in a proper state to favour their increase. The crystals may be either previously disposed in the vessel, and then the solution may be poured on; or having first introduced the latter, they may be afterwards distributed on the bottom of the vessel. And thus by continuing the same process, by taking care to change the position of the crystal from one side to the other frequently, and by keeping up the solution to a proper degree of strength, we may obtain crystals of any bulk we choose.

When the quantity of particles, which in a certain stage of concentration continue to be mutually attract-crease if left too long in the solution, diminishes in consequence of their accumulation on the crystals which are formed, at a certain stage of this diminution the crystals cease to enlarge or increase in bulk; it happens, on the contrary, if they are left in the fluid, that they begin to dissolve. It is usually on the corners and angles that this decrease takes place; and in some salts it seems to go on piecemeal, so as to present distinct layers of the particles; for in this case lines parallel to the sides may be observed, and these are disposed like steps of stairs. Should the accident, which is here alluded to, be allowed to go on too far, it may often require a long time to repair it; but it is in general easy to avoid this inconvenience, by watching the progress of the operation and the increase of the crystals. If their corners or angles are observed to become less sharp, they must be removed till the fluid is farther concentrated, or they must be introduced into a new solution of the same salt of the proper degree of strength. To prepare the new solution for the increase of the crystals, a quantity of the same salt is to be dissolved in a given portion of water, so that it shall be fully saturated. It is then allowed to cool and crystallize. The crystals being separated, the remaining solution is to be employed in such quantity as may be judged necessary to replace that in which the diminution of the crystals had commenced.

Sometimes it happens, from want of necessary precaution, that the new solution in which the process is to be conducted, either being too much saturated, or being disturbed by pouring from one vessel to another, exhibits many other points of attraction beside the crystals whose increase is proposed. In this case a great number of small crystals make their appearance, and cover the surface of the former with a kind of incrustation. The small crystals, provided they are taken in time, may be removed without injury to the others; if not, they will be unavoidably spoiled.

When the crystals have reached such a size as that they may be placed one by one, without being in contact with each other, we must still continue frequently to change their position. This may be done with a spatula, a glass rod, or with any instrument which will communicate nothing to the fluid. In this way the sides of the crystal which are alternately in contact with the bottom of the vessel will increase in equal proportion, and it will always remain complete.

It is chiefly in salts which furnish elongated prisms that the influence of position may be most distinctly seen. If, for instance, a crystal before it has acquired much volume is found to rest on one of its bases as well as on one of its sides, it will be observed to be compressed in the direction from base to base; and it will appear to be only a regular segment of the crystal, which having been placed on one of its sides has obtained a great bulk. If we take a six-sided prism whose summits are obliquely truncated, and if it be placed on one of its sides, it will enlarge in a greater or less degree, but always in such a manner that the distance from one base to the other shall never be less than the distance between the sides. But if the position be on one of its bases, then its principal increase will be in the direction of the sides, and it will appear to be compressed between the bases. At first sight, a crystal treated in this way will seem different from the former. For the corners form the summit of apparent pyramids which are separated by a four sided prism. This circumstance affords a sufficient explanation of one of the causes which produce varieties in the appearances of a crystal with regard to its relative extent; it shows that there is no foundation for the opinion of a supposed balance between the particles of the salt and that of the solvent; and it shows also, that if the force of attraction be the efficient cause of the saline particles coming into contact, the force of gravitation acts at the same time, and modifies in a greater or less degree the effects of the first.

According to these observations, and the different states in which crystallized substances are found, it has been supposed that we might conclude, that the force of adhesion between the particles of the salt and those of the solvent, varies according to circumstances, which depend on the degree of tendency to combination between the bodies, and the relative weight or bulk of the parts of which these bodies are composed. If a crystal in the incipient stage of its increase be placed on one of its bases, it enlarges in the direction of its sides; but if it be reversed and placed on one of its sides, it enlarges in the dimensions of an elongated prism.

An insulated crystal, placed on one of its sides on a smooth surface, and left undisturbed to enlarge in size, presents on this part a kind of hollow, which corresponds exactly with the side which it replaces. Here the saline particles which cannot reach this surface, are distributed on the neighbouring parts with which they come in contact, with this circumstance, that the edges of the surface on which the crystal rests increase in proportion, but without allowing the liquid to have access to this surface.

The hollows which are formed at the surface of liquids differ sometimes from each other even in the same salt. If we suppose that a particle forms the incipient point of the hollow, the latter will assume a configuration corresponding to the side of the particle presented to the surface of the liquid; but the part which it touches increases also; and if by any circumstance a change of position happens, the hollow, thus necessarily formed according to the arrangement of the part which corresponds exactly to the surface of the liquid, will change its form, because the new position of the side presented differs from the first.

When a neutral salt, in a state of purity, and after being crystallized, ceases to produce any effect on vegetable blues, it is not supposed that any of its constituent principles is in excess. But if in this state it is found to combine with other bodies, in such a manner as to produce solid and well defined crystals, we must admit that there exists an affinity between the salt and the body with which it has combined.

This subject, Leblanc observes, of the supra-composition, or compound combination, as it might perhaps be called, of which several salts are susceptible, has hitherto much occupied the attention of chemical philosophers. Some indeed have been pointed out by Bergman and others; but it has been remarked that these affinities are probably much more extensive than has been supposed; and not only with regard to neutral salts with each other, but also neutral salts with other bodies. Of this kind of combination is not to be reckoned that of one of the constituent parts of a salt being in excess, which frequently takes place in some salts, and is found to be more or less permanent. This circumstance seems to prove that certain salts have two different points in the combination of their constituent parts. Let us see what has been observed in this respect of the sulphate of alumina, which will perhaps explain the reason that this salt is almost always found in nature in the acidulous state. It is found that the more that alum approaches to the state of saturation by an additional portion of base, the less solid the new combination becomes; and in all cases, after a certain time, which is longer or shorter according to circumstances, the portion which was added separates. It will perhaps appear in the sequel, that this tendency to combination which is constantly in action, producing an immense multitude of different individuals, resides not only among the properties of the simple principles, Many of the sulphates are always found in the acidulous state; and all of them seem to be susceptible of combination with a new quantity of the same base, till they reach the point of saturation. For example, the sulphate of copper, in the state in which it is usually found, crystallizes in eight-sided oblique prisms, terminated by sides according to the obliquity of the prism. But if another portion of base be added, the crystals assume the form of pyramids of several faces, separated by a four-sided prism. The acidulous sulphate of zinc gives crystals of six-sided prisms, which are often very regular; but an addition of base produces a great change, for then the crystals are in the rhomboidal form, very little different from the cube. Alum in its ordinary state of combination crystallizes in the form of a regular octaedron; but in the intermediate proportions between this state and that of saturation, it assumes the form of a cube.

Haüy, as will be afterwards noticed, has demonstrated that the form of the primitive molecules is the same in all crystals of the same salt, and he has shown by calculation that the variations arise from the laws of decrement in the layers which surround the nucleus; but that the order according to which the secondary forms are produced may be interrupted, whether this form be complete or not; and the crystal may then, according to circumstances, return to its primitive form, or to some of those which are derived from it. But from the experiments of Leblanc, he thinks that these changes always depend on new conditions in the state of the fluid, as a different proportion of the principles of which the salt is composed.

If a crystal of octaedral alum be placed in a solution which forms cubic crystals of the same salt, the former will assume the cubic form, by giving up a series of molecules from the summits of the solid angles, so that the layers continue to decrease on the triangular faces till the crystal has completed its new form. In this process, the change may be stopped at any period, and crystals of every modification of form may be obtained. From this it follows, that the centre of each of the faces of the octaedron corresponds to a solid angle of the cube in which it is inscribed. But if a cubical crystal be introduced into the solution which yields the octaedron, its return to this latter form proceeds in the same order, by the subtraction of a series of molecules from the solid angles of the cube. It often happens, however, at the same time, that the subtraction of the molecules extends to the corners of the crystal; so that the layers of super-position decrease all at once, according to the order of the formation of the octaedron, and the dodecaedron with rhomboidal surfaces. This circumstance seems to suggest the possibility of obtaining crystals of alum of this latter form; but it seems to depend on a particular proportion which is not easily determined.

Thus we learn from experiment that salts which exhibit different forms of crystals can be made to assume each of these at pleasure. This phenomenon, which has not been much attended to, seems to merit particular investigation. The transition from one form to another may be explained according to the laws of diminution, by the successive and regular subtraction of series of molecules; so that the form actually obtained, the restoration of the preceding form, is easily explicable on the principle of restitution alone. It may be observed, that during this kind of metamorphosis, both operations, namely, that by which the crystal receives on the one hand a new form, and that by which on the other hand it increases on all its sides, constantly take place.

The particles of a salt which are in solution in a fluid, are attracted by it, particle by particle, without any separation or decomposition; but it is necessary that there be a balance of the attracting forces between the salt and the solvent. This is demonstrated by the following experiment. A vessel two feet high and two inches in diameter was filled with a solution of a proper degree of concentration for the growth of crystals, which were suspended at different heights from the bottom of the vessel to the surface of the fluid; and it was observed that the increase of the crystal was in proportion to its depth in the vessel, that which was nearest the bottom increasing most rapidly. When the liquid was deprived of saline particles by their accumulation on the crystals, by rest, and sometimes even by the influence of the atmosphere, the crystals decreased by similar gradations to those of their increase; so that it at last reached that state when the crystals near the surface of the liquid were dissolved, while those towards the bottom continued to increase; and sometimes it happened that the crystals at the bottom of the vessel continued to increase on the surface which was in contact with it, while the opposite upper surface was in a state of dissolution.

All the experiments which were made on salts of different degrees of specific gravity accord with this observation; and the difference in the degrees of saturation of the waters of the ocean, which depends on the difference of depth, seems to be in favour of this opinion. It is confirmed by the analysis of sea-water by Bergman and others, which was taken up in different places and at different depths. It receives still farther confirmation from a practice of the inhabitants of Salines in Bearn in estimating the degree of strength of a salt spring. An egg is thrown into the waters of the spring, and the whole water which covers the surface of the egg is thrown away, as it is not of a sufficient degree of concentration.

It is well known that a cold temperature is most convenient for the crystallization of salts. But it is not at the period when the salt begins to crystallize that it is most convenient to carry on the process; for then it sometimes happens, from too great concentration of the fluid, that the crystallization is too rapid and confused.

Hitherto saline substances, which are susceptible of regular crystallization, have been divided into two classes, according to the peculiarities in the formation of their crystals. The one class comprehends those crystals which are formed by cooling the fluid in which the solution is made. The other class includes those which are produced only during the evaporation of the solution. This distinction is no doubt well founded; but there are some exceptions to it which are necessary to be attended to in conducting the process of crystallization. If a saline solution which is too much saturated, be cooled, it furnishes a mass of crystals, which which are confused and irregular, and which present no determined form except on those sides which are in contact with the liquid. If in this state the remaining liquid is poured off, it will yield another set of crystals, but in very small number; and there are some salts which continue to form crystals after being several times successively treated in this way, the number of the crystals still diminishing from the first degree of concentration. It will be found too that this will take place whether the process be carried on in the open air or in close vessels. It follows from this that the increase or the formation of crystals, in this case depends solely on the mutual attraction of the particles, or on the attraction between the particles and the crystal; an attraction or affinity which is not destroyed by the cooling of the fluid, but is probably regulated by the distance of the particles, and the degree of force or affinity which exists between the particles and the solvent. In some saline solutions the increase of the crystals goes on in this manner for a long time. It is only in the interval between the cooling of the liquid to the temperature of the atmosphere, and that period when its degree of concentration is so diminished that the increase of the crystals ceases, that the latter proceeds with that degree of perfection of which it is susceptible.

It is not a property peculiar to dry substances to absorb moisture from the atmosphere. Liquids saturated with certain salts seem also to possess this property; for in some saline solutions, the liquids assume a solvent power which never fails to attack the crystals, and not only to prevent their increase, but to diminish the bulk which they had acquired. This accident can only be obviated by regulating the state of the atmosphere in which the evaporating vessels are placed, and preserving it free from an excess of moisture. From causes which produce a contrary effect, the evaporation becomes too rapid; this circumstance also requires to be attended to and properly regulated, to insure the full success of the operation.

From the preceding observations it will appear, that solutions of salts which are susceptible of crystallization have certain degrees of concentration which are necessary for the formation of crystals; and that they must be reduced nearly to that degree in which they begin to yield crystals, before it can be expected that they afford proper results. It is therefore necessary to attend particularly to the degree of concentration which each salt requires for the regular formation of its crystals, and to obtain them with that degree of transparency of which they are susceptible. We have seen that in the formation of crystals they may be removed from one vessel to another, and from one solution to another; and that in proportion to the slowness of the process they become more beautiful and more perfect. These operations, it may be added, require much patience and attention, but at the same time the observer is fully compensated for his trouble, by perceiving the progress of the crystallization, and by the interest which is excited in all its stages.

It is essential to know that neither the crystals formed during the artificial evaporation, nor those which are produced during the cooling of the solution, are proper to be made choice of for being increased and brought forward to the most perfect crystals. When a solution has become cold, that is to say, when it has acquired the temperature of the atmosphere, and it is deprived of the excess of saline particles which it held in combination during its increase of temperature, it is still in a condition to yield crystals, and as long as the distances between the particles are not too great to allow of mutual attraction. A solution saturated to excess affords on cooling a confused mass of crystals; but after the fluid is poured off, it will still produce more crystals, but in smaller number. The degree of concentration of the solution before it yielded the last product, may be considered as the term of saturation most proper to be employed for the species of salt which is thus treated. But by the repetition of these operations, and the observation of their progress, it will not be difficult to discover the proper proportions between the salt and the solvent.

It seems to be a mistake to suppose, with some, that the crystals which are placed in favourable circumstances to become larger and more perfect, are injured by coming in contact with each other during their increase. It is undoubtedly better that they should be kept separate; but it does not appear that they are hurt by touching each other, if the number in the vessel be not too great, and they are not heaped or pressed together. In that crystallization which results from the cooling of a solution too much saturated, the crystals are always confused and interlaced with each other; and the molecules which are arranged in this kind of disorder experience a kind of irregular distribution; and it may be observed, that in this case the summits only of the crystals which are elevated from the kind of cake which is formed on the surfaces of the vessel containing the solution, present regular and determined forms. The mass in which these crystals are implanted is a confused heap.

No cavities have been observed on the faces of crystals excepting those which are formed on the surface of fluids. Those which are produced on that side of a crystal which rests on the bottom of the vessel are more common in other salts. This phenomenon seems to merit more attention than has yet been bestowed upon it; as it explains easily the introduction of extraneous bodies which are sometimes detected in the interior of crystals. For when a cavity of this kind has acquired a certain depth, it is capable of receiving part of any foreign substance, and to be filled up by the change of position of the same crystal, retaining at the same time the extraneous matter. By a little art and dexterity, these fortuitous circumstances may be favoured, so that phenomena exhibited by such occurrences may be traced and observed at the pleasure of the operator. Experiments have been made with the view of ascertaining whether an extraneous substance could be substituted as the nucleus of a crystal; but from the result of these experiments, it does not appear that the particles of any salt have a tendency to combine with any foreign matter, and to form regular crystals. The portions of the salt which were attached to the extraneous substance were always separate and independent crystals.

There are some saline substances which retain in their solution an excess of particles even after cooling, and which being strongly agitated instantly deposit a great number of small crystals which render the solu-

The introduction of crystals of the same salt, it is well known, as in the case of a solution of Glauber's salt, promotes this sudden crystallization or separation of the excess of the salt. If, in this state of the solution, crystals are immersed with the view of having them large and regular, they are certain of being spoiled by the accumulation of a great number of small crystals on their surface, unless the precaution of immediately washing them with pure water when this happens is observed.

It may be remarked also that when the solution is diminished below a certain degree of saturation, the crystals not only cease to increase, but are also again in some measure dissolved; the corners and angles reduced and rounded. And if the crystals in this state be introduced into a solution of sufficient strength to promote their increase, supernumerary faces and truncatures, as they are denominated in technical language, are formed on the rounded corners and angles. But these faces always disappear as the increase of the crystals proceeds, and are replaced by corners and angles, which become at last sharp and distinct.

By attention to preserve the solutions of salt in perfect purity, we shall be more certain of obtaining the most beautiful and transparent crystals. Some fluids, after a certain time, are observed to deposit substances which are foreign to the salt held in solution, and were dissolved along with it. These substances sometimes appear in the form of earthy matters, which precipitate to the bottom of the vessel; in other cases they are diffused in the form of flakes, and sometimes they rise and swim on the surface. In all these cases, the crystals whose formation and increase are going forward must be removed, and the liquor must be filtrated before they are replaced.

A saline substance, which is capable of crystallization, possesses, in the state of minute division in which it is in solution, or in the condition of the molecules which compose it, a determined property which is uniform and constant, in which resides essentially the power of uniting in a certain symmetrical manner, and thus constructing regular solids. The results also are uniform and constant when the process is carefully conducted; but it is necessary to distinguish with accuracy the circumstances which accompany the operation, and may occasion a deviation from this uniformity. The sulphate of iron, for instance, usually crystallizes in the form of rhomboids; but sometimes it has been found to assume that of an irregular octaedron. And although it may be true that an elongated octaedron may be clasped with prismatic crystals, it does not on that account belong less to the octaedral form; but it seems probable that these different varieties, in the forms of crystals, depend on some changes which take place in the solutions themselves. The iron in the present case is constantly receiving new portions of oxygen from the atmosphere, and in this new combination it is precipitated in the fluid: this, therefore, occasions a change in the constituents of the salt.

Several sulphates are found to combine readily with each other: those of iron and copper are of this description; and the result of this compound crystal is always a rhomboid. It seems to be doubtful whether this should be considered as a case of simple interposition of one salt with the other.

When a liquid, which holds saline bodies in solution, is evaporated to a certain degree, a crust forms on the surface, acquires a certain thickness, and when this is removed, it is renewed. The point at which the liquid exhibits this appearance is known in chemistry, by the appellation of evaporation to a pellicle. When it has reached this point, the solution is in a formation state of complete saturation; and the smallest addition of dendrites of fluid cannot be withdrawn without a corresponding quantity of salt assuming the solid form.

On this principle Robinet has attempted to account for the formation of dendrites, or the arborescent appearance and efflorescence of some salts. Almost all the different species of fucus or sea-weed, he observes, are covered, in drying, with an efflorescence of white matter. In some species, this white matter was observed to possess a saccharine quality. A number of large roots of the fucus palmatus was hung up in the shade, and ten days had elapsed without the appearance of any thing on the surface. After that period it became white, and it was soon covered with a light downy substance, the filaments of which gradually increased to a considerable length. When this downy matter was brushed off with a feather, it was renewed till the plants were completely dry. This substance, it appeared on examination, was of a saccharine nature, mixed with a small portion of common salt, and a great quantity of mucilaginous matter. By solution and crystallization, the sugar was separated from the other substances.

In comparing the circumstances of this efflorescence with those of the formation of the pellicle, in the progress of evaporation, the former seems to be a modification of the latter. In a vessel which contains a liquid saturated with a salt, the surface subjected to evaporation has no sooner assumed a solid form, than the surface immediately inferior is exposed to the action of the same causes, and produces the same effect; and this effect continues till this crust has become so thick, or so compact, as to prevent the contact of air, and then the evaporation ceases. But, on the contrary, in the fucus, the air acting only on the surface of the plant, the liquid which it contains cannot undergo the process of evaporation, without coming to the surface. The attraction of the matter of the plant tends to promote this motion; for as the liquid is equally diffused through its whole mass, it rises constantly to the surface, in proportion as this surface is dried by the surrounding air; and it would appear that this is the process in the desiccation of all thick and massy bodies. Now, the saline matter which, in the present case, is in the state of efflorescence, having the same power of attraction on the liquid, the rudiments of each filament constitute, at the instant of their formation, part of the whole mass or body of the plant. They participate, therefore, of the same degree of moisture as that of the plant, and it is on their surface that the evaporation and crystallization of saline matter chiefly take place.

The mechanism of the dendritical or arborescent form of saline bodies seems to be in this way capable of explanation. The whole saline mass, which extends to the edges of the vessel, and even redescends externally, is constantly in the humid state, as long as any liquid remains in the vessel. It may be supposed, that the matter of the sides of the vessel determines, by its attraction, the external circle of the surface of the liquid to rise above the surface; a phenomenon which is sufficiently obvious, but especially in narrow vessels. This portion of liquid, which is more completely subjected to evaporation, gives origin to a circle of saline matter, which appears thus raised above the surface of the liquid, and which being the first rudiments of the dendrites, contributes afterwards to its increase, in the way which has been already explained. Thus the vegetation of salts bears a striking resemblance to the process of efflorescence, or the formation of the downy matter on the surface of the fucus.

There is yet another kind of crystallization which seems to depend on the same cause. This is the saline efflorescence, which occurs in different places on the surface of the globe, and is frequently in such quantity as to become an important object of manufacture. Without extending our observations to the efflorescence of soda on the surface of the soil in Egypt, or that of nitre in Asiatic countries, we may refer to the production of muriate of soda, or common salt, in different parts of Europe, in those places which are covered with the waters of the ocean during high tides. The waters of the sea, with which the sandy shores are twice periodically moistened in the course of a month, are far distant from the point of saturation which determines crystallization. They rarely contain more than 3 parts of salt in 100; and the sand at the degree of moisture, in which it is left by the sea, is not impregnated with a sufficient quantity of saline matter to be worthy the labour of manufacturing; but, during the interval between the tides, these circumstances are greatly changed. The dry air of summer, by evaporating the moisture on the surface, allows the matter of the sand to attract towards the surface a similar portion of water, which was in the lower part of the soil, and which always tends to diffuse itself equally through the whole mass. This liquid, carrying with it the salt, which it holds in solution, increases the quantity of saline matter which exists on the surface. This process continues without interruption, as long as there is no fall of rain. It reaches at last a certain point, at which the water subjected to evaporation is saturated with the salt; and this process cannot proceed farther without the deposition of crystals of the salt, which discover themselves by their shining appearance. After some days, the sand on the surface is collected, and about six times the quantity of saline matter is found in the same proportion of sand, when it was first moistened by the sea water (a).

Another phenomenon which takes place during the process of artificial evaporation, should not pass unnoticed. This is the formation of a saline crust at the bottom of the vessels in which the process is conducted. This seems to be the immediate effect of ebullition; for when the temperature of the liquid is kept under the boiling point, no such effect is produced. This crust is composed of all the saline substances which are held in solution in the liquid; and even these substances are found combined in the same proportion in which they actually exist in the solution. Whatever be the attraction of these substances for water, or even if they possess a deliquescent property, they are not less disposed to enter into combination during the formation of the solid crust on the bottom of vessels in which the process of evaporation is conducted with a temperature equal to the boiling point. A slight degree of attention will satisfy us, that the formation of this crust depends on the particular circumstances of the evaporation in the case of ebullition. It must be obvious, that in this case the stratum of liquid which is in immediate contact with the vessel, receives the caloric which penetrates its sides, is charged with it beyond its capacity, changes its state, and assumes the gaseous form, and by this change having entirely lost its solvent power, whatever saline matter is held in solution must assume the solid state in contact with the sides of the vessel, and consequently adhere to it. Thus it happens, according to a very judicious observation, that in different saline solutions, the results of which have been compared, these scales or crusts are more abundant in proportion as the degree of saturation is less.

To these observations we shall only add a short account of the phenomena of crystallization, as they were observed with the assistance of a microscope, by Mr. Baker, and of the appearances of different saline bodies which he hath described. This will not afford any scientific information to the philosopher, but it may perhaps be the source of amusement to some of our readers, and the means, by a minute observation of the phenomena, of leading to some useful discoveries. The method which he followed in conducting these experiments, is the following. The substance to be examined is to be dissolved in a quantity of pure water, so as to be completely saturated. For salts of easy solubility, cold water may be employed; but for salts which are dissolved with more difficulty, hot or boiling water may be found necessary. In preparing the solution, the same rule may be observed as in preparing solutions for obtaining large crystals, which has been given in the former part of this section. The solution should be allowed to remain at rest for some hours, so that the first crystallization, if too much saline matter has been added to the liquid, may be allowed to take place. Thus the solution will be always of the same strength, and the same appearances may be uniformly expected.

When the solution is thus prepared, a drop of it may be taken up with the point of a quill, cut in the form of a pen, and placed on a flat slip of glass, spreading it on the glass with the quill till the liquid is so shallow as to rise very little above its surface. It is then

(a) Common salt is manufactured in this way on the sandy shores of the Solway Frith, in Annandale in Scotland. These flat shores are covered with the waters of the ocean during spring tides; and in the interval of these tides the evaporation by the heat of the sun and the action of the air is so considerable, as to leave the sand impregnated with a quantity of salt, sufficient to defray the expense and trouble of manufacturing it by filtration and boiling. then to be held over the clear part of a moderate fire, or the flame of a candle, and such a degree of heat applied as is found from experience to produce the necessary evaporation. This will be known by observing the formation of saline particles at the edges of the drop of fluid. The microscope being previously adjusted, and a magnifier of moderate power being fitted on, the slip of glass is to be placed immediately under the eye, and brought exactly to the focus of the magnifier. After running over the whole drop, the attention is to be directed to that side on which the process of crystallization first commences, and proceeds from the circumference towards the centre. The motion is at first slow, if too much heat has not been applied, but becomes quicker as the evaporation continues. In some crystallizations the configurations are produced towards the end of the process with great rapidity, and exhibit an elegance, order, and regularity, which imagination only can conceive. When this rapid action has once begun, the eye must be kept fixed on the object, till the whole process is completed, because new forms appear, quite different from those which were first produced, and which have been properly ascribed to a quantity of different salts mixed with the substance to be examined, when the precaution has not been used of having it in a state of purity. When the configurations are fully formed, and the water evaporated, such salts as are deliquescent, it is scarcely necessary to observe, are soon destroyed by attracting the moisture from the air; but those which are more permanent, and not disposed either to deliquesce or to be deprived of their water of crystallization, may be preserved, by being enclosed between glasses, for a long time, as amusing objects for the microscope. To make the liquid spread readily on the glass, the surface of it may be moistened with a little of it, and rubbed with the finger. In this way, the repulsion which sometimes is observed between the liquid and the glass is completely removed. During the evaporation, the object-glass of the microscope is sometimes obscured by the condensation of the water from the saline solution on the slip of glass, and the vision is thus rendered indistinct. When this happens, if the circumstance be recollected, the glass must be wiped and replaced. In examinations of saline solutions, and in observing the progress of crystallization, Mr Baker recommends the light of a candle in preference to the light of day, which latter being of a whiter colour and nearly the same with the transparent crystals, they are less distinctly seen than with the brown light of a candle.

Fig. 1. is a representation of the microscopical crystals of nitre or saltpetre. They begin to shoot out from the edges with very moderate heat into flat figures of different lengths, with straight parallel sides, and exceedingly transparent. They appear in different states of their progress at the letters, \(a\), \(b\), \(c\), \(d\), and \(e\); \(a\) exhibits the appearance when they first begin to form. When a number of crystals have made their appearance they sometimes dissolve under the eye, and disappear entirely: but, by continuing to watch the changes which go on, the process is frequently observed to recommence, and new shoots push out. The first crystals sometimes become larger without undergoing any change of figure; and sometimes form in the way which is represented in the figure. When the heat is too great, as might be expected, the process goes on with great rapidity, and numerous ramifications are formed. This arises no doubt from the confused crystallization.

Fig. 2. shews the microscopical crystals of blue vitriol (sulphate of copper), which appear first round the edges, short at the beginning, but gradually increasing, as they are represented at the letters \(a\), \(b\), \(c\), which denote their difference of form, and the progress of their growth. These crystals, which are transparent, assume a solid regular form, and reflect the light from their polished sides and angles. As the evaporation proceeds, a great number of filaments as fine as hairs make their appearance, some crossing each other, as at \(d\); and others exhibiting a stellated form with many radiations, as at \(e\). The crystallization of this salt proceeds slowly. Towards the end of the process the regular crystals appear, and are finely branched as at \(f\).

Fig. 3. is a view of the crystals of distilled verdigrise, or acetate of copper. When it is immediately applied to the microscope, the regular figures \(1\), \(2\), \(3\), \(4\), \(5\), \(6\), \(7\), make their appearance; but if the solution is allowed to remain at rest for a few hours, and a drop of it is then heated on a slip of glass till it begins to concrete about the sides, sharp-pointed solid figures are formed, and shoot forwards. These crystals are often striated obliquely, frequently arise in clusters, or shoot from a centre. Sometimes, towards the end of the process, and in the middle of the drop, they assume a foliated form, and have the appearance of four leaves of fern united by their stems.

Fig. 4. shews the microscopical crystals of alum. These are more or less perfect according to the strength of the solution, and the temperature employed. To prepare this salt for examination, the saturated solution may remain for some days. In that time crystals will form, and if what remains liquid should be found too weak, heat may be applied, which will again dissolve the crystals.

In fig. 5. is a view of the crystals of borax, or the subcarbonate of soda. The drop of this solution should not be held too long over the fire, as it hardens on the slip of glass, and no crystals appear. A brisk heat for about a second is recommended as the best method. It is then applied to the microscope, and the crystals will form as in the figure.

Fig. 6. shews the microscopical crystals of sal ammoniac, or muriate of ammonia. Great numbers of thick, sharp, and broad spiculae shoot from the edges, and from their sides are protruded others of the same form, which are parallel to each other, but perpendicular to the main stem. The formation of these crystals, unless the heat employed be very moderate, is very rapid.

Fig. 7. exhibits the appearance of the crystals of acetate of lead (sugar of lead). After a little of this salt is dissolved in hot water, and allowed to remain at rest for a short time, it is fit for being examined with the microscope. A drop of it put on a slip of glass, and heat being applied, will be seen forming round the edge, a regular border of a clear and transparent substance, which with a strong heat runs over the whole of the drop, and hardens on the glass; but when the heat employed is moderate, bundles of lines, arranged Structure of ranged in a radiated form, make their appearance.