For confining the vapour of acid, or highly corrosive substances, the fat lute is well adapted. It is formed by beating perfectly dry and finely sifted tobacco pipe-clay, with painters' drying oil, to such a consistence that it may be moulded by the hand. The same clay, beat up with as much sand as it will bear, without losing its tenacity, with the addition of cut tow, or of horse-dung, and a proper quantity of water, furnishes a good lute, which has the advantage of resisting a considerable heat, and is applicable in cases where the fat lute would be melted or destroyed. Various other lutes are recommended by chemical writers; but the few that have been enumerated I find to be amply sufficient for every purpose.

On some occasions, it is necessary to protect the retort from too sudden changes of temperature, by a proper coating. For glass retorts, a mixture of moist common clay, or loam, with sand, and cut shreds of tow or flax, may be employed. If the distillation be performed by a sand heat, the coating needs not be applied higher than that part of the retort which is bedded in sand; but if the process be performed in a wind furnace (fig. 63), the whole body of the retort, and that part of the neck also which is exposed to heat, must be carefully coated. To this kind of distillation, however, earthen retorts are better adapted; and they may be covered with a composition originally recommended by Mr. Willis. Two ounces of borax are to be dissolved in a pint of boiling water, and a sufficient quantity of slaked lime added, to give it the thickness of cream. This is to be applied by a painter's brush, and allowed to dry. Over this a thin paste is afterwards to be applied, formed of slaked lime and common linseed-oil, well mixed and perfectly plastic. In a day or two the coating will be sufficiently dry to allow the use of the retort.

For joining together the parts of iron vessels, used in distillation, a mixture of the finest China clay, with solution of borax, is well adapted. In all cases, the different parts of any apparatus made of iron should be accurately fitted by boring and grinding, and the above lute is to be applied to the part which is received into an aperture. This will generally be sufficient without any exterior luting; otherwise the lute of clay, sand, and flax, already described, may be used.

In every instance, where a lute or coating is applied, it is adviseable to allow it to dry before the distillation is begun; and even the fat lute, by exposure to the air during one or two days after its application, is much improved in its quality. The clay and sand lute is perfectly useless, except it be previously quite dry: In applying a lute, the part immediately over the juncture should swell outwards, and its diameter should be gradually diminished on each side. (See fig. 13, where the luting is shown, applied to the joining of the retort and receiver.)

Beside the apparatus already described, a variety of vessels and instruments are necessary, having little resemblance to each other, in the purposes to which they are adapted. Glass vessels are requirVOL. IE

ed for effecting solution, which often requires the application of heat, and sometimes for a considerable duration. In the latter case, it is termed digestion, and the vessel, fig. 4, called a matrass, is the most proper for performing it. When solution is required to be quickly effected, the bottle, fig. 5, with a rounded bottom, may be used; or a common Florence oil flask serves the same purpose extremely well, and bears, without cracking, sudden changes of temperature. For precipitations, and separating liquids from precipitates, the decanting-jar (fig. 14), will be found useful; or, if preferred, it may be shaped as in fig. 26, f. Liquids, of different specific gravities, are separated by the vessel, fig. 3; the heavier fluid being drawn off through the cock b, and air being admitted by the removal of the stopper a, to supply its place. Glass rods, of various lengths, and spoons of the same material, or of porcelain, are useful for stirring acid and corrosive liquids; and a stock of cylindrical tubes, of various sizes, is required for occasional purposes. It is necessary also to be provided with a series of glass measures, graduated into drachms, ounces, and pints. The small tube, fig. 15, called a dropping tube, which is open at each end and blown in the middle into a ball, will be found useful in directing a fine stream of water upon the edges of a filtre, or any small object. The same purpose may, also, be very conveniently effected by fixing a piece of glass tube of small bore, two or three inches long, and bent at one end to an obtuse angle, into a hole bored in a cork, which may be used as the stopple of an eight ounce vial filled with water, fig. 25, a. On inverting the vial, and grasping the bottom part of it, the warmth of the hand expels either a few drops or a small stream of water, which may be directed upon any minute object. When the flow ceases, it may be renewed, if required, by setting the bottle, for a moment, with its mouth upwards (which admits a fresh supply of cool air), and then proceeding as before.

For the drying of precipitates, and other substances, by a heat not exceeding 212°, a very useful apparatus is sold in London. It is represented, supported by the ring of a lamp-stand, by fig. 27. The vessel a is of sheet-iron or copper japanned and hard-soldered; c is a conical vessel of very thin glass, having a rim, which prevents it, when in its place, from entirely slipping into a; and d is a moveable ring, which keeps the vessel c in its place. When the apparatus is in use, water is poured into a about as high as the dotted line; the vessel c, containing the substance to be dried, is immersed in the water, and secured by the ring d; and the whole apparatus set over an Argand's lamp. The steam escapes by means of the chimney b, through which a little hot water may be occasionally poured, to supply the waste by evaporation. By changing the shape of c to the segment of a sphere, still retaining the rim, I have d it a most convenient vessel for evaporating fluids.

curate beams and scales, of various sizes, with corresponding ts, some of which are capable of weighing several pounds, the smaller size ascertains a minute fraction of a grain, are es instruments in the chemical laboratory. So also are mor

tars of different materials, such as of glass, porcelain, agate, and metal. Wooden stands, of various kinds, for supporting receivers, should be provided*. For purposes of this sort, and for occasionally raising to a proper height any article of apparatus, a series of blocks, made of well seasoned wood, eight inches (or any other number) square, and respectively, eight, four, two, one, and half an inch in thickness, will be found extremely useful; since, by combining them in different ways, thirty-one different heights may be obtained.

The blow-pipe is an instrument of much utility in chemical researches. A small one, invented by Mr. Pepys, with a flat cylindrical box for condensing the vapour of the breath, and for containing caps, to be occasionally applied with apertures of various sizes, is perhaps the most commodious formt. One of a much smaller size, for carrying in the pocket, has been contrived by Dr. Wollastont. A blow-pipe, which is supplied with air from a pair of double bellows, worked by the foot §, may be applied to purposes that require both hands to be left at liberty, and will be found useful in blowing glass, and in bending tubes. The latter purpose, however, may be accomplished by holding them over an Argand's lamp with double wicks. Occasionally, when an intense heat is required, the flame of the blow-pipe, instead of being supported by the mouth, may be kept up by a stream of oxygen gas, expelled from a bladder or from a gas-holder. The blow-pipe invented by Mr. Brooke, consists of a small square box of copper or iron, into which air is forced by a condensing syringe, and from which it is suffered to rush, through a tube of very small aperture, regulated by a stop-cock, against the flame of a lamp or candle ¶. By means of a screw added to the syringe, the receiver may be filled with oxygen gas, or, as will be described in chap. v. sect. 5, with a mixture of hydrogen and oxygen gases. Blow pipes on this construction may be had of Mr. Newman, and of most of the other makers of philosophical instruments.

In the course of this work, various other articles of apparatus will be enumerated, in detailing the purposes to which they are adapted, and the principles on which they are constructed. It must be remembered, however, that it is no part of my object to describe every ingenious and complicated invention, which has been employed in the investigation of chemical science: but merely to assist the stu dent in attaining apparatus for general and ordinary purposes. For such purposes, and even for the prosecution of new and important

See a representation of the apparatus for this purpose, in the Chemical Conversations. pl. ix.

Thomson's Annals, vii, 367; or, Journal of Science and the Arts, i. 65.

inquiries, very simple means are sufficient; and some of the most interesting chemical facts may be exhibited and even ascertained, with the aid merely of Florence flasks, of common vials, and of wine glasses. In converting these to the purposes of apparatus, a considerable saving of expense will accrue to the experimentalist; and he will avoid the encumbrance of various instruments, the value of which consists in show, rather than in real utility.

In the selection of experiments, I shall generally choose such as may be undertaken by persons not possessed of an extensive chemical apparatus. On some occasions, however, it may be necessary, in order to complete the series, that others should be included, requiring, for their performance, instruments of considerable nicety. The same experiment may, perhaps, in a few instances, be repeatedly introduced in illustrations of different principles; but this repetition will be avoided as much as possible. Each experiment will be preceded by a brief enunciation of the general truth which it is intended to illustrate.

CHAPTER II.

OF CHEMICAL AFFINITY.

ALL bodies, composing the material system of the universe, have a mutual tendency to approach each other, whatsoever may be the distances at which they are placed. The operation of this force extends to the remotest parts of the planetary system, and is one of the causes that preserve the regularity of their orbits. The smaller bodies, also, that are under our more immediate observation, are influenced by the same power, and fall to the Earth's surface when not prevented by the interference of other forces. From these facts, the existence of a property has been inferred, which has been called attraction, or more specifically, the attraction of gravitation. Its nature is entirely unknown to us; but some of its laws have been investigated, and successfully applied to the explanation of phenomena. Of these, the most important are, that the force of gravity acts on bodies directly in proportion to the quantity of matter in each; and that it decreases in the reciprocal proportion of the squares of the distances.

From viewing bodies in the aggregate, we may next proceed to contemplate them as composed of minute particles. Of the nature of these particles, we have no satisfactory evidence. It is probable that they consist of solids, which are incapable of mechanical division, but are still possessed of the dimensions of length, breadth, and thickness. In simple bodies, the particles must be all of the same nature, or homogeneous. In compound bodies, we are to understand, by the term particles, the smallest parts into which bodies can be resolved without decomposition. The word atom has of late been revived, to denote both these kinds of particle; and we may,

therefore, speak, with propriety, of simple atoms and of compound atoms. When two atoms of different kinds unite to form a third or compound atom, we may term the two first component atoms; and if these have not been decomposed, they may be called elementary or primary atoms.

The atoms or particles of bodies are also influenced by the force of attraction, but not unless when placed in apparent contact. Hence a distinction has been made between gravitation, and that kind of attraction which is effective only at insensible distances. The latter has been called contiguous attraction or affinity; and it has been distinguished, as it is exerted between particles of matter, of the same kind, or between particles of a different kind.

By the affinity of aggregation the cohesive affinity, or more simply cohesion, is to be understood that force or power, by which particles or atoms of matter of the same kind attract each other, the only effect of this affinity being an aggregate or mass. Thus a lump of copper may be considered as composed of an infinite number of minute particles or integrant parts, each of which has precisely the same properties, as those that belong to the whole mass. These are united by the force of cohesion. But if the copper be combined with another metal (such as zinc), we obtain a compound (brass), the constituent parts of which, copper and zinc, are combined by the power of chemical affinity. In simple bodies, therefore, cohesion is the only force exerted between these particles. But in compound bodies, we may distinguish the force, with which the component atoms are united, from that which the compound atoms exert towards each other; the former being united by chemical affinity, and the latter by the cohesive attraction.

SECTION I.

Of Cohesion, Solution, and Crystallization.

The cohesive affinity is a property, which is common to a great variety of bodies. It is most strongly exerted in solids; and in these it is proportionate to the mechanical force required for effecting their disunion. In liquids, it acts with considerably less energy; and in aëriform bodies we have no evidence that it exists at all; for their particles, as will afterwards be shown, are mutually repulsive, and, if not held together by pressure, would probably separate to immeasureable distances. Its force is not only different among different bodies, but in various states of the same body. Thus in the cohesion of certain metals (steel for instance), important changes are produced by the rate of cooling, by hammering, and by other mechanical operations. Water, also, in a solid state, has considerable cohesion, which is much diminished when it becomes liquid, and is entirely destroyed when it is changed into vapour.