muriatic acid, was formerly sold at a much higher price than oil of vitriol, and could only be very sparingly procured; but at present it is so abundantly produced in certain processes of manufacturing chemistry, that it is of less value than that of the glass bottles necessary to contain it; therefore torrents of muriatic acid are daily allowed to run to waste, and it is a matter of no small difficulty with the manufacturer to find out a place where to throw it away. A few years ago carbonate of soda was a valuable preparation, and sold for twelve shillings per pound; it may now be bought, dry and pure, for ninepence, or, in the crystalline state, for twopence per pound.

About thirty years ago a single grain of the metal potassium was a wonder; there were but few chemists who could elicit it from its combination with oxygen, and it could not be purchased at any rate, therefore its marvellous properties were almost exclusively exhibited in the laboratory or lecture-room of the noble Institution in which its discovery was made. As analytical chemistry advanced, the facilities for procuring potassium somewhat increased, and it could be sparingly purchased at the rate of about five shillings per grain, although many persons refused to part with it even at that price. It gradually became more abundant, and its price lowered in proportion, until at the present day it is kept in the shops as a common preparation, and sold for about fifteen shillings per ounce; a pound of potassium is now no uncommon quantity.

Although it has been stated that the room destined for the laboratory should be well stored with chemical preparations and apparatus, yet no extensive or extravagant outlay of money in their purchase is meant; on the contrary, a multitude of the most beautiful and instructive experiments can be made, with very limited means, and without any costly showy apparatus. It is better for the juvenile student to obtain a few "chemicals" as he requires them, rather than to fill his bottles and shelves at once with a host of articles selected from a catalogue. Those who are desirous of knowing how a regular laboratory should be fitted up, will find ample directions in Faraday's excellent work on Chemical Manipulation, which is also replete with valuable instructions concerning the art of making experiments.

To perform experiments with neatness, safety, and success, requires long and laborious study. A beginner must not expect anything like great precision in the results of his first attempts; nor must he be discouraged by repeated failures, but endeavour assiduously to ascertain the cause of them, and thus he will gradually, yet securely, acquire much valuable practical information. Chemistry is a science of experiment; facts are the data from which conclusions are to be drawn, for no reasoning, à priori, can enable us to judge as to the result of any chemical operation. But now, to commence the more immediate subject of the present paper; and this must be done by referring for a moment to Mechanical Attraction.

Mechanical Attraction principally exists between bodies of similar natures; or, where it takes place between solids and fluids, the bodies undergo no change of their respective natures; thus two pieces of lead by strong pressure unite with each other, and form one mass, which has precisely the same characters and habitudes of either piece of metal

separately. If a piece of lead be dipped into water, oil, or spirits of wine, and withdrawn, a drop of any of these fluids will be attracted by the lead, but they are not changed in their characters by this proceeding, for the attraction is merely mechanical, and mechanical means will instantly destroy it; thus the capillary attraction of a cloth or piece of blotting paper, will instantly draw any of these fluids from the surface of the solid lead.

Melt some lead in a ladle, and lay a little lump of tin upon its surface, it will float there, because it is much lighter than the lead; it will soon melt, and when this happens remove the ladle from the fire and let the metals cool. It will be found that, although the tin was the lightest metal of the two, it is not discoverable as a distinct stratum at the surface; that, cut the lump of mixed metals where you will, it presents an uniformity of composition, and nowhere can the tin and the lead be separately seen.

Take an ale-glass nearly full of water, and cautiously pour upon its surface some spirits of wine, coloured red with cochineal; this will float upon the water as a distinct stratum, (which the colour renders very evident, and it is employed for no other purpose,) for spirits of wine is lighter than water. Cover the glass with a card, or saucer, so as to prevent evaporation; take another ale-glass, about one-quarter full of water coloured with cochineal, and fill it up cautiously with colourless spirits of wine, which is best done by letting it run from the pipe of a funnel over which a bit of muslin is tied, this pressed against the sides of the glass, and nearly touching the water, will enable you to pour on the spirit without disturbing the water. Cover up this glass, as well as the former, and leave them for a day or two. At first the liquids preserve their respective situations, in virtue of their different relative weights; but, in the course of time, the lighter spirit will be drawn down through the heavier water, and the heavier water will rise through the lighter spirit, as will be evident from the red colour being diffused throughout the whole contents of both glasses, and when the action has arrived at its maximum, no repose will cause the spirit and water to separate into two distinct strata.

Now these are cases exactly analogous to the experiment with lead and tin, which has been already mentioned, some power of Attraction, very different from mere Mechanical Attraction, is here operating, to cause lighter substances to descend through heavier, and vice versa. It is Chemical Attraction,—the lead has a chemical attraction for the tin, the water has a chemical attraction for the spirits of wine; the substances, therefore, although of opposite weights and properties, are enabled mutually to penetrate and combine with each other, in opposition to the laws of gravity, and, when thus chemically combined, they will not separate, by any mechanical means. Every part of the lead, upon analysis, is found to contain tin, and the strength of the diluted spirit is found to be the same from whatever part of the glass it is taken for examination. Chemical Attraction is often called Heterogeneous Attraction, its distinguishing feature being, that it takes place between the particles of dissimilar bodies, causing their union, and producing a new and distinct class of compounds; when substances thus attract each other, they are

said to possess mutual affinity,-hence chemical attraction is far more commonly called Chemical Affinity.

To take another instance of this power of attraction. Let us refer to the metals gold and mercury; the one a solid metal of a fine yellow colour, the other a liquid metal of a silvery whiteness. Bring them into contact, they attract each other with a very considerable force; and that this is not merely mechanical, but chemical, is soon proved by the change of physical characters which the metals undergo by being left for a short time in contact; the gold loses its solidity and colour, and the mercury its liquid form,—the resulting compound is a soft, unctuous mass, of the colour of mercury. That form of gold-leaf called "dentists' gold" is convenient for this experiment, it is very much thicker than common gold-leaf, and therefore more tangible; a little disc of it, about half an inch in diameter, presented to a globule of mercury about the size of this letter (0) will present the result; or several leaves of ordinary gold-leaf may be rubbed with a similar globule in the palm of the hand, with the point of the finger, and the desired compound is produced. Such compound is called an amalgam*, and no mechanical means enable us to separate the metals after they are thus once combined; but heat will destroy the chemical attraction existing between the two metals; the mercury, being volatile, flies away in fumes, leaving the fixed gold in its metallic state. The amalgam, when rubbed upon the surface of a clean plate of copper, adheres to it, and presents a silvery-looking surface, but no gold will appear: expose the amalgamated plate to heat, the mercury volatilizes, and leaves the gold in firm and close contact with the copper; and upon this principle depends the art of water-gilding†. Gilt metal buttons are an example of it, they are made of discs of clean copper, to which the amalgam of gold is applied, then heated to expel the mercury, and the noble metal is left in the state called "dead gold," which may be polished to the full extent of its splendour, by rubbing it forcibly with a smooth hard steel, or agate, tool, called a burnisher‡.

Silver has a strong attraction for mercury, and forms an amalgam which is not distinguishable in appearance from that of gold, but yielding up silver by heat, and therefore applied for silvering the surface of copper. In experiments with mercury it often happens that coin and plate become accidentally spotted with it, thus a sovereign instantly becomes white, and no longer passable; heat it carefully in the flame of a spirit-lamp, the mercury volatilizes, and the coin will assume its proper lustre by a little friction. When mercury is accidentally spilt, persons often endeavour to collect the scattered globules in a silver tablespoon, which becomes of course totally spoiled by uniting with the mercury; but heat will volatilize it as in the case just mentioned, and

* The term amalgam is used to denote the compound of mercury with other metals; but when they unite with each other to the exclusion of mercury, as in the case of lead and tin, the term alloy is applied to the compound.

+Mercury was called hydrargyrum by the alchymists, the term signifying water of silver; perhaps the term water-gilding

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originated from the circumstance of this "solutive water" being employed to dissolve gold,

The highly-ornamental buttons worn on full-dress coats, furnish examples of gold in its dead and burnished state, the former is the appearance as it comes from the fire, the latter the result of friction.

the spoon may then be burnished until it acquires its usual lustre; if, however, the metals have been for some time in contact, the solid texture of the silver is destroyed throughout its whole mass, and then all that can be done is to heat the amalgam in an iron ladle, and preserve the silver for some other experiment. When mercury is accidentally spilt, the globules may be collected in a wooden, or horn spoon, or upon a piece of bent card; rings, watches, and trinkets, should always be laid aside during all chemical experiments, for not only will mercury spoil them, but various acids and gases are sad enemies to ornamental metalwork. This chemical attraction of mercury for the nobler metals is however of much practical utility, as has just been shown when speaking of "water-gilding," also in several metallurgical operations; the ores of silver are reduced to powder, and agitated with mercury for a considerable time, an amalgam of silver is formed, and this exposed to heat leaves the fixed metal in a state of considerable purity.

Tin and mercury also form an amalgam of nearly the same colour and texture as the two former; it has very important uses, for all our mirrors and looking-glasses owe their lustre to it. A thin sheet of tin, called tin-foil, is laid upon a smooth solid table, and amalgamated with mercury, thus presenting a brilliant metallic surface; upon this a perfectly clean and dry plate of glass is slid gently and carefully; equable pressure is then applied, and the glass forcibly adheres to the amalgam in virtue of attraction of cohesion. The superfluous mercury is afterwards drained away, and none is left excepting in actual combination with the tin; this hardens by time into a crystalline texture, as may be seen upon inspecting the back of a looking-glass.

Now it matters not how large or solid the pieces of gold, silver, or tin may be, they will all combine with mercury, and lose their states of aggregation in time; but it will be remarked, that if they are employed in the form of fine leaves, their union with the mercury is vastly expedited: we therefore come to the conclusion that attraction of cohesion influences chemical action. This fact is remarkably exemplified with regard to platinum: in the compact solid state, mercury has no action upon it whatever, and it might be hastily concluded that the two metals have no attraction for each other; but if the mechanical aggregation of the platinum be destroyed, so as to reduce it into very minute particles, mercury will then unite to it and form an amalgam*. Chemists are well aware of such facts, and therefore almost invariably destroy the attraction of cohesion of the substances which they submit to experiment; hence the variety of mortars, mills, shears, and files, with which a well-fitted laboratory abounds. Heat and solution are also resorted to when mechanical means cannot be conveniently used. There are numberless familiar examples of cohesion opposing chemical action; thus the fire-grate does not burn away, because of the strong cohesive force of the iron resisting the degree of heat to which it is ordinarily exposed; nor does a lump of coal suddenly and bodily start into combustion, because opposes cohesion to the chemical attraction of the oxygen of the air. Reduce an iron bar into fine filings, and a lump of coal into fine powder, * The finely-divided state of the metal required, is known as spongy platinum,”

it

which is obtained by heating its ammonia muriate to redness.

aggregation is thus to a great extent destroyed: sift them into a fire, the iron will burn with brilliant sparks, like the firework called " a gerbe;" the coal will suddenly burn with a very luminous flame. A lump of rosin held in the flame of a candle will not take fire; destroy its aggregation by powdering, and then dust it through a flame, and it produces an enormous blaze. The sudden combustion and flash of powdered rosin is often used at the theatres for producing what is called "artificial lightning." The dry vegetable powder called Lycopodium is employed for a similar purpose, and does not evolve so much smoke. In all these instances the substances are enabled rapidly to attract oxygen and burn, because their aggregation is overcome by mechanical means.

Again, a lump of rock-salt, alum, or sugar-candy, thrown into water, will be some time in dissolving, because the attraction of crystallization opposes chemical solution; reduce them to fine powder, and they all rapidly dissolve.

In some instances, even when the aggregation of bodies is destroyed, they refuse to exert any chemical attraction for each other, until a third agent is added to them. Thus, ink-powders consist of gall-nuts, and sulphate of iron; perfectly dry and in fine powder, they exert no attraction for each other, but remain a mere mixture, of a brownish colour: add water to this, it overcomes the aggregation of the powder by dissolving the sulphate of iron, which exerting its chemical attraction for the matter of the gall-nuts, unites with it to form a new and distinct compound, of a black hue, viz. writing-ink. Soda-water powders, or saline powders, are instances of the same kind: they consist of tartaric acid and carbonate of soda, both perfectly dry, powdered and mixed together, and in this state they will remain for years without showing any tendency to combine; their respective particles are not endowed with freedom of motion so as to come into close contact: add water, it overcomes their remaining cohesion, they dissolve, attract each other chemically, producing a solution, in which, if proper proportions are employed, the taste of neither substance is perceptible, and this union is attended with the escape of a vast quantity of gaseous matter, forming the well-known effervescc_ce.

If sand, carbonate of soda, and red-lead be reduced to fine powder, and intimately mixed together, they show no tendency to combine. If water is added, the carbonate of soda dissolves; but no other result takes place, excepting that the sand and red-lead sink as an insoluble mixture to the bottom of the vessel containing the experiment. We must seek then another agent to cause the union of these three distinct and opposite bodies: expose the powder to a very strong heat, the cohesion of its constituent particles will be overcome; they melt, and in this fluid state begin to exert a strong chemical attraction for each other, and ultimately produce a compound which is solid, hard, transparent, and brittle, namely glass.

The whole art of making this truly wonderful and important substance depends upon Chemical Attraction; it is another instance of the application of science to purposes of practical utility. There are few compounds in which the properties of the components are more completely disguised than in glass. Who would imagine that glass,