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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 it 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, 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.
* The finely-divided state of the metal which is obtained by heating its ammonia required, is known as spongy platinum, muriate to redness.
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 effervesce 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,
transparent and beautiful as it is, consists of three opaque bodies of such distinct and opposite natures? Water poured into a glass will no longer be able to dissolve away the soda, as it did from the mixture before fusion; and this is a remarkable instance of aggregation preventing chemical action, for if the glass be reduced to a moderately-fine powder, water will then instantly dissolve out the soluble soda*.
Soda presents an example of what chemists call an alkali: there are several such bodies, and they are all characterized by changing the yellow colour of vegetables to a reddish-brown, the yellow of the turmeric particularly. Paper stained with the watery infusion of this vegetable is called test-paper. Place a bit of glass upon it, moistened with water, no change of the yellow colour results: it, however, instantly changes to brown if powdered glass be so treated, thus indicating that alkali is abstracted by the water when aggregation is to some extent destroyed. Acids are opposed to alkalies, and, generally speaking, they change vegetable blue colours to red: paper stained with watery infusions of violets or litmus, is used as a test-paper for acids. Thus the tartaric acid instantly reddens litmus paper; but carbonate of soda, or soda, will restore the blue colour: in other words, it is said to neutralize the acid; hence when soda-water powders are dissolved in water, the resulting solution is neutral, neither acid nor alkaline.
The production of colour, or its modification by chemical agents, is the foundation of the arts of colour-making, dyeing, and calico-printing. Thus, lead exposed to heat absorbs oxygen, forming a red oxide, which is employed as a pigment, under the name of red-lead. If a piece of white cloth be immersed in a solution of sulphate of iron, afterwards dried, and then soaked in an infusion of galls, these two substances will attract each other in the fibre of the cloth, and form ink, with which compound the cloth is permanently dyed. If any design, such as a letter or flower, be drawn upon white calico, with a solution of prussiate of potash, which is bright yellow, allowed to dry, and then soaked in the solution of sulphate of iron, the two substances will attract each other, and produce a beautiful blue (prussian blue), wherever the design extends. This is the principle upon which one department of the beautiful art of calico-printing depends. If a strong solution of the well-known chloride of lime be spotted over the black cloth, the dye is destroyed wherever it touches, and white spots appear upon a black ground; and if the dyed or printed cloths be immersed in this solution, they become perfectly bleached, in consequence of the destruction of dye and colour by the agency of chlorine. This is the principle of the whole modern art of bleaching.
Substances which are useless, or of little value alone, form, by chemical attraction, compounds of vast utility. The skins of animals in a recent state do not admit of many useful applications; they are gelatinous, and prone to putrefaction and decay. The bark of trees is of little value, excepting for fuel; it is also apt to become mouldy and rotten: but if an infusion of bark be made, and a recent skin thrown into it, and suffered to remain for some weeks, the animal and vegetable
* A fact discovered by Griffiths, Quarterly Journal, xx, 262,
matters will be found to have entered into mutual combination, producing a substance, having the form of the skin, it is true, but neither its texture or habitudes; unlike its components it is insoluble in water, and, so far from being prone to putrefaction, it is remarkably permanent and of extreme utility, admitting of manifold applications in the arts and manufactures, and known as leather, produced by the art of tanning*.
A solution of glue or other animal jelly, added to infusion of oak-bark, will instantly form a solid mass, which is, chemically speaking, leather, wanting only texture.
Leather, as it comes from the tanners, is of a nutmeg-brown or russet-colour, and was formerly exclusively employed in this state, for harness, sandals, &c. Accident, perhaps, first showed that the contact of iron discoloured it, a sword might have been left in a wet leather scabbard, or a spur upon a wet riding-boot, which would both become black wherever there was contact with the steel or iron ; this, probably, gave the first hint of dyeing leather black, and gradually solutions of iron were employed for the purpose, under the name of “ Copperas Waters.” Copperas is sulphate of iron, and a little of its solution washed over russet leather will instantly dye it black, on account of chemically uniting with the matter of the oak-bark which the leather contains, and forming ink.
All black leather, excepting Japan leather, is dyed with solutions of iron,-generally the sulphate of iron; other colours are produced by different metallic solutions, as will be fully shown, when particularly discussing the chemical arts.
The action of various acids upon the metals presents some very important and pleasing instances of chemical affinity, a great number of compounds are thus produced which are called salts, all of which have their uses, either in chemistry, medicine, the arts, or manufactures t.
If a piece of pure silver be placed in a glass containing nitric acid, diluted with three parts of water, (and distilled or pure water is in all cases to be understood 1,) a violent action and a vast evolution of dark orange-coloured vapours takes place; the silver rapidly diminishes in bulk, and ultimately completely dissolves, forming a transparent colourless solution, (if the silver be thrown into the acid in fine filings, the action is still more energetic, and part of the materials fly suddenly out of the vessel and are lost, it is therefore better to employ the metal in its more aggregated state.) Now this is a curious and remarkable phenomenon, consequent upon the attraction which the two substances have for each other; no ordinary solvent will act upon silver, and therefore its solution in this instance is totally unlooked for; it proves the impossibility of anticipating the result of chemical action, by any reasoning à priori, it
* Probably the most ancient art on # Distilled water may be abundantly record.
obtained from the waste-pipe of a hot
house, or other building heated by steam: + In making these solutions of metals the waste steam from any boiler, if conin acids, and indeed in all cases where densed in a worm-pipe, will furnish disfumes are evolved, the vessels containing tilled water of purity sufficient for all ordithe experiment should be placed beneath nary purposes. Recent rain-water may a chimney, so that the fumes may be car- be substituted for distilled, if there is much ried away without annoying the operator. difficulty about the distillation.
can only be determined by experiment. Add silver to the solution until the acid refuses to dissolve any more, or is saturated with the metal, then pour off the clear solution into a cup or glass, set upon hot sand, and allow it to cool very gradually; determinate figures will soon make their appearance throughout the liquid, they will gradually increase in size until a considerable mass of white and beautiful crystals form, which are those of the nitrate of silver. Thus you have an example of the union of a liquid acid with a solid metal, producing a solid crystalline body, which is a salt of silver. A piece of copper acted upon by the diluted nitric acid, will readily dissolve with the disengagement of the same orange-coloured vapours; but the solution, instead of being colourless, has a beautiful deep blue tint, and if saturated and allowed to cool, as just directed, it yields blue crystals of nitrate of copper. Dry both crystalline products between two or three folds of blotting-paper, and leave them exposed for a few hours to light and air, the salt of silver will soon become purple, dark purple, and lastly, black, the salt of copper
will lose its solid form, and gradually pass into the liquid state, but undergo no change of colour. Place portions of the two salts in a dark room for a similar length of time, the nitrate of silver will remain solid and white, that of copper will still deliquesce exactly as before. We find, then, that the presence of light, which is an imponderable element, affects the nitrate of silver ; but what causes the change in the other? Nothing more than the watery vapour in the atmosphere, for which it has a strong attraction, and which therefore dissolves it when exposed.
The blackening effect of light upon nitrate of silver, is applied to domestic purposes, for marking linen. Permanent ink is a strong solution of it, which has a powerful attraction for vegetable fibre; many dyes for the hair, also, contain this salt; ivory, bone, and leather, are also frequently stained black by its employment; if any portion chances to touch the fingers in the above experiments, they will be indelibly dyed black. The crystals of nitrate of copper are highly soluble in spirits of wine; and if the solution be kindled, it burns with a lovely emerald-green flame, hence it is abundantly employed in pyrotechny, which is completely a chemical art; sponges soaked in the alcoholic solution of nitrate of copper, and suspended by fine wires over the stage of theatres, produce the lambent green flames now so common in incantation-scenes ; strips of flannel saturated with it, and applied round copper swords, tridents, &c., produce, when lighted, the flaming-swords and fire-forks, brandished by the demons of such scenes : the chief consumption of nitrate of copper is for these purposes. In theatrical displays, the skill of the chemist is everywhere called into activity, producing not only various-coloured flames and magical appearances, but also contributing to the splendour of the scenery, dresses, and decorations, the perfection of the gas-lights, and to the efficiency of the ventilation. Many other amusements depend mainly upon chemistry for their interest and perfection. The nitrates of silver and of copper
further present some curious information. Take the solution of the first, diluted with about half its weight of water in a glass, and place in it a slip of bright VOL. II.