After printing, the picture is toned and fixed. Toning consists in giving it a rich color by replacing part of the Ag2Cl with gold from a neutral solution of AuCl3. 3 Ag2Cl+ AUCl3 = 6AgCI + Au. Fixing removes the unaffected AgCl, as in the negative, the same substance being used. Describe the action. 2 AgCI + 3 Na2S203 = Ag2Na4(S203) + 2 NaCl. Both the positive and the negative must be well washed after each process, particularly after the last. The picture is then ready for mounting. In fine portrait work both the negative and the positive are retouched.

This consists in removing blemishes with colored pencils or India ink.

The negative--No. 1. Dissolve: sulphite soda crystals, 2 oz. (57 g) in 8 oz. (236 cc.) water (distilled); citric acid, 60 grains (4 g) in 1/2 oz. (15 cc.) water; bromide ammonium, 25 grains (1 1/2 g) in 1/2 oz. water; pyrogallic acid, 1 oz. (28 g) in 3 oz.

(90 cc.) water. After dissolving, mix in the order named, and filter. No. 2. Dissolve: sulphite soda, 2 oz. (57 g) in 4 oz.

(118 cc.) water; carbonate potash, 4 oz. (113 g) in 8 oz. (236 cc.) water. Dissolve separately, mix, and filter. To develop plates, mix 1 dram (3 2/3 cc.) of No. 1 and 1 dram of No. 2 with 2 oz. (60 cc.) water. Cover the plate with the mixture, and leave as long as the picture increases in distinctness. Remove, wash, and put it into a saturated solution of alum for a minute or two, then wash and put it into a half-saturated solution of hypo.

Leave till no white AgCl is seen through the back of the plate.

Wash it well.

The positive.--1. Dissolve 30 grains (2 g.) pure gold chloride in 15 oz. (450 cc.) water. This forms a stock solution. 2. Make a saturated solution of borax. 3. Prepare a toning bath by adding 1/2 oz. (15 cc.) of the gold chloride solution and 1 oz. (30 cc.) of the borax solution to 7 oz. (210 cc.) water. After printing the picture, wash it in 3 or 4 waters, put it into the toning bath, and leave it till considerably darker than desired; wash, and put it for 15 minutes into a hypo solution that has been, after saturation, diluted with 3 or 4 volumes of water. Then wash repeatedly.

CHAPTER LII.

PLATINUM AND GOLD.

PLATINUM.

Examine platinum foil and wire.

285. Platinum is much rarer than gold, and is about two-thirds as costly as the latter. It is found alloyed with other metals, as An, and is obtained from sand, in which it occurs, by washing.

Aqua regia is the only acid which dissolves it, and the action is much slower than with Au. Pt is one of the heaviest metals, having a specific gravity three times that of Fe, or twenty-one and a half times that of water. Its fusing-point is about 1600 degrees, or just below the temperature of the oxy-hydrogen flame.

Like Au it has little affinity for other elements, but alloys with many metals. Pt is so tenacious that it can be drawn into wire invisible to the naked eye, being drawn out in the center of a silver wire, which is afterwards dissolved away from the Pt by HNO3. Noting its valences, 2 and 4, write the symbols for the ous and ic chlorides and oxides.

286. Uses.--Pt is much used in chemistry in the form of foil, wire, and crucibles. On what properties does this use depend?

Describe its use in making H2SO4.

PtCl4 is made by dissolving Pt in aqua regia, and evaporating the liquid. On heating PtCl4, half of its Cl is given up, leaving PtCl2. If it be still more strongly heated, the Cl all pa.s.ses off, leaving spongy Pt. By fusing this in the oxy-hydrogen flame, ordinary Pt is obtained. Spongy Pt has a remarkable power of absorbing, or occluding, O without uniting with it. This O it gives up to some other substances, and thus becomes indirectly an oxidizing agent. What other element has this property of occluding gases?

GOLD.

Examine auriferous quartz, gold chloride, yellow and ruby gla.s.s colored with gold. 287. Gold is rarely found combined, and has small affinity for other elements, though forming alloys with Cu, Ag, and Hg. Its source is usually either quartz rock, called auriferous quartz, or sand in placer mines. The element is widely distributed, occurring in minute quant.i.ties in most soils, sea water, etc. California and Australia are the two greatest gold- producing countries. That from California has a light color, due to a slight admixture of Ag. Australian gold is of a reddish hue, due to an alloy of Cu. Gold-bearing quartz is pulverized, and treated with Hg to dissolve the precious metal, which is then separated from the alloy by distillation. Compare this with the preparation of Ag.

Such is the malleability of Au that it has been hammered into sheets not over one-millionth of an inch thick; it is then as transparent as gla.s.s. Gold does not tarnish or change below the melting-point. On account of its softness it is usually alloyed with Cu, sometimes with Ag. Pure gold is twenty-four carats fine.

Eighteen carat gold has eighteen parts Au and six Cu. Gold coin has nine parts Au to one part Cu. The most important compound is AuCl3. Describe a use of it. This metal is much employed in electroplating, and somewhat in coloring gla.s.s.

CHAPTER LIII.

CHEMISTRY OF ROCKS.

288. Cla.s.sification.--Rocks may be divided, according to their origin, into three cla.s.ses: (1) Aqueous rocks. These have been formed by deposition of sedimentary material, layer by layer, on the bottoms of ancient oceans, lakes, and rivers, from which they have gradually been raised, to form dry land. (2) Eruptive or volcanic rocks. These have been forced, as hot fluids, through rents and fissures from the interior of the earth. (3) Metamorphic rocks. These, by the combined action of heat, pressure, water, and chemical agents, have been crystallized and chemically altered. The rocks of the first cla.s.s, such as chalk, limestone, shale, and sandstone, are distinguished by the existence of fossils in them, or by the successive layers of the material which goes to make up their structure and to give them a stratified appearance. The rocks of the second cla.s.s are recognized by their resemblance to the products of modern volcanoes and their non-stratified appearance. Rocks of the third cla.s.s are composed of crystals, which, though often very minute, are minerals having a definite chemical composition. Examples of the third cla.s.s are gneiss, slate, schist, and marble. The last two cla.s.ses abound on the Eastern sea-board, while the interior of our continent is composed almost exclusively of stratified sedimentary rocks.

289. Composition.--Rocks are not definite compounds, but variable mixtures of minerals. Some, however, are tolerably pure, as limestone (CaCO3) and sand-stone.

Granite is mainly made up of three minerals,--quartz, feldspar, and mica. Quartz, when pure, is SiO2. Feldspar is a mixed silicate of K and Al, and often several other metals, K2Al2Si6O16 (=K2O, Al2O3, 6 SiO2) symbolizing one variety, while a variety of mica is H8Mg5Fe7Al2Si3O18.

The pupil should learn to distinguish the different minerals in granite. Quartz is gla.s.sy, mica is in scales, usually white or black, and feldspar is the opaque white or red mineral.

290. Importance of Siliceous Rocks.--Slate and schist are also mixed silicates. Pure sandstone is SiO2, the red variety being colored by iron. Igneous rocks are always siliceous. Obsidian is a gla.s.sy silicate. A mountain of very pure gla.s.s, obsidian, two hundred feet high, has lately been found in the Yellow-stone region. We see how important Si is, in the compounds Si02 and the silicates, as a const.i.tuent of the terrestrial crust. Limestone is the only extensive rock from which it is absent. Always combined with O, it is, next to the latter, the most abundant of elements. Silicates of Al, Fe, Ca, K, Na, and Mg are most common, and these metals, in the order given, rank next in abundance.

291. Soils.--Beds of sand, clay, etc., are disintegrated rock.

Sand is chiefly SiO2; clay is decomposed feldspar, slatestone, etc. Soils are composed of these with an added portion of carbonaceous matter from decaying vegetation, which imparts a dark color. The reddish brown hue so often observed in soils and rocks results from ferric salts.

292. Minerals, of which nearly 1000 varieties are now known, may be simple substances, as graphite and sulphur, or compounds, as galena and gypsum. Only seven systems of crystallizations are known, but these are so modified as to give hundreds of forms of crystals. See Physics. A given chemical substance usually occurs in one system only, but we saw in the case of S that this was not always true.

Crystals of some substances deliquesce, or take water from the air, and thus dissolve themselves. Some compounds cannot exist in the crystalline form without a certain percentage of water. This is called "water of crystallization"; if it pa.s.ses into the air by evaporation, the crystal crumbles to a powder- and is then said to effloresce.

293. The Earth's Interior.--We are ignorant of the chemistry of the earth's interior. The deepest boring is but little more than a mile, and volcanic ejections probably come from but a very few miles below the surface. The specific gravity of the interior is known to be more than twice that of the surface rock. From this it has been imagined that towards the center heavy metals like Fe and Au predominate; but this is by no means certain, since the greater pressure at the interior would cause the specific gravity of any substance to increase.

294. Percentage of Elements.--Compute the percentage of O in the following rocks, which compose a large proportion of the earth's crust: SiO2, Al2SiO4, CaCO3. Find the percentage of O in pure water. In air. Taking cellulose, C16H30O15, as the basis, find the percentage of O in vegetation.

An estimate, based on Bunsen's a.n.a.lysis of rocks, of the chief elements in the earth's crust, is as follows:--

O, 46 per cent Ca, 3 per cent Si, 30 per cent Na, 2 per cent Al, 8 per cent K, 2 per cent Fe, 6 per cent Mg, 1 per cent

More than half the elements are known to exist in sea-water, and the rest are thought to be there, though dissolved in such small quant.i.ty as to elude detection. What four are found in the atmosphere?CHAPTER LIV.

ORGANIC CHEMISTRY.

295. General Considerations.--Inorganic chemistry is the chemistry of minerals, or unorganized bodies. Organic chemistry was formerly defined as the chemistry of the compounds found in plants and animals; but of late it has taken a much wider range, and is now defined as the chemistry of the C compounds, since C is the nucleus around which other elements centre, and with which they combine to form the organic substances. New organic compounds are constantly being discovered and synthesized, so that nearly 100,000 are now known. The molecule of organic matter is often very complex, sometimes containing hundreds of atoms.

In organic as in inorganic chemistry, atoms are bound together by chemical affinity, though it was formerly supposed that an additional or vital force was instrumental in forming organic compounds. For this reason none of these substances, it was thought, could be built up in the laboratory, although many had been a.n.a.lyzed. In 1828 the first organic compound, urea, was artificially prepared, and since then thousands have been synthesized. They are not necessarily manufactured from organic products, but can be made from mineral matter.

296. Molecular Differences.--Molecules may differ in three ways: (1) In the kind of atoms they contain. Compare CO2 and CS2. (2) In the number of atoms. Compare CO and CO2. (3) In the arrangement of atoms, i.e. the molecular structure. Ethyl alcohol and methyl ether have the same number of the same elements, C2H6O, but their molecular structure is not the same, and hence their properties differ.

Qualitative a.n.a.lysis shows what elements enter into a compound; quant.i.tative a.n.a.lysis shows the proportion of these elements; structural a.n.a.lysis exhibits molecular structure, and is the branch to which organic chemists are now giving particular attention. '

A specialist often works for years to synthesize a series of compounds in the laboratory.

297. Sources.--Some organic products are now made in a purer and cheaper form than Nature herself prepares them. Alizarine, the coloring principle of madder, was until lately obtained only from the root of the madder plant; now it is almost wholly manufactured from coal-tar, and the manufactured article serves its purpose much better than the native product. Ten million dollars' worth is annually made, and Holland, the home of the plant, is giving up madder culture. Artificial naphthol-scarlet is abolishing the culture of the cochineal insect. Indigo has also been synthesized. Certain compounds have been predicted from a theoretical molecular structure, then made, and afterwards found to exist in plants. Others are made that have no known natural existence. The source of a large number of artificial organic products is coal-tar, from bituminous coal. Saccharine, a compound with two hundred and eighty times the sweetening power of sugar, is one of its latest products. Wood, bones, and various fermentable liquids are other sources of organic compounds.

298. Marsh-Gas Series.--The chemistry of the hydro-carbons depends on the valence of C, which, in most cases, is a tetrad.

Take successively 1, 2, and 3 C atoms, saturate with H, and note the graphic symbols:--

H H H H H H | | | | | | H-C-H, or CH4.H-C-C-H, or?H-C-C-C-H, or ?

| | | | | | H H H H H H

Write the graphic and common symbols for 4, 5, and 6 C atoms, saturated with H. Notice that the H atoms are found by doubling the C atoms and adding 2. Hence the general formula for this series would be CnH2n+2. Write the common symbol for C and H with ten atoms of C; twelve atoms; thirteen. This series is called the marsh-gas series. The first member, CH4 methane, or marsh gas, may be written CH3H, methyl hydride, CH3 being the methyl radical. C2H6, ethane, the second one, is ethyl hydride, C2H5H.

Theoretically this series extends without limit; practically it ends with C35H72.

In each successive compound of the following list, the C atoms increase by unity. Give the symbols and names of the compounds, and commit the latter to memory:--

Boiling-point.

1. CH4methane, or CH3H,methyl hydride, gas.

2. C2H6ethane, C2H5H,ethyl hydride, gas 3. C3H8propane, C3H7H,propyl hydride, gas 4. ?butane, ? ? 1 degree 5. ?pentane ? ? 38 degrees 6. ?hexane, ? ? 70 degrees 7. ?heptane, ? ? 98 degrees 8. ?octane, ? ? 125 degrees 9. ?nonane, ? ? 148 degrees 10.?dekane, ? ? 171 degrees