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INTRODUCTION TO LECTURE VI.

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CARBON-ITS HISTORY, NATURAL HISTORY, VARIETIES, AND PROPERTIES.

HISTORY.-Carbon, under most of the numerous forms which it can assume, has been known from time immemorial. Under the different conditions of charcoal, diamond, plumbago or black-lead, and plumbagine, it furnishes an instance of that interesting quality allotropism; although this fact was for a long period overlooked, or allowed to pass under the eyes of observers without reflection. Carbon exists in both kingdoms of nature, although it may be considered as especially belonging to the organic world-for the coal deposits, which are the main store of carbon in the inorganic kingdom, are well known to be of vegetable origin. The purest form of carbon, as ordinarily procured, is charcoal; which is developed by exposing animal or vegetable substances to heat, atmospheric air being excluded. The forms of apparatus for conducting this operation are various the most efficient consisting of retorts or distillatory cylinders, by means

of which, at the present time, all the charcoal employed in the manufacture of gunpowder is produced.

The means commonly had recourse to for the preparation of charcoal are illustrative of a leading chemical quality of this body-its complete fixity even at the highest temperature, provided the accession of air, or oxygen, be prevented.

Even when prepared from wood of different species the resulting charcoal differs as to its density, its power of electrical conduction, and certain other characters; but on examining other forms of black carbon, such as anthracite coal, coke, Kilkenny coal, plumbago, and plumbagine, other points of characteristic difference will be recognised. Common bituminous or caking coal is not carbon, but an association of many complex unions of carbon and hydrogen, from which heat expels the volatile parts, leaving coke behind, which is a mixture of carbon with small quantities of metallic oxides.

Amongst the most interesting forms of black carbon is plumbago, or black-lead-formerly considered to be a carburet of iron. The best specimens of plumbago, however, are altogether free from iron: hence this metal can only be regarded as an accidental impurity. Lead is never present in plumbago: hence the appellation black-lead is altogether a misnomer, and appears to have been given merely on account of the black mark, somewhat similar to that of lead but deeper, which plumbago leaves on white surfaces.

The employment of plumbago in the manufacture of pencils is too well known to require comment. For this purpose the best quality of plumbago was the produce of Borrowdale, in Cumberland; but the vein. is now quite exhausted, and other sources of plumbago have had to be sought. Most of the ordinary pencils now used are manufactured from a factitious paste, made of powdered plumbago, antimony, and sulphur fused together, cast into blocks, and these blocks sawn into bars of the required length and size. The great disadvantage of these pencils is their harsh grittiness, and the difficulty with which their marks are effaced by Indiarubber. A better sort of pencil-the best of all for certain effects, is made by subjecting the powder of plumbago to extreme hydrostatic pressure simultaneously with the abstraction of all remaining traces of air by means of the air-pump. The result of this treatment is a block, which, when cut by the usual method furnishes the bars that, properly mounted, constitute Brockedon's pencils;-preferred by Harding over all others, and which are undoubtedly the best for the purposes to which they are applied by that gentleman.

PLUMBAGINE is a term which has been applied to a material very much like plumbago in appearance, and which is formed, under certain circumstances, in gas-retorts. Ivory and bone black are varieties of charcoal which result from the concentration of ivory and bones in retorts. They are employed for a variety

of purposes. Ivory black forms a constituent of the finer kinds of printing-ink—especially that used for copper and steel plate engraving. The preparation of bone black now constitutes an enormous manufacture, its chief use being the decoloration of raw sugar in the operation of refining. For this purpose the bone black is prepared in the state of grain, packed into large cylinders, and the coloured sugar solution allowed to percolate through. This operation illustrates a function of charcoal, especially the variety termed animal charcoal, to bleach or decolourise. Still, in the case of bone black, it would be a mistake to assume that charcoal possesses the sole decolourising agency manifested by that substance, of which almost 90 per cent. is not charcoal, but carbonate and phosphate of lime, mixed with iron salts and some other bodies; yet if this 90 per cent. of matter not charcoal be removed by digestion with an acid, then the remainder, which is pure charcoal, does not decolourise, weight for weight, equal to the original and impure mixture.

THE DIAMOND.-By far the most extraordinary and beautiful, as well as the most valuable, form of carbon is the diamond, a gem which has been known and valued on account of its resplendent beauty from the earliest ages. Since the year 1720, when diamonds were first discovered in Brazil, that country has been their chief source; anterior to which period they were exclusively brought from India and Borneo.

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The composition of this gem is undoubtedly carbon, seeing that the sole result of its combustion in oxygen is carbonic acid gas: but the origin of the diamond is a subject of much curious speculation. Seeing that its structure is crystalline, the diamond should have been at some early period in a liquid or semi-liquid condition; a state which presupposes fusion by fire, or solution in some menstruum. Opposed to the first hypothesis is the circumstances that within the structure of many diamonds are seen remains of organic beings-appearances scarcely consistent with the assumption that the diamond was once in a state of igneous liquidity. Sir David Brewster inclines to the opinion that the diamond is a drop of fossilised gum.

Although the diamond has been celebrated for its beauty in all countries and all ages of which we have any notice, yet the extreme beauty which this gem is capable of assuming can only be developed by an ingenious and tedious process of cutting unknown even to this day in its full perfection by Eastern nations, and of somewhat modern introduction to Europe, viz., in the year 1456 by Louis Berghen, of Bruges, who accidentally discovered, that by rubbing two diamonds together a new face was produced. In other words the diamond is so hard that it can only be abraded by portions of its own substance: hence, diamond powder is universally employed for that purpose; such stones as, on account of their inferior colour or their flaws, are valueless as gems, being broken down into powder for the purpose of

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