WHAT IS PLUMBAGO ?

Hawke's Bay Times, Volume V, Issue 248, 5 April 1865, Page 2

WHAT IS PLUMBAGO ?

In the Mineralogical section of the Chronicles of Science, published in the Quarterly Journal of Science, appears the following notice of the character and composition of this mineral; — “One word on the formation, still so little known, of graphite (plumbago, pencil lead). The presence of graphite in granite, gneiss, and diorits has renewed the disputes between the -Neptunists and Pluntoniists. Graphite is well known to be nearly pure carbon, for it leaves'in burning but a very small quantity of a,sh. .Now, if these primitive crystalline rocks are of igniops. formation, it is impossible to explain how graphite could coexist with silicates of protoxide of iron without having reduced these ‘ salts. Judging merely by what takes place ip blast furnaces, carbon reduces all oxides of iron at a high temperature. It must then be admitted that granite, gneiss, and diorite did not contain graphite when the mineral elements of these rocks, such as mica, hornblende, and other ferrous silicates, were in a stale of fusion. Graphite then must have been subsequently introduced into those rocks, but how? Questions such as these are very difficult to answer eatisfao-

gorily. The most plausible hypothesis is that graphite.has been introduced by the wet way into the crystalline rocks an substituted for one of the mineral compoments. Thus.in the gniess of If as* sau*(Bavaria) it takes the place of mica. “ Graphite is frequently to be met with with in granuiated’limestoue, a fact particularly interest* ing-to geologists. Is limestone a product of eruption, or is it a sediment transformed by the action of heat ? The presence of graphite is explicable by neither hypothesses. For at a certain tempe-' rature, which need not be very high, it decompose* carbonate of lime. This salt may, no doubt, under strong pressure be heated to the melting point without losing its carbonic acid ; this is a laboratory experiment often cited by the Piutonists; .but it is quite a different tiling with a mixture of carbon and carbonate of lime at a high temperature. If we reject the Neptunian origin of granulated limestone, we must then, as with crystalline rocks, suppose that graphite has been introduced by the wet way at a more recent recent period. The same remark applies to mageutic pyrites (sulphide of iron) often very rich in plumbago kerns. Does graphite, like all carbon, belong to the organic kingdom P It is certain that anthracite, lignite, coal, are the result oftheslowdecompositiou of an enormous quantity of vegetables. The impressions found on them often indicate the kind of vegetables, most of them extinct, which have contributed to these carbonaceous. formations.. Graphite, if not formed in precisely the same way as coal and anthracite, nevertheless bears signs of an organic origin. The formation of nuclei and veins of graphite in crystalline rocks is sufficiently explained by the decomposition of carbonized hydrogen gas at a high temperature; this gas, disengaged from organic matters, and penetrating the fissures of the burning rock, would undergo decomposition into hydrogen and carbon. “It is this deposited carbon which forms graphite. If in our laboratories we do not obtain exactly the same product, it must be remembered that nature has means at her command which escape our researches. We find it it impossible to make coal from wood. The wood may be car bonized by the dry or by the wet way. In the first case the carbonization is very vapid ; in the latter it is extremely slow', as is shown by the the blackened points of piling sunk in water. Finally, graphite has been found in meteorites or aerolites. Attempts have been made to explain its presence here by the continuance of these stones in soli more or less rich in carbonized princiules. But with regard to newly-fallen stones, this explanation is inadmissible. If it be maintained that graphite is an organic product, it must be admitted that in the case of newly-fallen meteoriees jt can proceed only from organic matters belonging to another world than our own.”