April i, 1885.] 



THE TROPICAL AGRICULTURIST, 



767 



PLUMBAGO : ITS GEOLOGIC AGE AND 

 ORIGIN. 



The writer in the local " TimeB " who, over the 

 initial R., wrote recently on the geology of Uva, in 

 which he included " trap " rock, a volcanic product 

 in a country believed as yet to show no definite signs of 

 volcanic action, now expreises doubt regarding ourd 

 quoted statement that plumbago is amongst the oldest 

 formations in the earth's crust. If belonging to the 

 primitive serifs of rocks, then, R. asserts, it cannot be 

 of vegetable origin : — 



On the other hand, the existence of plumbago in th e 

 oldest known formations being admitted would lead me 

 to the supposition that it is some chemical elimination 

 formed in the bowels of the earth, from the fact that 

 graphite or plumbago is produced in the manufacture of 

 cast iron, separating from the molten pig-iron in the form 

 of scales. 



As. R. desiderates the latest information regarding 

 rhe mineral of which, in an available state, Ceylon 

 has almost a monopoly, we may ask him if he bas 

 tead Professor Armstrong's article on Carkon in the 

 new edition of the Encyclopedia Britannica ; because 

 in that article, natural plumbago and that obtained 

 from iron furnaces are stated to be essentially 

 different. On the other hand the identity in constiiu- 

 ents of plumbago and the most b eantiful and 

 precious of all crystals, the diamond, seems to be 

 assumed at the very commencement of the article : — 



Carbon (symbol, C; atomic weight, 12) is one of the most 

 important of the eemical elements. It occurs pure in the 

 diamond, and nearly pure as graphite or plumbago ; it is 

 a constituent of all animal and vegetablo tissues and of 

 coal ; and it also enters into the composition of many 

 minerals, such as cbalk and dolomite. 



Carbon is a solid substance, destitute of taste and 

 odour ; but it occurs in several modifications which exhibit 

 very diverse physical properties. Thus, it is met with 

 in the form of the diamond in transparent crystals belong- 

 ing to the regular or cubical system, which conduct 

 electricity but slowly ; and in the form of graphite in 

 opaque crystals belouging to the hexagonal system, which 

 conduct electricity nearly as well as the metals. The diamond 

 is the hardest substance known, and has a relatively high 

 specific gravity (3'33 to 3"55), but graphite is comparat- 

 ively soft, producing a black shining streak when rubbed 

 upon paper and has a much lower specific gravity (2' 15 to 

 2"35). In addition to these two crystalline modifications of 

 carbon there are a number of varieties of non-crystalline 

 nr amorphous carbon, which, however, exhibit the greatest 

 differences in physical properties. 



By heating to the high temperature afforded by a power- 

 ful galvanic battery, both the diamond and amorphous 

 carbon are converted into graphite. In the election are 

 carbon appears to be converted into vapour ; but the 

 temperature which is required to volatilize it is extremely 

 high ; in fact, it has been calculated that the boiling- 

 point of carbon is not less than about 7,000 ° on the centi- 

 grade scale. 



Experiments are then described under which the 

 diamond remained intact, but the an orpbous carbons 

 were converted into " a /iMJnuo'-like substance" : — 



Or " humic acids," soluble in water, whereas the differ- 

 ent varieties of graphite furnish '* graphite oxides," which 

 are insoluble in water, and especially characterized by the 

 property of undergoing decomposition with deflagration 

 when heated. 

 Then follows the statement : — 



Berlhelot has examined a very large numberof varieties of 

 carbon in this manner with the following results. The carbon 

 of wood charcoal, animal charcoal, coke, the so-called metallic 

 carbon obtained by decomposing hydrocarbons by passing 

 their vapours through a red-hot tube, gas-retort carbon 

 and various specimens of anthracite from different sources 

 all dissolved entirely with more or less readiness when 

 treated in the above manner; lamp black, however. furn- 



ished a small amount of graphite oxide. The amorphous 

 carbon of the meteorite of Craubourne (Australia) furn- 

 ished a graphite oxide identical with that obtained by 

 similarly treating graphite from cast-iron, but the carbon 

 of the Orgueil meteorite was entirely soluble. The carbon 

 of the Greeidaud meteoric rock discovered by Norden- 

 skiod also dissolved entirely with the exception of a very 

 insignificant residue. 



On examining the pencils of carbon employed in pro- 

 ducing the electric light it was found that the spongy 

 muss of carbon collected on the negative pole contained 

 a large proportion of graphite, but that only traces 

 were present in the pencil employed as positive pole, 

 which appears to indicate that it is necessary for the 

 carbon to undergo volatilization in order that it may be 

 converted into graphite. The graphite thus prodaced is 

 not identical with that contained in cast-iron, nor with 

 natural plumbago ; the same variety of graphite is pro- 

 duced, however, when the diamond is heated in the 

 electric arc. 



Then comes a separate notice of " graphite " which 

 we quote in full : — 



Graphite. — Graphite is found native near Travancore, in 

 Ceylon, and near Moreton Bay in Australia, in several 

 parts of the United States, in South Siberia, and in Ger- 

 many, principally at Griessbach near Passau, always in 

 rocks belonging to the earliest formations. It occurs in two 

 distinct modifications, one of which, like the Borrowdale 

 graphite, is fine-grained and amorphous ; the other, like 

 the Ceylon variety, is composed of small flat plates. Native 

 graphite contains from 95 to nearly 100 per cent of carbon, 

 the impurity being usually small quantities of silicates. 

 Graphite, also called plumbago or black lead, is used for 

 makiug so-called lead pencils, for polishing iron work, for 

 lubricating machinery, for making crucibles, and in the 

 electrotype process for coating the surfaces of wood, 

 plaster-of-paris, gutta-percha, and other non-conducting 

 materials, so as to render them conductive. 



The behaviour of graphite on treatment with a mixturr 

 of potassic chlorate and nitric acid has beeu carefully 

 studied by Brodie ; but our knowledge of its oxidation 

 products is still very incomplete. He has shown that it is 

 converted into a body to which he attributes the compos- 

 ition indicated by the formula C|,H, 5 ; graphitic acid, 

 as this compound is termed, forms yellow silky plates, in- 

 soluble in water and acids. It doos not form salts, and 

 Berthclot therefore prefers to call it graphitic oxide. When 

 this substance is heated it decomposes almost with ex- 

 plosive violence, leaving a residue which still contains 

 hydrogen and oxygen, but which is not distinguishable 

 from finely divided graphite. When the graphite which 

 crystallizes from cast-iron and that obtained by heating 

 amorphous carbon in the electric arc are similarly treated, 

 graphitic oxides are produced which differ from each other, 

 and from the oxide formed from native graphite ; it is 

 therefore supposed that these graphites are distinct sub- 

 stances (Berthelot, Ann. Clt. fliys. [41 xix. 399). 



" Always in rocks belonging to the earliest form- 

 ations " writes Professor Armstrong, and he quotes 

 from Bertholet the statement that the graphite which 

 ciystalizes from cast iron, that resulting from heating 

 amorphous carbon in tbe electric arc, and native 

 graphite, are different substances. There is certainly 

 no room to dogmatize on the subject, but we stil 

 I n an strongly to the opinion that graphite is as mue! 

 derived from vegetable matter as is ,-iuthracite or 

 true coal. Who can ray bow early n geologic time 

 vegetation in the shape of ferns, mosses and similar forms 

 appeared on the earth, and how completely chemical 

 changes enabled carboniferous matter ti resist com- 

 bustion ? 



Of the intimate bearing of plumbago, ifs constitu. 

 flits and formation, en the theory cf tbe crystal. 

 ization of amorphous rock, we can have no etoubt, 

 any more than that the two gre t agents at work 

 were beat and pressure, with resulting nio°netie 

 at traction. If plumbago e'oulel but speak it coulel a 

 wondrous tale unfold of changes sudden and violent 

 ami processes continued through centuiies of ages, 

 before it assumed its present shape and character. 



