OF TRANSMITTING LIGHT. 339 



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I shall offer, before concluding, a few remarks on carbon, considered in its 

 varieties. How paradoxical are these ! The diamond most remote in its cha- 

 racter from a metal, perfectly transparent a nen-conductor of electricity placed 

 at the head of the class of gems, and resembling not a little, in its general cha- 

 racter, those oriental ones, of which a metallic oxide, alumine, is the chief consti- 

 tuent part. What a contrast is there between it and charcoal, a conductor of elec- 

 tricity, possessed of peculiar properties, especially in regard to absorption, differ- 

 ing in this respect from almost every other substance, excepting, indeed, the hy- 

 drate of alumine a resemblance the more remarkable, considering their similarity 

 in their crystalline state ! And, farther, what a contrast is there between both 

 these substances and plumbago, which possesses the perfect metallic lustre, is 

 sectile, slightly malleable, admits of incorporation, as it were, by welding, and, in 

 brief, has very much the character of a metal ! 



To what the marked differences of these substances are owing remains to be 

 ascertained. It may be to the presence of minute portions of foreign matters, 

 which have hitherto escaped detection, although diligently sought after. The dif- 

 ference in their specific gravities is in favour of this view.* Or, it may be, that 



* Whilst the diamond is comparatively of the high specific gravity 3.5, I find that of charcoal, coke, 

 and anthracite (making allowance for the ash yielded by the latter) is only about 1.5. This is the result 

 of some trials made with considerable care. The method employed was briefly the following. In the 

 instance of the charcoal, whilst hot from the crucible in which it had been prepared, it was weighed in 

 air; then, with distilled water, it was subjected to the air-pump, till it sunk and ceased to give out any 

 air, when it was weighed in water ; after which it was dried, ignited, and again weighed in air. 



A piece of charcoal of the oak, weighing 12 grs., thus treated, appeared to be of sp. gr. 1.519 at 

 53 F. ; a piece of charcoal of deal, weighing 5.67 grs., of sp. gr. 1.54, and reduced to powder of the sp. 

 gr. 1.45. 



In the instances of anthracite and coke, the same method was used, omitting the weighing in air the 

 second time, after the weighing in water, as they yielded nothing soluble in water, and ascertaining the 

 quantity of ash, or foreign fixed matter, which each afforded on incineration. 



A portion of a specimen of anthracite, for which I was indebted to Professor JAMESON, weighing 

 65.2 grs., appeared to be of the sp. gr. 1.57 ; it contained 4.7 per cent, of ash. A portion of coke, weigh- 

 ing 18.02 grs., obtained from bituminous coal, appeared to be of the sp. gr. 1.70; it contained 6.8 per 

 cent, of ash. The ash of the anthracite consisted chiefly of silica, with a little alumina, coloured light red 

 by peroxide of iron ; the ash of the coke, of silica, with only a trace of alumine and peroxide of iron, of the 

 latter not sufficient to colour it ; both were without lime or alkali. The difficulty of extracting the whole of 

 the air from the anthracite, charcoal, and coke, was considerable, especially from the coke. After three days' 

 exposure to the action of the air-pump, the effect was produced on the anthracite ; in about the same time 

 on the charcoal ; but not in less than eleven on the coke ; it floated eight days, and this notwithstanding 

 that the pump was frequently worked the mercury in the gauge standing steadily, after the first day, at 

 .25 inch, and although the total quantity of air to be disengaged was equal only to the volume of 4.48 grs. 

 of water. In one instance, the charcoal was boiled in distilled water, after it had ceased to give out air 

 under the exhausted reservoir ; but without effect in increasing its specific gravity. 



The 

 VOL. XV. PABT III. 4 Y 



