8 Dr. Colquhoun on a new Form of Carbon. [July, 



scratch of a knife, and broke with a smooth conchoidal fracture, 

 the lustre of which resembled in quality that of the finest speci- 

 mens of indigo. Some of the harder varieties exhibited on their 

 exterior surface a beautiful mammillated structure, and a very 

 brilliant and perfect metallic lustre. 



Such are the leading characters of the different forms of 

 aggregation under which the carbon presented itself on different 

 occasions in the steel chest. It would be easy to enter into 

 much more minute details respecting them. I consider it suffi- 

 cient, however, to have particularized them, only in so far as 

 appears necessary for the purpose of illustrating such of their 

 counterparts, as are of the most frequent occurrence in coal gas 

 manufactories. 



The manner in which the analogous carbonaceous formations 

 occur is well known to the mechanics employed in that depart- 

 ment of the arts. Within the elliptical retort, there is a gradual 

 deposition and accumulation of them, first at the farther end, 

 and next along the sides and towards the mouth of the retort. 

 Such is the strong adherence of the carbon to the metal, that 

 this process goes on notwithstanding all the endeavours of the 

 workman to check it by breaking away the carbon as it forms, 

 and in the end it causes such a waste of fuel before the fresh 

 coal within the retort can be distilled, that the retort becomes 

 useless. It generally happens long before this, however, that as 

 carbon and iron expand with a very different rapidity on the 

 application of equal heat to both, the sudden changes of tem- 

 perature which are invariably consequent on the introduction of 

 fresh coal into a retort in the process of gas making, occasion 

 the cracking of the vessel long before the " burning out," as it 

 is called by the workmen, or complete oxidation of the metal, 

 has taken place. 



The carbonaceous matter thus formed within the gas retorts, 

 possesses considerable variety of appearance, a variety, how- 

 ever, which is completely parallel to that of the carbon found 

 ■within the steel chest. Its structure, in the great, is stratified, 

 large, and very irregular slaty, with a powdering of a black 

 substance, like lamp-black or soot, between the strata ; and it 

 breaks more easily in the direction of the strata than across 

 them. In the small, the stratified portions seem to the eye quite 

 compact ; they break with a smooth earthy fracture, and are 

 almost always devoid of lustre. Their colour is iron grey, and 

 their texture hard. They are difficult to reduce to powder, but 

 are easily scratched by a knife. Occasionally specimens are 

 found possessing a mammillated structure, with a brilliant and 

 perfectly metallic lustre on their surface. In general, however, 

 there is no peculiarity below the siirface, which distinguishes 

 this from the more common carbon formation. 



When a retort has been cracked in the gas manufactory, as 



