1-823.] Rev. J. Conybeare on Plumbago in Coal Gas llelorts. 51 



The cast-iron retorts used in the Bath gas works, being, on 

 an average, somewhat less than one inch and a quarter in thick- 

 ness, are burned through, as it is technically termed, in the part 

 exposed to the greatest heat, in about IS months. After this 

 they are of course no longer serviceable. This destruction is 

 effected by the progressive oxidation and scaling off of the 

 metal. The scales consist chiefly of peroxide of iron, and the 

 powder which they afford by grinding and washing might be 

 applied to any of the purposes for which the crocus mortis is in 

 request. It is observable that the portion of cast-iron which 

 remains unoxidated in the parts exposed to a strong heat is 

 found to present, on being broken, a texture much more highly 

 crystalline than that of the portion further removed from the 

 action of fire.* Is this simple development, or is it to be 

 ascribed to that mysterious re-arrangement of particles which 

 in some other cases appears to take place at a heat short of 

 actual fusion ? 



The unserviceable retorts on being withdrawn from their beds 

 are found lined with a coating of plumbago averaging the thick- 

 ness of four inches. This coating is thickest towards the bot- 

 tom of the retort, nearer to its mouth it becomes scantier and 

 more intermixed with coaly and fuliginous matter. The general 

 aspect of the predominant variety may be thus described : 

 Colour, iron-grey, somewhat lighter than that of native plum- 

 bago ; texture, scaly ; sti'uclure, mainmillated, usually in very 

 close aggregation ; some specimens exhibit this structure on 

 the larger scale ; generally it is discoverable only by the use of 

 the lens ; hardness, somewhat variable, but always greater than 

 that of the best native plumbago ; scratches gypsum, but is 

 scratched by calc-spar; lustre of the exterior surface (especially 

 where the mammillated structure is distinct), sometimes very 

 considerable : lustre of the fracture usually but small: the powder 

 uniformly resembles that of common plumbago, excepting that 

 it is somewhat less brilliant. 



In another variety, the structure of the mass is stalactitic, and 

 its texture, where broken, perfectly compact and uniform; even 

 under the lens, this variety closely resembles some of the stalac- 

 titic grey ores of manganese, and occasionally exhibits on the 

 surface a considerable degree of iridescence. The mass for the 

 greater part gives evidence of its gradual formation, by its slaty 

 aspect, and tendency to break with greater readiness in the 

 direction of what maybe termed its strata. In many parte, it is 

 rifty, and the rifts are usually coated with a brilliant deposition 

 of a lighter colour than the general mass. Fragments projected 



• For this observation, as well as for most of the facts contained in this letter, I am 

 indebted to the kindness of Mr. Eastwick, the intelligent and obliging superintendent of 

 the Bath Gas Works. 



E2 



