Dlvt/tm of Bituminous Suhjlaneis. 20 J 



MOUNTAIN OR MINERAL TAR, Bitumen PETaotEUM tarde fluens. 

 This fubftance is vifcid, and of a reddifh or blackifli brawn or black. When burned, it 

 emits a difagreeable bituminous fmeli, and by expofure to the air itpaflss into 



MOUNTAIN OR MINERAL PITCEI— Bitumen TvIaltha. 



The mineral pitch much refembles common pitch, and, when heated, emits a ftrono- un- 



pleafant odour, like the former fubftance. When the weather is cold, it may be broken, 



and then exhibits, internally, a glaffy luftre; but when warm, it is foftened, and poffcfTes 



feme tenacity. It is however fufccptible of a fuperior degree of induration, and then becomes 



ASPHALTUM — Bitumen Asphaltum — Petroleum induratum. 



This is a light, brittle fubftance, of a brownifli black, or black. When broken, it ftiews 

 a<:onchoidal fradlure with a glafty luftre. It has little of the bituminous odour, unlefs it is 

 rubbed or heated. It eafily melts, is very inflammable, and, when pure, burns without' 

 leaving any aflies. 



In this manner, naptha, by infpifTation, paff<s fucceflively through different ftates until it 

 becomes afpbaltum, which appears to be the ultimate- degree of induration which the pure 

 bitumens derived from naptha can receive. 



I have at this time fpecimens before me which prove thefe gradations ; and I Jiave ktn a 

 remarkable inftance in a bitumen brought from the Ifland of Trinidad, which exhibits 

 mineral tar pafling into mineral pitch, and laftly into afphaltum*. 



SECT. III. 



THE divifion which comprehends the fimple bituminous fubftances derived from naptha, 

 may therefore be confidered as terminating in afphaltum ; but nature appears to have glided 

 on by an uninterrupted chain which conne£ls the ftmple bitumens with thofe which we have 

 called compound ; and this effe£l is produced by the gradual increafe of the carbonic princi- 

 ple, and the introdu£lion of extraneous matter, the different quantity of which, together 

 with the greater or lefs degree of mixture or of chemical union, occafion confiderable changes 

 in thefe fubftances, fo that they are gradually rernoved from thofe characters which 

 diftinguifti the pure bitumens. 



To form an accurate table of thefe gradations, it would be neceffary to have comparative 

 analyfes of the different bituminous fubftances, and alfo to contraft the analyfes with the 

 properties of thefe bodies. But at prefent thefe analyfes, for the greater part, are wanting ; 

 and although at fome future time I intend to attempt a feries of fuch experiments, I muft 

 now content myfelf with the obfervations and fa^ts which I have been able to colledlf. 



* The progreffive changes of naptha into petroleum, miperal tar, mineral pitch, and afplialtum, appear to be 

 caufed by the gradual diffipation of part of the hydrogen of the bitumen, and the conl'equent development or 

 difengagement of carbon. Hence, I am inclined to believe, arife the changes of colour, the degrees ef in- 

 fpiffation, and the increafed proportion of carbon found in thofe fubftances by chemical analyfis. I would be 

 underftood however to mean that the carbon is only relatively increafed, in relpeft to the other ingredients, in 

 a given quantity of thefe bitumens, and that it predominates in proportion to the di/fipation of a certain 

 portion of the hydrogen, which was originally neceffary to the forming of the bitumen in conjunction with the 

 carbon. 



f This paper was written and read before I had feen the ingenious experiments which the celebrated 

 Mr. Kirwan has publifhed, in the laft edition of his Elements of Mineralogy.— Vide vol. ii. p. 514, (or 

 Pbilof. Joum. I, 4«7.) 



D d a Ff on» 



