John MacCulloch

“ The sensible qualities of the bitumens (their taste and smell) are in all states utterly and entirely different from those of the vegetable tar. Petroleum is also much less soluble in alcohol, and further differs from the vegetable tar in being perfectly soluble in naphtha. In their solubilities in oil of turpentine they resemble, each other, as well as in their habitudes with acetic acid and the alkaline lixivia, although the vegetable tar will be found the more readily soluble of the two. I need not repeat the circumstances in which the essential oil of wood differs from naphtha. ”

“ The power of yielding naphtha on distillation, is rather to be resorted to as the distinction between the one end and the other of the series; and it would be surely equally correct to call coal a compound of charcoal and naphtha, as a compound of charcoal and asphaltum.
Its several varieties will moreover be found to vary from each other by containing greater or less proportions of carbon, compared with their other ingredients; just as in asphaltum the relative proportions of the hydrogen, azote, and oxygen, to the carbon, are found to differ from those which constitute petroleum or naphtha. ”

“ All these bituminous compounds may therefore properly be said to belong to one genus or family, of which the principal chemical ingredients are carbon and hydrogen, and it is chiefly to the difference in the relative proportions of those two substances that we are to look for the differences which characterize the several bitumens, from naphtha placed at one extreme, to anthracite placed at the other. The chasm in this series from asphaltum to fat coal, is in fact, rather apparent than real, being more properly a mechanical or accidental, than a chemical or essential one. ”

“ So the last result of the distillation of asphaltum is charcoal, and the intermediate steps through which asphaltum passes in its progress to charcoal, resemble precisely the corresponding changes which occur in the distillation of coal till coak is formed, and confirm by their chemical analogy the view here held forth of the chemical composition of coal, and the gradation to be traced in nature from fat coal to anthracite. If asphaltum be subjected to distillation it gives petroleum. ”

“ It is already known that a substance resembling plumbago is formed in water, it having been discovered by Fabroni in the country round Naples. It is equally known to be formed in the iron foundries, and the advocates for the igneous origin of coal have also contended for that of plumbago, and have supposed it to have been produced by the contact of melted greenstone with beds of coal. But even if we admit this cause of its formation, something else seems necessary for the production of the substance, and some other mode of applying the heat required before it can be produced. ”

“ For the same reason also petroleum is easily rectified into naphtha. Asphaltum, in its ordinary state, contains the two ingredients in a ratio in which the carbon bears a large proportion to the hydrogen, and that ratio is reduced to the minimum, or becomes evanescent, when by the continuance of distillation, charcoal alone remains behind. A large portion of the oxygen, and also of the azote is disengaged during this process, but not the whole, since the darker compounds still give it over on repeating the process. The naphtha is probably entirely exempt from oxygen. ”

“ If the same turpentine be exposed to the action of the fire, a colourless volatile oil is separated, and resin remains in the retort. This however is not a mere case of the separation of a more volatile from a fixed substance, for a decomposition takes place, and acetic acid is generated. Nor can turpentine be again reproduced by mixing together the essential oil and the resin—it then forms a varnish. The essential oil is in fact a new compound, produced from the vegetable elements by the action of fire ”

“ The softest specimens of the pitch are nearly as soluble, leaving only a small residuum, which is infusible and powdery. The harder specimens become in proportion less soluble, and leave a larger residuum; and those which have been the longest exposed to heat scarcely give a stain to the alcohol, resembling in this respect the driest specimens of asphaltum. The analogy is here very apparent, for asphaltum may approach more or less to petroleum, and the various specimens of it are found to exhibit various degrees of solubility in alcohol. ”

“ The foregoing experiments show that the fire of our furnaces does not convert wood into bitumen, and the processes of Nature seem to prove that water can produce this effect, and that jet, the bituminous lignite which approaches nearest to coal in its chemical characters, is the result of this action. ”

“ From this to the harder lignites, surturbrand and jet, the transition is so gradual, that there seems no reason to limit the power of water to produce the effect of bituminization in all these varieties, nor is there aught in this change so dissonant from other chemical actions, as to make us hesitate in adopting this cause. In the ordinary process of vegetable putrefaction and destruction, a variety of compound gases are formed by the reaction of their elements, and carbon alone, or rather carbon united to a portion of hydrogen, remains behind. ”

“ Assuming the tar as the medium form, it is seen that when exposed to heat it gives over oil, and that pitch remains. Thus, petroleum yields naphtha and asphaltum; and thus too, asphaltum exhibits all the gradations which I have described in the pitch, in properties varying in a similar manner, according to its particular state. In the process of distillation the principal difference will be found to consist in the relative quantities of acetic acid and ammonia, which they severally yield ”

“ It will be in vain that we attempt to combine iron with charcoal for this end, unless the charcoal or carbon be in that state of very minute division in which it exists when precipitated by a new affinity from some previous combination. ”

“ By distilling or evaporating the oil from the pitch, according to the process described above, a colour may be produced varying in tone from the warmest bistre brown down to black. At the same time the substance loses a great portion or the whole of its disagreeable tenacity, according to the degree of boiling it has undergone. By treatment in alcohol, results in some measure similar are produced, and the residuum of this solution is equal in colour to seppia, and totally void of tenacity. ”

“ The disproportion of acetic acid and carbonic oxide produced from the wood pitch, when compared with the produce of the bitumen, proves that it contains oxygen and azote in proportions different from those in which the same substances exist in the bitumens ”

“ The causes of these varieties will be very evident to those who have read the foregoing experiments. An imperfect separation of essential oil and a consequent tenacity arising from its too near alliance to the tar, will appear to be its most common vice, and it is this which gives it that disagreeable gumminess and disposition to return to the pencil which is destructive of its best qualities. ”

“ Retaining as they do, the traces of organization, and that sometimes in great perfection, it may be often questioned whether they do not more properly rank with the fossil remains, than with the minerals properly so called. ”

“ When it is subjected to distillation in a heat sufficient to keep it in a gentle ebullition, an oily looking matter passes over in considerable proportion, which sinks to the bottom of the water into which the tube is inserted. ”

“ I may also add that it forms a substitute for asphaltum in drying oil where such a coloured varnish is wanted, and that it makes a very good japan varnish for metal if dissolved in spirit of wine, and heated strongly after its application. It is for practical men to see whether by combining it with asphaltum, lac, or the gums, some more useful and cheap compounds of this sort may not be produced. ”

“ With subcarbonate of potash it thus forms in the first instance an uniform solution of a brown colour, but a continuance of trituration or boiling renders it pitchy and tenacious, after which it forms no further union with the mild alkalies. ”

“ At times also from the same causes it is offensively yellow. So valuable is a brown colour that will work freely and with transparency, that the artists will be much obliged to him who shall render bistre equal in freedom and force to seppia. ”