m the Distillatio7i of Wood, &c. 209 



oil is separated, and resin remains in tlie retort. Tliis however 

 is not a mere case of the separation of a more volatile from a 

 fixed substance ; for a decompositioTi takes place, and acetic acid 

 is generated. Nor can turpentine be again reproduced by mix- 

 ing together the essential oil and the resin — it then forms a var- 

 nish. The essential oil is in fact a new compound, produced from 

 the vegetable elements by the action of fire; and, although pro- 

 perly enough classed Vv'ith tliose essential oils which are vegetable 

 secretions, differs from them in some of its chemical properties. 

 It is, for example, difficultly soluble in alcohol; but on exposure 

 to air it becomes thick and yellow, and is then easy of solution 

 in the same substance. 



If the resin, which is the residuum of this distillation, be still 

 further heated, it gives over a thick and high-coloured oil, gra- 

 dually increasing in v/eight, till it equals, and at length exceeds, 

 the specific gravity of water. The residuum becomes ultimately 

 black, and very brittle, remaining soluble in ether and in lixi- 

 vium of potash, but refusing to dissolve in alcohol. 



Common tar differs from turpentine in containing a portion of 

 the vegetable tar now mider review, mixed with conmion turpen- 

 tine and with the acetic acid whi^h is formed in the distillation 

 to which the wood is subjected for the purpose of obtaining it. 

 Evaporation converts this into pitch, by decomposing it. 



In this process, an essential oil, compounded of the ol! of tur- 

 pentine and the oil of vvood, together with a portion of acetic 

 acid, is separated, and the resrduum, or common pitch, is a com- 

 pound of rciin and the v.-ood pitch which I have beeii describing. 

 To this admixture, and not to that of adventitious charcoal 

 j)roduced in combustion, is the black colour of comrr.on pitch 

 owing. 



The analogy between this wood jjitch and the bitumens is 

 equally striking, and the preceding history of tlicse compounds 

 will throw light on the several varieties of the bituminous sub- 

 stances. 



Assuming the tar as the medium form, it is seen that when ex- 

 posed to heat it gives over oil, and that pitch remains. Thus, 

 petroleum yields naphtha and asphaltum ; and thus too, asphal- 

 tum exhibits all the gradations which I have described in the 

 pitch, its properties varying in a similar manner, according to 

 its particular state. In the process of distillation, the principal 

 dirTerencc will be found to consist in the relative quantities of 

 acetic acid and ammonia, which they severally yield ; t!ie former 

 chiefly characterizing the wood tar, and the latter the petroleum. 

 From the same chemical cause which produces this effect arises 

 also the difference in the nature of the inflammable gases which 

 are produced from these different substances. 



Vol. 15. No.203. 3;«R7«»18ir>. O The 



