OIL

OIL

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flight degree of cold is fufficient for their con-gelation, at which time they affume the appear-ance of cryftalline plates, which however muftnot be confounded with the prifmatic cryftalsthat molt of them depofit by long expofure tothe air. The colours of eflential oils are various;fome are blue, others green, but the ufual colouris light yellow verging more or leis by longkeeping to reddifh brown.

Eflential oil is compleatly volatile withoutdecompofition at a heat lefs than that of boilingwater; (hence if a few drops are fpread on apiece of paper and then held for a minute ortwo before the fire, any fophiftication with ex-prefled oil may be detected by the greafe fpotthat will remain after the volatile oil has beenentirely exhaled). It may however be detainedin a higher heat by mechanical mixture withdry clay or fand, and then it undergoes a partialdecompofition, carburetted hydrogen being givenout and a little charcoal remaining in the re-ceiver : the undecompofed refidue if fubjedfedthree or four times fucceflively to fimilar treat-ment will be entirely deftroyed.

Eflential oils are highly combuftible ; theytake fire on the application of an ignited bodyor by the eleftric fpark, and burn with a largewhite flame and a denfe black fmoak; a largerproportion of oxygen is required for their corn-bullion than of the other oils, and a greaterquantity of water is produced.

The eflential oils, efpecially the more volatileof them, deoxygenate atmofpheric air very com-plcatly, as was firft afcertained by Prieltley,which in part at leaft accounts for the unealyfenfations experienced by moft perl'ons when ina clofe newly painted room. In proportion tothe oxygen abforbed the oil becomes thick andcoloured, more efpecially if it is at the fametime expofed to the fun, and at length nearlylofes its peculiar odour and approaches in con-fidence to a foft refin. In thofe oils that arenaturally thiek a change of another kind takesplace : a weakly acidulous water is producedand prifmatic cryftals are depofited, the refidueyof the oil becoming in the mean time concrete.Thefe cryftals, which have been occafionallymiftaken for camphor, are flightly foluble inhot water, and more fo in alcohol, to which{ hey communicate the property of reddeningvegetable blues : when gently heated they meltan cryftallize in needles by cooling; whentreated by the blowpipe they evaporate, but donot inflame: from thefe characters they havebeen confidered as an acid very analogous to

k An. Chim. xxi, 174.

the benzoic. It appears from the experimentsof Margueron h that fome oils when expofed toa freezing mixture undergo the fame change:at the moment of congelation a portion of highlyodorant gas is difengaged, a watery acid is pro-duced and cryftals are depofited. Some effen-tial oils however afford real camphor by evapo-ration, as Prouft has fhown, and as the readerwill find fully detailed under the article Cam-phor.

Oil that by long keeping in a half-ciofedbottle has become thick and nearly fcentlefs,may be rectified by rediftilling it with waterand fome of the frefh vegetable from which itwas originally extracted, and this was oftencited as a proof that the fubftances called effen-tial oils were in reality compounds of refin andthe fpiritus reBor , or aromatic principle. Butprecifely the fame effe£t may be produced bymeans of alcohol or ftill more readily by ether.This curious and important fa£t was firft Hatedby De Roover', from whofe experiments it^appears that if thick fpoiled oil be mixed withT V of fulphuric ether for a few days and thendiftilled with pure water, more than half theoil will come over perfectly limpid and poffefiedof its peculiar odour, while the remainder willbe left behind in the ftill in the ftate of a darkcoloured refin. The united weights of the dif-tilled oil and refin exceed that of the originaloil, whence it is probable that the ether actuallycombines with part of the oil, and probably byfupplying it with hydrogen, reftores to it theliquidity and odour which it poffeffed at firft.It is to be regretted that M. De R. did not fol-low up his experiments by treating the refinousportion with frefh quantities of ether, fo as toafcertain whether it was poflible to convert itin part or entirely into volatile oil as at firft.

Eflential oil is fparingly foluble in water,from which it may be feparated by diftillation,but is taken up much more abundantly by alco-hol ; this folution however is decompofable bywater, the greateft part of the oil being fepa-rated.

Both fulphur and phofphorus are foluble ineflential oils equally well as in the fixed andwith nearly the fame phenomena : phofphorizedoil is luminous in the dark, has a fetid odour,and gives out phofphuretted hydrogen by diltil-lation : fulphurized oil is of a yellow or browncolour, and gives out on diftillation fulphurettedhydrogen; it is often called balfam of fulphur.

The ftronger acids a£t more vigoroufly onthe eflential oils than they do even on the fixed.

Nich, Journ, 8vo. vii. p. 68.