O I L

OIL

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clafs of infects, contain oil; the quantity how-ever of which, as well as the fituation which itprincipally occupies in the body, is fubject toconfiderable variety. In all cafes it is containedin peculiar receptacles of cellular membrane ;but' thefe receptacles in quadrupeds are for themolt part but fparingly difperfed among themufcular fibre, are more abundant in the bones,.and moft fo of all in the cavity of the abdomen,and efpecially attached to the kidneys. Thehog however is an exception to this, the prin-cipal part of his fat being depofited between thelkin and the mufcles. In birds the chief featof the oil is immediately below the lkin, andin water-fowl it is particularly fecreted by acollection of glands in the rump. In the warmblooded fifh, as the whale, the oil is chieflycontained in the head and jaw-bones, and is in-terpofed in vad abundance between the lkinand the mufcular flefli. In the cold bloodedfilh it is contained in the liver, as in the {hark,the cod, and the ling ; or is difperfed throughthe whole body, as in the fprat, the herring,,and pilchard.

While the fat remains in the living body it isalways in a fluid or femi-fluid hate, but after ithas been extradited and is expofed to the com-mon temperature, a remarkable difference in itsconfidence is obferved. The oil or fat inveftingthe kidneys of quadrupeds is called fuet ortallow, and is the molt folid and hardeft of any;the next in hardnefs is the fat of the bones,and that in which the mufcles are imbedded isthe next in degree; the fat of the hog (calledlard) is the lead folid. The fat of birds isfeldom fo folid as hog’s lard, and in manyfpecies is aftually fluid. The fat or oil of fifhis almod always fluid at the common tempera-ture. Befides the, above varieties of animal«il, there is yet another contained in the yolkof eggs, and which may be extracted by Amplepreflure after the yolk has been coagulated byheat.

Animal oil in its pured date is. obtained bycutting fredi fuet into direds and liquefying itin boiling water, and then palling it through apiece of thin gauze, in order to fepavate thecellular membrane. When thus purified it isef a yellowifh-white colour, moderately hard,of a mild tade, and nearly deditute of odour orflavour: it is combudible, like the fixed vege-table oils, and agrees alfo with thefe in thechanges produced upon it by the alkalies andother chemical re-agents. Ad the animal oilshowever belong to the clafs of unctuous or fat

oils, none of them being either drying in itfelfor capable of becoming io by means of lithargeor other fubdances.

When fat is expofed to dry didillation, asfoon as it acquires the. temperature of about400 0 it emits a white acrid and difagreeablevapour; as the heat increafcs iome of the oilconies over into the receiver, and what remainsin the retort acquires a blackifli tinge; empy-reumatic, acetous, and Sf.bacic acids, thenmake their appearance, together with carbu-retted hydrogen and carbonic acid of a modremarkably offenfive odour. This eircumdance,fliews that there is a real difference betweenanimal and vegetable oils, though it has not yetheen pointed out. by chemical analyfis. Thecoarfer kinds of animal oil being extracted bymeans of putrefaction and a drong heat, poffeis,a much more difagreeable odour than any ofthe vegetable oils, and when rancid dilengageammonia by the aCtion of the fixed caudic alka-lies, in which alfo they differ from the latter.

The fidi oils when rancid,, and from, the modeof their extraction they never are otherwife,are for the mod part thick and glutinous, whichin fome mcafure difqualifies them for burningand for fome other ufes to which they are ap-plied. Many attempts, for the mod part with-out fuccefs, have been made to bring them to,a better date, but as appears from the experi-ments of Mr. Doffie, k they may be confiderablyimproved by means of fixed alkali and chalk,,by which the albumen and gluten, are thrown-down, and the fupernatant oil after due redmay be poured od’ in a fluid date, and veryfetifibly amended both in confidence, in odour,and in fitnefs for burning.

Animal oils are fubdances of great economi-cal importance. They, are u.fed as food,, and inmedicine as the bafe of various unguents: theylargely employed in the manufacture of foap,and for burning either in lamps or in the formof candles.

Animal Oil Volatile,. or Dippel’s OIL

As in the vegetable kingdom oil is producedby the dedruCtive didillation of various fub-dances that contain none in their natural date ;fo it is with refpeCt to the animal kingdom.If albumen., or gluten be diddled at a dry heat,there arifos together with the ammonia, andcarburetted hydrogen, a quantity of fetid blackoil; this was made the fubjeCt of various expe-riments, fird by Dippel, a chemift of Berlin,and afterwards by Rouelle. From the concur-rent labours of thefe enquirers it appears that

* Phil. Mag. xv. p. 105.