CHEMISTRY. 



71 



the heat necessary to distil them over appears to be 

 greater. When cooled they congeal; but the tern - 

 prnture necessary to produce this effect varies in dif- 

 ferent oils. Some become solid at 50, others not 

 till cooled down to 17. Several of them yield 

 crystals of camphor and of benzoic acid when thus 

 treated. 



When exposed to the light, they acquire consis- 

 tency, and assume a brown colour. Dr Priestley as- 

 certained, that they imbibe oxygen with avidity. 

 Probably these changes are connected with that ab- 

 sorption. By long exposure, several of them assume 

 the form of resins. 



When heated sufficiently, they take fire, and burn 

 with a strong yellow flame, emitting a great quantity 

 of smoke. The. products of the combustion, besides 

 the soot deposited, are water and carbonic acid. 



When agitated in water, they render it milky, and 

 communicate their peculiar odour. They dissolve in 

 alcohol, ether and fixed oils. 



They dissolve a little phosphorus and sulphur, but 

 do not act on hydrogen or charcoal. 



The alkalies and earths act but feebly on the vola- 

 tile oils. The compounds formed have been called 

 taponules. It is probable that they assume the form 

 of resins before they combine with these bodies. 



Sulphuric acid decomposes them, converting them 

 first into a kind of resin, and at last into charcoal. 

 Concentrated nitric acid sets them on fire. The di- 

 luted acid converts them into a kind of yellow resin. 



They scarcely act upon metals ; but they have a 

 tendency to reduce some of the metallic oxides. 



From the products obtained when the volatile oils 

 are burnt, it has been concluded that they are com- 

 pounds of carbon and hydrogen. But no exact ana- 

 / lysis of any of them has hitherto been made. 



SECT. IV. Of Fixed Oils. 



Fixed oil*. The fixed oils, which are of o extensive utility, 

 have been known from the remotest ages. They may 

 be distinguished by the following properties. 

 Their pro- 1. Liquid, or easily become so when exposed to a 

 pertiet. gentle heat. 



2. An unctuous feel. 

 S. Very combustible. 



4. A mild taste. 



5. Boiling point not under 600. 



6. Insoluble in water and alcohol. 



7. Leave a greasy stain upon paper. 



How ob- These oils, called also fat, or expressed oils, are ob- 

 tained, tained partly from animals, partly from vegetables, by 

 simple expression. They occur chiefly in the seeds 

 of bicotyledinoui plants, and in the livers of animals. 

 Whale oil, spermaceti oil, olive oil, and linseed oil, 

 may be mentioned as examples of fixed oils. 



Fixed oil is usually liquid, with a certain degree of 

 viscidity. It has usually a yellowish or greenish tinge. 

 Its taste is sweet, or nearly insipid. When fresh it 

 has no smell. 



Many solid bodies also are obtained from vegetables 

 which nave been hitherto confounded with the fixed 

 oils, as palm oil, shea butter, &c. From the late ex- 

 periments of Dr Bostock, these substances eem to 



approach the nature of wax rather more than that of Elements 

 fixed oils. . f 



The fixed oils are all lighter than water. Their < ^^; 

 specific gravity varies from 0.968 to 0.892. 



Fixed oil does not begin to evaporate till heated 

 above 212. As the heat increases, a pretty copious 

 vapour may be seen to rise ; but the oil does not be- 

 gin to boil till heated nearly to 600. It may be dis- 

 tilled over, but it is always altered by the process. 

 Some water and acetic acid seem to be formed ; heavy 

 inflammable air is given out. The oil deepens in co- 

 lour, and acquires a disagreeable taste and smell. 



Fixed oil, when kindled, burns with a yellowish 

 white flame, and is decomposed. The products arc 

 carbonic acid and water. When exposed to cold, they 

 congeal or crystallize, and at the same time their 

 bulk diminishes very considerably. 



When exposed to the action of air, they undergo 

 different changes according to the nature of the oil. 

 They gradually absorb oxygen and become solid. 

 Now there are some that retain their transparency 

 after they have become solid, while others assume the 

 appearance of talloiu or wax. Those that remain 

 transparent are called drying oils ; those that become 

 opake, are called fat oils. 



The drying oils are used as a vehicle of paints and Dryin -. 

 varniihes. Linseed, nut, poppy, and hempseed oils oils, 

 belong to this class. They acquire the property of 

 drying oils more completely after they have been 

 boiled. For some purposes it is common to set them 

 on fire, and, after they have burnt for some time, to ex- 

 tinguish them, and continue the boiling till they have 

 acquired the requisite viscidity. By this process, they 

 lose the property of leaving a greasy stain upon pa- 

 per, and acquire many properties in common with the 

 resins. In this way, nut-oil and linseed- oil are pre- 

 pared for printers ink. The oil, thus altered, still 

 continues insoluble in water and alcohol, but it readily 

 unites with fixed oil. 



The fat oils, when exposed to the air, gradually p at oils, 

 become thick, opake, and white, and assume an ap- 

 pearance very much resembling wax or tallow. Olive- 

 oil, oil of sweet almonds, of rape-seed, and of ben, 

 may be mentioned as examples of this class. 



The action of the simple combustibles on the fixed 

 oils is not very remarkable. Hydrogen has no action. 

 Charcoal renders them purer when they are filtered 

 through it ; but separates from them with such diffi- 

 culty that it cannot bo employed for that purpose 

 with advantage. 



They dissolve a little phosphorus and sulphur when 

 assisted by heat. 



They are insoluble in water, alcohol, and ether ; 

 but they unite readily with each other, with volatile 

 oil?, with bitumens, and with resins. 



The fixed alkalies combine with them readily, and 

 form with them the important compound called soap. 

 Potash forms with them only soft soap, while soda 

 forms hard soap. The earths likewise and metallic 

 oxides combine with the fixed oils, and form a kind of 

 soap insoluble in water. 



Sulphuric acid gradually decomposes the fixed oils, 

 blackening their colour, and at last evolving charcoal. 

 Nitric acid acts with still greater energy. When 



