72 



CHEMISTRY. 



Element! poured suddenly on the drying oili, it sets them on 

 , f tin-. When suddenly diluted, it converts them all 

 ._""JT' into substances similar to resins or tallow. 



The fixed oils oxidize some of the metals, as cop- 

 per and mercury. They combine with various me- 

 tallic oxides, as those of arsenic, lead, and bismuth ; 

 and are capable of forming with; several, the viscid 

 compounds called plattert. 



They are liable, by keeping, to become rancid. 

 They become thick, acquire a brown colour, an acrid 

 taste, and a disagreeable smell. The oil, thus altered, 

 converts vegetable blues to red, and of course contains 

 an acid. This change is, at present, ascribed to a 

 decomposition of the mucilaginous matter which is 

 dissolved in all oils, or to the action of that matter 

 in the oil. 



When oils are burnt, the only products are carbonic 

 acid and water. Lavoisier, from a set of experiments 

 made in this way on olive-oil, deduced its composition 

 as follows : 



Carbon, 79 



Hydrogen, 21 



100 



But the experiments of Thenard and Gay-Lussac, 

 made in a different way, give its constituents as fol- 

 lows : 



Carbon, 77.213 



Hydrogen, 13.360 

 Oxygen, 9.427 



100 



Many oils occur in the vegetable kingdom, which 

 are intermediate in their properties between the fixed 

 and volatile oils. Like the volatile oils, they are 

 soluble in alcohol ; but, like the fixed, they cannot 

 be distilled over with that liquid. Hence they may 

 be obtained by digesting the substance containing 

 them in alcohol^ and then separating the alcohol by 

 evaporation. 



Many oils are formed when animal and vegetable 

 bodies are exposed to a heat above that of boiling 

 water. These oils are called empyrcumatic. They 

 are usually dark coloured, have an acrid taste and a 

 disagreeable smell, and possess most of the properties 

 of the volatile oils. 



SECT. V. Of Bitumens. 



Bitumens. The term bitumen has been often applied to all the 

 inflammable substances which occur in the earth. But 

 it is better to limit it to those fossil bodies only 

 which have a certain resemblance to oily and resinous 

 substances. They may be divided into two classes. 

 The first set possess nearly the properties of volatile 

 oils ; while the sicond set posses* a peculiar character. 

 The first class may be called bituminous oils. The 

 second, bitumens proper. 



1. Bituminous Oils. 



Bituminous Only two species of bituminous oils have been hi- 

 m l, therto eximiiv d, namely, naplitlia t>T petroleum, and 



inuliliu or tea wax. 

 Naphtha. ' Naphtha, r petroleum, is an oil of a brownish 



yellow colour. When pure, fluid as water, and pretty 



rolatilc. Its specific gravity varies from 0.730 to Element* 

 1.S7S. It has a peculiar smell. When hi-ated, it " f 



Empyrcu- 



mitic oil*. 



volatil 



It has a pecu 



may be distilled over without alteration. It unites 

 with alcohol, ether, volatile and fixed oils, and, as far 

 as is known, posscseM all the characters of volatile oil;. 



When found in the earth pure, it is distinguished 

 by the name of naphtha ; when less fluid and darker 

 coloured, it is called pelrolrum. When petroleum is 

 distilled, naphtha is obtained from it. 



'2- Maltha, or sea-wax, is a solid substance found Maltlu. 

 on the Baikal lake in Siberia. It is white, melts 

 when heated, and on cooling assumes the consist- 

 ence of white cerate. It dissolves in alcohol, and 

 seems to possess the character of a solid volatile oil. 



2. Proper Bitumeru. 



The true bituminous substances may be distin- Proper 

 guished by the following properties : bitumens 



1. They are cither solid, or of the consistence of 

 tar. 



2. Their colour is usually brown or black. 



3. They have a peculiar smell, or at least acquire 

 it when rubbed. This smell is known by the name 

 of the bituminous odour. 



4. They become electric by friction, though not 

 insulated. 



5. They melt when heated, and burn with a strong 

 smell, a bright flame, and much smoke. 



6. They are insoluble in water and alcohol, but 

 dissolve most commonly in ether and in fixed and vo- 

 latile oils. 



7. They do not dissolve in alkaline leys, nor form 

 soap. 



8. Acids have little action on them ; the sulphuric 

 scarcely any : the nitric, by long and repeated diges- 

 tion, dissolves them, and converts them into a yellow 

 coloured substance, soluble both in water and alco- 

 hol. 



The pure bitumens at present known are three, 

 namely, a.ij)/ialtiim, mineral lui; and mineral ciwiit- 

 C/IOHC. United to resin, it forms a curious substance 

 called rctinasphaltum. United to charcoal, it form* 

 the various species of pit-coal so important as articles 

 of fuel. 



1. Asphaltum. This substance occurs in great 

 abundance in the island of Trinidad, on the shores of 

 the Dead Sea, in Albania, and in other places. Its 

 colour is black, with a shade of brown, red, or grey. 

 It is heavier than water. It is insoluble in acids, al- 

 kalies, water, and alcohol ; but soluble in oils, petro- 

 leum, and sulphuric ether. See ASPHAI.TUM. 



2. Mineral tar. This substance is lound in Bar- Mineral 

 badges and other places. It is named from its con ur> 

 sistence and appearance. It seems to be a mixture 



of petroleum and asphaltum. Accordingly, when 

 distilled, abundance of petroleum is obtained, of a 

 brown colour, but very fluid. 



3. Mineral cnoutchouc is a singular substance, hi- vii nfra i 

 therto found only in Derbyshire It is soft and caoui- 

 elas.tic, not unlike common caoutchouc or Indian chuuc. 

 ru'uber. Its colour is dark bn.wn, with a shade of 



green or red. It resists ihc action uf almost all li- 

 quid menstrua. Neither alcohol, alkalies, nur nitric 

 acid, affect it. Ev< n cnlb and petroleum are incapa- 

 ble of dissolving it. When heated it melts, and con- 



