Part 7 (2/2)

Oil not readilyater, it will diffuse itself over its surface, and, notwithstanding its tenacity, it will do this very rapidly, and to a great extent; and then it has the extraordinary effect of preventing the action of the wind upon the water, so as to prevent the for of waves If a quantity of oil and water be put into a glass vessel and swung, the surface of the water below the oil will be seen to change with respect to the vessel, but not that of the oil If spirit of wine be put upon them, that will be at rest, and both the lower fluids in etable oil is of two kinds, the _soft_, or _mild_, which has little or no taste or smell, and the _essential_ oil, which is thin, and retains the smell and taste of the plant from which it was extracted

Mild or sweet oil is expressed froetables, and requires a considerable degree of heat to convert it into vapour, in which state alone it is capable of being infla water, and is generally obtained by means of distillation from the most odoriferous sorts of plants; but is soe The strong taste of this kind of oil arises froed acid which abounds in it; and by this means it is soluble in spirit of wine, which sweet oil is not; but it losesexposure to the air it loses its more volatile parts, and thereby approaches to the nature of a resin This volatile odoriferous principle has been called the _spiritus rector_ of the plant

The essential oils of different plants differ ravity, and also in theheavier and others lighter than water, and soealed Though the differences with respect to _weight_ and _consistency_ in these oils is probably owing to the state of the acid that is combined with them, these two properties are wholly independent of each other; so very thin and yet heavy, and others thick and yet light Essential oils are used in perfu powerfully the nervous system

Essential oils are very apt to be adulterated If it be with sweet oil, it may be discovered by evaporation on white paper, or by a solution in spirit of wine, which will not act upon the sweet oil If spirit of wine be mixed with it, it will be discovered by awith the spirit, will leave the oil much divided If oil of turpentine, which is the cheapest of essential oils, be mixed with any of the more valuable kinds, it will be discovered by evaporation; a strong s left on the paper, or cloth, upon which the evaporation was etable, is of two kinds; the first _butter_, or _fat_, which is easily congealed, owing to the quantity of acid that is intietables in having no smell or taste The other kind of animal oil is extracted by distillation from the flesh, the tendons, the bones, and horns, &c of animals It differs essentially fro an alkali instead of an acid By repeated distillation it becohly attenuated and volatile; and in this state it is called the _oil of Dippel_, the discoverer of it

All oil exposed to reeable smell; and in this state it is said to be _empyreumatic_: but this property is lost by repeated distillations

Besides the vegetable and animal oils above described, there is a fossil oil called _bitumen_, the several kinds of which differ much in colour and consistence; thefound in the cavities of rocks, and the more solid kinds are _amber_, _jet_, _asphaltum_, and _pit-coal_ When distilled, the principal component parts of all these substances are an oil and an acid But all fossil oil is probably of vegetable or ani buried in the earth Their differences fro to _time_; the co bitu their insolubility in spirit of wine

That the most solid of these, as amber, has been formerly in a liquid state, is evident, fro frequently found in them; and pit-coal has been often found with both the internal texture and external appearance of wood; so that strata of pit-coal have probably been beds of peat in some former state of the earth

LECTURE XIX

_Of Solid Substances_

All solid substances are capable of beco fluid by heat, and most of them may thereby be reduced into a state of vapour, or air; and in passing from a fluid into a solid state their coe to the nature of the substance; so that all solids, especially if they be suffered to concrete slowly, may be called _crystals_

Exclusive of _salts_, which have been considered already, as foreneral have obtained the names of _earths_, or _stones_, which differ only in their texture; and they are distinguished into those that are _ called _ores_, and the latter simply _earths_; the principal of which are the _calcareous_, _siliceous_, _argillaceous_, _nesia_, _terra ponderosa_, and a few others which have been discovered lately, but have not been much examined

_Of Calcareous Earth_

Calcareous earth is found in the shells of fishes, the bones of aniypsuin; and beds of chalk, liht to have been beds of shells formed in the sea, in some pristine state of the earth

The calcareous earth which is found in shells, lime-stone, andwith acids To obtain it perfectly pure, the earth must be pounded and washed ater, in order to free it from any saline substance which ar, and precipitated by mild alkalies Liht, in fixed air and water, and the remainder, called _quick-lime_, attracts water very powerfully, and their union is attended with much heat, after which it dissolves into a fine powder called _slaked lime_

If it be left exposed to the at moisture, fall into the state of powder

Water dissolves about one seven hundredth part of its weight of quick-lime, and is then called _lime-water_ Exposed to the air, a crust will be formed on its surface, which is found to consist of calcareous earth and fixed air

Lime and water mixed with sand make _mortar_, by which means different stones may be made to cohere as one mass, which is the most valuable use of this kind of earth

Calcareous earth, united with vitriolic acid, ypsum_; and this substance pounded and exposed to heat, parts with its water, and is then called _plaister of Paris_ In this state, by iain, it beco moulds, &c

The earth of animal bones is calcareous united to the phosphoric acid