100 



UNDULATORY FORCES. LIGHT. 



[PROPEKTIKS OP olt.s. 



which huliU the impurities. At other time* it U boiled 

 with weak alkaline solutions, or with lime water, and 

 allowed to repoM. Dome's process for purifying fish- 

 ciU U ai fullowi: To each gallon of oil an ounce of 

 powdered chalk, and half an ounce of recently slaked 

 lime, are added : after standing a short time, about half 

 a pint of water is carefully stirred in, and the mixture is 

 agitated at intervals of two or three hours, for a period of 

 several day*. At the expiration of that time, a solution 

 of one ounce of pearljuh in four ounce* of water in stirred 

 in, and the stirring U kept up at intervals for several 

 hour* : aft.-r this, two ounces of salt dissolved in a ]>int 

 of water are poured in, and the mixture is agitated occa- 

 sionally for the next two days. It is then allowed to 

 stand until it becomes clear; and if the oil be not suffi- 

 ciently pure and free from odour, the process is to be 

 repeated, taking care that the quantities of the ingre- 

 dients are reduced one-half. 



The plan that is usually adopted for the purification of 

 oils, is founded on the fact, that concentrated sulphuric 

 acid, in small quantity, has the power of decomposing 

 the impurities of oil without touching the oil itself : but 

 in carrying out the process, care must be taken that oil 

 of vitriol ia not added in too great quantity, or allowed 

 to act for too long a period. The process is generally 

 conducted in the following way : About one per cent, of 

 commercial sulphuric acid is cautiously added, little by 

 little, to the impure oil ; in those cases where the oil 

 clots with strong acid, as happens with linseed oil, the 

 vitriol acid must be diluted with about half its bulk of 

 water before it is added. The mixture is then stirred 

 for several hours, in order that the acid may be brought 

 into contact with all the impurities in the oil ; the stirring 

 is generally effected by means of a sort of churn or 

 Kpindlc with cross pieces, which is kept slowly revolving 

 in the vat containing the mixture. In the course of a 

 few minutes after the vitriol is added, the oil becomes 

 discoloured ; and after the agitation has been kept up for 

 some time, the impurities dot together in the form of 

 flakes. When this happens the oil is allowed to stand 

 quiet ; and in the course of a few hours the flakes subside, 

 and leave the oil in a clear and almost colourless condi- 

 tion. The oil is now run off into vats, and boiled up with 

 about half its bulk of water ; this removes any acid that 

 may be adhering to the oil : the fire is then withdrawn, 

 and in a short time the oil floats upon the water, and 

 may be run off into casks ready for the market. 



Other processes have been recommended for the ^.iri- 

 fication of oils ; as, for example, the nitration of them 

 through animal or peat charcoal, or the exposure of them 

 to the action of light, or to the chemical influence of a 

 weak solution of chloride of lime ; but all these processes 

 are more or loss difficult of management, and conse- 

 quently they have given place to the more speedy and 

 certain method of refining by means of oil of vitriol. 

 The filtration of oil through charcoal is, however, a very 

 effectual plan for removing bad colour or bad odour. 



General Properties of Oil. Every variety of fixed oil 

 communicates a greasy stain to paper or cloth, and has 

 an unctuous feel. It is also perfectly insoluble in water ; 

 and when mixed therewith, speedily rises to the surface. 

 This shows that it is lighter tlian water ; in fact, a 

 bottle which, when full, holds 1,000 grains of distilled 

 water, will contain only 884 of sperm oil, or 965 of 

 castor oil. These are the two extremes, for sperm oil 

 is the lightest of all fixed oils, and castor the heaviest. 

 Chemists have applied the term tpecific gravity to these 

 numbers ; and as it U found that each oil has its own 

 particular weight, specific gravity is made the means of 

 discovering the nature and quality of any doubtful oil. 



With respect to the mode of taking specific gravity, 

 we may state, that solids are weighed first in air, and 

 afterwards in distilled water. The weight in air is then 

 divided by the loss of weight sustained by the body on 

 Ireing weighed in water; and the quotient in the specific 

 gravity. Liquids, such as water, spirits, ether, oils, AT , 

 may have their specific gravity taken by weighing them 

 in a glass globe with a narrow neck, in the manner already 



ntioned. 



The following table represent* the relative weights or 

 specific gravities of different fixed oils : 



Sperm 884 



Tallow 900 



Rape-seed .... 913 



rium-kemel ... 913 



Henbane-seed. . . 913 



L 



Cioimd nut . . . 915 



Olive 916 



Sessamum .... 916 



Almond '.M7 



White mustard-seed . 918 



Radish-seed ... 919 



GrajHs-seed ... 920 



Poppy-seed ... 922 



Whulu (train) ... 923 



Black mustard-bead . !-:! 



Walnut 923 



Cucumber-seed . . 923 



Tobacco-seed . . . 923 



924 



990 

 998 

 936 



Cherry-stone . , 

 Fish .... 

 Camelino . . 

 Nightshade 

 Sunllower-seed 

 Hemp-seed . . 

 Cocoa-nut . . 



Walnut '.'- 1 ; 



Fill-ert 926 



Anda 927 



Horse-chesnut . . '.'-'7 



Cod-liver .... !US 



Seal 929 



A (new) ... 930 



\\rl.l-seed .... 9'M 



Orange-seed . . . 940 



Linseed (old) ... 960 



Castor .... 965 



Most of the fixed oils are affected by the atmosphere 

 the oxygen of which they slowly absorb. In some 

 cases the oil becomes thick ; or, if exposed in a thin 

 layer, it dries. In this manner a skin is frequently 

 formed over the surface of the oil ; and if it be left 

 for some time in a lamp, it will assume a jelly-like 

 appearance. Other oils do not become so thick by 

 exposure to the air ; but they acquire an unpleasant 

 smell, and get rancid. The former is the property of 

 the drying, and the latter of the fat or unctuous oils. 

 These changes are due to the action of atmospheric 

 oxygen ; indeed, Saussure found that a layer of nut-oil, 

 one-fourth of an inch in thickness, absorbed as much as 

 145 times its bulk of oxygen in the course of eleven 

 months, of which quantity 142 volumes were taken up 

 during three months' exposure to the sun. It appears, 

 also, that it is the carbon of the oil which undergoes 

 oxidation ; and, as it were, slowly burns, evolving car- 

 bonic acid. This change is always accompanied with an 

 elevation of temperature ; and hence it sometimes hap- 

 pens that rags, tow, or cotton, that have been smeared 

 over with oil, and then thrown aside as useless, have; in 

 the course of a few days, generated heat enough to produce 

 spontaneous combustion. Fires have frequently origi- 

 nated in this manner in warehouses and dockyards, 

 where such materials have been allowed to accumulate. 

 This points to the danger that is attendant on the careless 

 and slovenly trick of throwing greasy rags, and other 

 such matters, into out-of-the-way corners. Those oils 

 which absorb oxygen with great facility, and thereby 

 become thick, are not well-suited for combustion in 

 lamps, unless the lamp is cleaned out every day, and 

 a fresh wick adapted to it. The quick-drying oils are 

 Unseed, poppy, walnut, hemp-seed, and nut all of 

 which are employed by painters on this very account; 

 while rape, colza, sessama or gingilie, cocoa-nut, grape- 

 seed, cameline, sunflower, cotton, mustard, <fcc., only 

 dry after very long exposure to the air; and sperm, 

 olive, almond, seal, and whale oils are not much dis- 

 posed to dry at all. 



Heat and culd act upon the oils, and produce changes 

 in them which are more or less hurtful. In cold 

 weather that is, at a temperature below 32 most 

 of the oils become thick, from the congelation of their 

 solid constituents ; and we then find great difficulty in 

 making the oil burn in a lamp. At a temperature of 

 600, or thereabouts, the several fats begin to boil ; and 

 if the temperature be carried a little higher, the oil 

 undergoes decomposition, and gives forth a most irri- 

 tating and unpleasant vapour, which is called acroline. At 

 still higher temperatures, the fixed oils are resolved into 

 combustible gases, which burn with a very bright light 

 and a sooty flame : indeed, the object of all the arrange- 

 ments for the combustion of oil in lamps for illumi- 

 nating purposes, U that of bringing the oil slowly into 

 contact with the bunting wick, so as to generate tin- 

 gas in question, the supply of which ought to be duly 

 apportioned to that of the air which consumes it. When 



