TESTING OILS. ] 



UNDULATOBY FORCES. LIGHT. 



101 



oils are heated to near their boiling-point, they fre- 

 quently become thick, and acquire au increased dis- 

 position to absorb oxygen. It is on this account that j 

 inferior drying oils cannot be employed for any length | 

 of time in a lamp, without closing the channels that 

 lead to the wick ; and hence it is found necessary to ' 

 clean the lamp very frequently, or else to pour away 

 the residue of the last consumption before fresh oil is 

 supplied. The same circumstance leads to the stoppage 

 of the pores of the wick, and finally to a total closing of 

 all the fine capillary channels by which the oil ascends 

 to the flame ; so that a new wick and a clean lamp are 

 indispensable to the perfect combustion of an inferior 

 oil. 



Although the fixed oils are not soluble in water, yet 

 they combine very readily with fluids which contain 

 alkaline matters, as soda or potash. In this way they 

 form white emulsions of the nature of soap ; and the 

 alkalies, or their carbonates, may be at all times resorted 

 to for the purpose of cleaning out the half-gelatinised 

 oils from old lamps. Ammonia, also, or strong sal 

 volatile, is a very eifective agent for removing grease 

 from the clothes. 



Concentrated sulphuric acid acts on the fixed oils 

 in the same way as it operates on the solid fats : it first 

 combines with the fatty acids, and then with the basic 

 constituent to form composite acids, named sulpho- 

 , sulpho-margaric, sulpho-steric, and sulpho glyceric 

 acids. These are not permanent compounds, and have 

 not, therefore, been isolated ; but when they are treated 

 with hot water, they are decomposed ; and as the 

 sulphuric acid separates, the other fatty constituents 

 resolve themselves into four new acids namely, meta- 

 uiargaric, hydromargaritic, metoleic, and hydroleic. The 

 first two are solid, crystallisable, and partly volatile, 

 while the last two are liquid and oily. Besides these 

 changes, there are others effected by the action of the 

 sulphuric acid on the sugar, mucilage, colouring matter, 

 itc. , contained in the oils ; and these are so peculiar and 

 characteristic, that they have been made the means of 

 discovering the nature of a fixed oil This fact was 

 first recognised by Heidenreich of Strasburg, in 1841; 

 but it has been greatly developed by the investigations 

 of Penot, Marchand, and others. The mode of pro- 

 ceeding is as follows : Put eight or ten drops of the oil 

 on a white plate, and then let fall into the centre of 

 the oil a single drop of concentrated sulphuric acid : in 

 the course of a few seconds or minutes, the characteristic 

 tints will be observed. Another mode of conducting 

 tin- experiment is to stir the drop of acid so as to mix it 

 with the oil. The following are the changes observed : 



(a.) With olive oil there is a yellow colour produced at 

 the points of contact ; this rapidly passes into orange, 

 and at last into a bright chesnut-brown, while the sur- 

 rounding parts of the oil gradually acquire a dirty gray, 

 and finally a smoky tint ; but there is never any shade of 

 blue or lilac. If the mixture be stirred, it speedily 

 becomes dirty-brown or brownish-gray. 



(6.) Poppy oil immediately acquires a fine lemon- 

 yellow colour, which soon becomes darker in some parts. 

 Tha surrounding portion gradually assumes a rose tint, 

 which speedily passes into violet, and then into a violet- 

 blue. This requires a period of from half-an-hour to 

 three-quarters ; and finally the yellow colour becomes 

 dirty-brown. If the oil has been kept for some months, 

 or if it has been expressed by the aid of heat, it will 

 then assume a greenish tint. When stirred it acquires 

 a brownish-olive colour. 



(c.) Nut oil produces almost exactly the same tints as 

 olive oil, but it becomes brown more rapidly, so that 

 within ten minutes it acquires a chesnut-brown colour, 

 and the gray border changes into olive-green. If it be 

 stirred, the oil clots and becomes dirty-brown. 



(d.) Cottar oil assumes a yellow tint, which slowly 

 panes into grayish-red, and at last, after many hours, 

 into purple-brown. When the oil is very old it becomes 

 brown at once ; and if the oil and acid be stirred, the 

 tint is pale yellow, and then brown. 



(t.) Almutid oil produces nearly the same reactions as 



olive oil ; but the surrounding tint is pale grayish, and 

 the yellow has a greenish tint. If stirred it becomes 

 dirty-green. 



(/.) Sessama, gingtiie, or teal oil, assumes a yellowish 

 colour, which, in the course of a few minutes, becomes 

 orange, and then chesnut-brown, the surrounding oil 

 acquiring, in the course of half-an-hour, a purplish tint. 

 If it be stirred it acquires an orange-brown colour directly. 



(g.) Orange-seed oU takes on a yellow shade, which 

 rapidly passes into brown, and then into black. If 

 stirred, it becomes black almost immediately. 



(h.) The, oil of black and white mustard-seed is Crst 

 yellow, then rich yellow-orange, and finally, in the 

 course of a few minutes, brownish-black, the surrounding 

 oil being of a greenish tint. By stirring, the dark tint 

 is brought out almost immediately. 



(i.) Linseed oil becomes rich chesnut-brown almost 

 immediately, and it soon coagulates, or clots into a hard 

 tenacious spot. When stirred it immediately thickens, 

 and acquires a brownish-black appearance. 



(t.) Hemp-seed oil presents nearly the same character 

 as Unseed, except that this oil assumes a greenish-yellow 

 tint at the edges of the acid ; and if stirred it acquires a 

 greenish-brown colour. 



(I.) Cocoa-nut oil acquires a pale purplish-brown colour, 

 which gradually darkens; aud when stirred it is first 

 ochre-brown, and then deep violet-brown. 



(m.) Refined rape-teed oil assumes the same appear- 

 ance as the last; and it is charred, as it were, by the 

 acid. 



(n.) Fresh-drawn, or raw rape-seed oil becomes bright 

 green; and then, after the lapse of ten minutes or a 

 quarter of an hour, the tint deepens, and finally becomes 

 olive-green or dirty greenish-brown; the surrounding 

 edges retain a bright green colour. If the oil and acid 

 be stirred, the tint is blueish-green, and at last olive- 

 green. 



(o.) Oleic acid changes into a sepia brown, which at 

 length darkens almost to a black. 



(p.) Scat's- foot oil becomes yellow, and then, after 

 some time, brownish. If stirred, it changes into a dirty- 

 brown at once. 



(</.) Whale or (rain oil acquires a reddish-brown 

 colour, with the edges of a violet tint ; and, if stirred, it 

 passes into a dark violet-brown, like that of the lees of 

 wine. 



(r.) Cod and other liver oil instantly changes to a rich 

 violet, with the edges of a carmine colour ; in the course 

 of a few minutes this passes into orange, and, finally, 

 into dark-brown. When stirred, the violet tint is very 

 remarkable, but very transient. 



(*.) Seal oil assumes a yellow colour, which passes 

 into rich orange, and then into a blackish-brown, having 

 a number of purple spots about the mixture. When 

 stirred, it acquires a lively yellow tint, which soon 

 changes to orange-brown. 



If the acid be diluted with one-third its bulk of water, 

 the reactions are slower, and the effect, in some cases, 

 is more marked. With such a mixture, olive, almond, 

 and castor oils show but little action ; while poppy, 

 orange, and mustard become dirty-brown ; gingilie, 

 yellowish with a pink border ; linseed, brown ; rape, 

 green ; cocoa-nut and refined rape, pale purple-brown ; 

 sessama, lavender ; oleic acid, dirty-brown ; sperm, pale 

 lavender; cod-liver, rose, passing into rich violet, and 

 then into brown ; common whale, black-brown ; seal, 

 dirty-brown ; and tallow-oil, blackish-brown. 



Mixtures of the above-mentioned oils give reactions 

 which are compounded of the preceding ; and thus the 

 fraud of adulteration may be easily detected. 



Another reaction which has been noticed during the 

 admixture of the oil with concentrated sulphuric acid, is 

 that of a great elevation of temperature ; and, as each 

 oil produces its own amount of heat, M. Maumene has 

 proposed that this reaction should be used as a means of 

 discovering the quality of an oil. Maumene's results have 

 been confirmed in the laboratory of Professor Fehling, 

 by Faisst and Knauss, who state, that when 225 grains 

 of oil are quickly mixed in a thin glass vessel, with 75 



