11, 12. DETECTION, ETC., OF FIXED OIL. 11 



quantity of water. " I do not think it improbable that in the juice 

 of fresh plants oil is held in solution by carbohydrates and 

 does not show itself until separated by dilution with water. And 

 in examining the expressed juice of fresh plants, or concentrated 

 infusions of the same, it is well to bear this peculiarity of oils in 

 mind. 



To determine the total amount of fixed oil, the residue from 

 the evaporation of part or all of the petroleum-spirit solution is 

 dried at 100 till the weight remains constant, which may then 

 be noted. For further information respecting the estimation of 

 fixed oils, and especially the apparatus to be used, see 120. J 

 Compare also 36. 



The fatty residue so obtained may be kept for some time, 

 to observe whether partial or complete solidification does not 

 gradually take place. The solubility in absolute alcohol, spirit 

 of 95 and 90 per cent., may also be tested, to ascertain whether 

 free fatty acids, cholesterin, resinous bodies, caoutchouc, or such 

 compounds, can be isolated. (Of. 125, 126, 127, 130.) It may 

 also be observed whether the oil is easy or difficult to saponify, 

 whether the soap is soft or hard, colourless or coloured, whether 

 glycerine is separated during saponification, and the fat con- 

 sequently contain glycerides (cf. 13), and whether the oil 

 resinifies readily on exposure to the air ( 121). Finally, the 

 melting and solidifying points may be taken. Concerning this 

 determination see 17. 



12. Composition. If a further insight into the composition of 

 the fixed oil is required, larger quantities must be prepared either 

 by extraction, or by expression followed by extraction, accord- 

 ing to the nature of the material and the quantity of oil it 

 contains. 



A few qualitative experiments may first be made with a portion 

 of this oil. If it remains fluid at ordinary temperatures the 

 action of nitrous acid may be tried. The solidification of the oil I 

 would prove the presence of oleic ( 19, 130) or an allied \ <U- 

 acid capable of conversion into elaidin ( 122). In this case, on / 

 mixing the oil with about one-fifth of its volume of concentrated 

 sulphuric acid, but little heat will be evolved, whilst compounds 

 of the drying linokic acid ( 130) and its allies generally cause 

 a considerable rise in temperature ( 123). For comparison 

 parallel experiments may be made with linseed and almond or / 



