10 SUBSTANCES SOLUBLE IN PETROLEUM SPIRIT. 



Shallow evaporating dishes, which can be enclosed between 

 clamped glasses and weighed, may also be used if ethereal oil is 

 present ; but they must be placed in other larger dishes during 

 the evaporation of the petroleum spirit. It is, however, preferable 

 even in these cases to use the glass dishes with vertical sides pre- 

 viously described. 



10. Treatment of Fresh Plants. Fresh, very aromatic parts of 

 plants may be examined as stated in 5, without being previously 

 dried. 1 They should be as finely divided as possible by pressure 

 and trituration, then packed in a small percolator, and the moisture 

 present displaced by the smallest possible quantity of petroleum 

 spirit or ether ; the latter is, perhaps, in this case to be preferred. 

 The menstruum itself must subsequently be displaced by water. 

 The liquids may be received in a graduated burette fitted with a 

 glass stop-cock and long fine point ; in this the ether or petroleum 

 spirit may be allowed to separate, and an aliquot part measured 

 off for evaporation. (See also 22 and following.) 



EXAMINATION OF THE FIXED OIL. 



11. Detection and Estimation. We will first consider the 

 simpler case in which the petroleum spirit (or ether) dissolves fixed 

 but not ethereal oil. The absence of the latter may be recog- 

 nised by the light colour of the petroleum-spirit solution and its 

 residue after evaporation, and by the absence of any aromatic 

 odour which would otherwise be given off during the evaporation 

 of the last traces of solvent, the operation being conducted at the 

 ordinary temperature. That we really have a fixed oil to deal 

 with may be shown by the uniform character of the spot left on 

 evaporating a drop of the petroleum-spirit solution on a sheet of 

 blue notepaper. 



On examining vegetable substances under the microscope, fixed 



011 is seen in the form of small globules of high refracting power, 

 which dissolve in petroleum spirit, ether, and bisulphide of carbon, 

 and are saponified by a dilute solution of soda. If the objects 

 examined are fresh it is advisable to treat the section with a 

 relatively large quantity of water. Concentrated solutions of 

 sugar and similar substances have the power of dissolving oil, 

 which is, however, again separated on the addition of a large 



1 For information concerning the so-called dietheralysis, see Legrip, Union 

 Pharrn. V. vi. p. 65 (1876). 



