8 SUBSTANCES SOLUBLE IN PETROLEUM SPIRIT. 



II. 



Examination of the Sotbstances Soluble in Petroleum 



Spirit. 



§ 8. Peiroleum Spirit — I have proposed the use of petroleum 

 spirit in the analysis of plants on account of its being a relatively 

 good solvent for most ethereal and fatty oils, but not for the 

 majority of resins and allied substances which ttouM have 

 been simultaneously brought into solution had ether been used. 

 We have therefore in this liquid a means of more accurately 

 estimating ethereal and fatty oils than ■was formerly possible 

 with ether. Another advantage which petroleum spirit possesses 

 over ether is that it does not, like ether, cause a coagula- 

 tion of soluble albuminous compounds in substances rich in 

 such bodies. As it is desirable to deprive the material of fat 

 before extracting the soluble albuminous substances for their 

 quantitative determination, the whole or part of the residue 

 after treatment with petroleum spirit may be very well employed 

 for this purpose^ A chief condition for the successful application 

 of petroleum spirit is that it be very volatile. It must therefore 

 be piirified by repeated fractional distillation, and care taken that 

 it contains no compound boiling above 45°. It is, moreover, 

 desirable to distiLit over fat (lard) to free it from some of the 

 impurities of more powerful odour. 



§ 9. Extraction with Petroleum Spirit. — It has already been men- 

 tioned in § 6 that vegetable substances to be extracted with 

 petroleum spirit must be reduced to the finest powder possible. 

 It is advisable in such extractions to employ a known quantity 

 of petroleum, spirit — say five to ten times that of the substance 

 to be treated ; or, better etill, for every gram of the latter 10 cc. 

 of the former. A snuU narrow cylinder with glass stopper may 



