§ 9. EXm ACTION. 9 



be used for this purpose. It should be weighed immediately after 

 the introduction of substance and menstruum ; or, if graduated, 

 the volume only occupied by both need be noted. They may he' 

 macerated for about eight days, shaking several times daily, and 

 then made up to the original volume or weight by the addition of 

 petroleum spirit, to replace any that may have been lest by 

 evaporation. This having been done, it is sometimes only neces- 

 sary to evaporate an aliquot part of the solution, and caJculate 

 from the residue the weight of the substances which have been 

 brought into solution. i 



The supernatant liquid frequently becomes so perfectly clear on 

 standing, that all trouble of titration may be avoided by removing 

 with a pipette a definite volume, which may then be evaporated 

 and weighed.^ 



This method of procedure is especially to be recommended if the 

 object under examination contains ethereal oil, in which case all 

 washing of the residue, or any dilution whatever of the petroleum- 

 spirit solution, should be carefully avoided. The more concen- 

 trated the petroleum -spirit extract is, the more accurate will be 

 the gravimetric estimation of the ethereal oil. If, however, the 

 petroleum-spirit solution is to be filtered off and the residue on 

 the filter washed, care should be taken that a funnel with ground 

 edges be employed and kept well covered. 



For the evaporation of the petroleum-spirit solution no porce- 

 lain basin or round-bottomed platini^m or glass dish should be 

 use<^ on account of the loss easily caused by the capillarity of its 

 sides. It is expedient, as a rule, to use a flat-bottomed glass dish 

 with vertical sides and well-ground edges, a ground-glass plate 

 acting as a cover. If the presence of a rapidly resinifying oil is 

 suspected, the petroleum-spirit solution may be . evaporated in a 

 tared flask by passing a current of carbonic acid gas through it 

 whilst kept siu-rounded with warm water. (See also § 138.) 



' In this case, a slight error is introduoed into the calculation, by the in- 

 creased volume of the petroleum spirit due to di3.iolVed oil. But this will, as 

 a rule, be ao small that it may be entirely neglected ; or, if desirable, a correc- 

 tion may be made after weighing the residual oil, since we know that the 

 specific gravity of the fatty oil? hitherto examined ranges from 0'91to 0"925. 



^ Even when the petroleum-spirit solution does not become quite clear on 

 standing, aa is often the case when seeds are under examination, it i? better 

 to measure off a quantity with -a pipette, filter it, and wash the filter and the 

 mouth of the funnel (on the outside) with petroleum spirit, than to filter off 

 the whole of the liquid and measure off a quantity for evaporation. 



