Detection of Rosin when dissolved in the Fixed Oils. 289 



the electric current was in the same direction, whether the 

 metal were brought near to, or caused to recede from the 

 magnet, provided that the direction of its movements were 

 unaltered. In Arago's revolving disc the electricity that I 

 was able to obtain from one of these parts in a multitude of 

 experiments always agreed with these results (F. 92. 95. 96.), 

 and consequently (F. 119. &c.) I recapitulated them in a 

 short description, as presented in Arago's disc, establishing 

 more particularly (F. 123.), that the currents produced near 

 or under the poles are discharged or return into the parts of 

 the metal situated on each side of and more distant from the 

 place of the pole, where the magnetic induction is necessarily 

 " weaker." 



[To be continued.] 



XL 1 1 1. On the Detection and Estimation of Colophony 

 (common Rosin] 'when dissolved in the Fixed Oils. By J. 

 DENHAM SMITH, Esq. 



To Richard Phillips, Esq., F.R.S., L. $ E., $c. 



MY DEAR SIR, 



QOME samples of linseed oil were sent to me for analysis 

 ^T in July last, the bulks of which had been exported, but 

 were found on their arrival to be unsaleable and perfectly 

 useless, for when mixed with white-lead in the usual manner 

 for making paint, the mixture became quite hard at the ex- 

 piration of a few hours, it having set as plaster of Paris does 

 when moistened with water. 



At first sight it was obvious that all these oils had been 

 considerably adulterated ; for not only was their colour much 

 deeper than that of ordinary linseed oil, but they were all 

 extremely viscid, resembling castor oil, in this respect, much 

 more than the comparatively thin and fluid commercial lin- 

 seed oil. Suspecting both from smell and taste, particularly 

 the latter, that the adulterant was common colophony (black 

 rosin), I endeavoured to ascertain whether my conjecture 

 was well-founded, and if so, to determine the proportion of 

 colophony contained in the several samples ; especially as the 

 adulterated oils were likely to become the subject of legal 

 proceedings. 



I am not aware that there is any mode on record for se- 

 parating, or even detecting common rosin when dissolved in 

 the fixed oils, so that I was obliged to make numerous ex- 

 periments, before a method was discovered which appeared 



Phil. Mag. S. 3. Vol. 17. No. 110. Oct. 1840. U 



