of Colophony when dissolved in the Fixed Oils. 291 



cipitate and filter was found to be 29'4 grs. IT? grs., weight 

 of filter, = 17'7 grs. of the compound of resin and oxide of 

 lead obtained from 30 grs. of common colophony, which 

 would indicate that 59 grs. of this compound are equal to 

 100 grs. of rosin. 



This experiment was repeated with 40 grs. of rosin dis- 

 solved in a little oil, and this mixture treated in the manner 

 above described. In this instance 26*7 grs. of the precipitate 

 were obtained, which is equal to 66*7 per cent, of the resin 

 employed. It appears, from these experiments, that although 

 this method is incapable of affording strictly accurate results, 

 I presume from the slight solvent action of the spirit upon the 

 precipitate, yet that an approximation to the quantity of colo- 

 phony contained in oils adulterated with this substance, may 

 be obtained, which perhaps by some subsequent modifications 

 of and precautions in the process, may lead to a mode ca- 

 pable of yielding not merely a tolerable approximation, but 

 rigidly accurate results. 



When unadulterated commercial linseed oil is treated with 

 rectified spirit in the manner described, the alcoholic solution 

 affords no precipitate on the addition of a spirituous solution 

 of acetate of lead, but it merely becomes turbid, exhibiting 

 appearances very different from the bulky precipitate which 

 is produced if colophony be present. The samples of adul- 

 terated oil when submitted to the process I have described, 

 respectively afforded 27*7, 21 and 26*3 per cent, of this 

 compound of resin and oxide of lead, which would indicate, 

 according to the average of the two experiments with resin 

 and oil already mentioned, 44-1, 33'4 and 41 '7 per cent, of 

 common rosin respectively contained in these specimens of 

 oil. 



When this white precipitate is suspended in rectified spirit, 

 and a current of hydrosulphuric acid gas passed through it, 

 decomposition takes place, sulphuret of lead is formed, and a 

 light yellow-coloured solution, which reddens litmus, is ob- 

 tained ; this, on evaporation, leaves a brown and brittle resi- 

 duum exactly resembling common rosin. For this mode of 

 separating the acid resin from the oxide of lead, I am in- 

 debted to my friend Dr. Brett. From the circumstance of 

 this alcoholic solution affording no precipitate with nitrate of 

 silver until the addition of a little ammonia, I conclude the 

 resin acid which is combined with the oxide of lead in the 

 precipitate obtained in the foregoing experiments, to be the 

 sylvic acid. When ignited in a covered crucible, so as to 

 avoid the access of atmospheric air, and consequent oxidation 

 of the lead, 1 find that the precipitate, whether obtained from 



U2 



