'260 CN THE NATURE OF DRYING OIL*. 



Drying oils ; ' . Of the firft, which I {hall call drying oils, there are three? 



poppy/ ' * n common u ^ e > linfeed, nut, and poppy oil. The firft is 

 darkeft coloured, and dries the fooneft ; the fecond is lighter, 

 but does not dry fo foon ; and the third has leaft colour, and 

 dries flower than either of the others. 



Drying oils con- By a procefs, which it is perhaps needlefs to defcribe here 



tain mucilage. . ( ^ y^ ^ ^ ?g |j,* T ^ fucceeded in f e p ara ting 



from each of thole oils, a mucilage or gum, in a liquid ftate, 

 and capable of mixing with water in every proportion, though, 

 when thoroughly dry, it would not diflolve in cold water ; 

 but my experiments on this head were not carried to any 

 great Jength. It is to be remarked that linfeed oil afforded 

 moll of this gum, nut oil the next'largeft quantity, and poppy 

 oil the lead: of all. 

 Olive o!l con- . Olive oil, when treated in the fame manner, afforded none 

 tains none: ©f this mucilaginous fubftance ; whence I was led to conclude 

 .that the effential difference between the drying oils, and thofe 

 which do not dry, confifts in this :— that the latter either 

 contains no mucilage or gum, or that it is fo intimately com- 

 bined with its other principles, that it cannot be feparated 

 from them in that peculiar manner which always takes place 

 in oils which dry by themfelves, or when mixed with con- 

 tours. 

 Drying oil affords If drying oil is expofed to the air, in a fhallow vefTel, and 



drieffilrft *& * Ieft at reft> a P ellicIe is r ° on formed on the top, and becomes 

 externally perfectly dry. If this be removed, a fecond will 

 ■be formed in the fame mariner ; and if this experiment be re- 

 peated many . times on the fame quantity of oil, without 

 moving or fliaking the veflel, it will be found that the fecond 

 pellicle will require more time to form it than the firft, and fo 

 on, till it will be found difficult to get it fairly fkinned over 

 in a confiderable time. The fame effect takes place, in a 

 lefs vifible manner, in every quantity of drying oil which is 

 united with colours in a picture. 



. . From this experiment it is to be concluded, that drying 



oils exert that faculty by throwing up their mucilaginous 

 ' • parts, which become (olid when at reft, and in contact with 

 the air. 



* Or Philof. Journal, quarto, III. 345, 



The 



