PAINTING. 257 



added. Raw oil requires from five to six days in drying, 

 while the boiled oil dries in about one-fifth the time. 



No other known oil has the power for absorbing oxygen 

 that is possessed by liriseed-oil, but in the process it has 

 been shown by Muelder that the oil gives off carbonic acid, 

 acetic and formic acid, and possibly water-vapors, the slow es- 

 cape of which probably accounts for the well-known porosity 

 of the dried film, and on account of which the film has 

 remarkable absorbent capacity, acting like a sponge in the 

 presence of moisture, which Dr. Dudley considers the primary 

 cause of the decomposition of the material, although not sat- 

 isfied that the water itself is the cause of the decay. 



Like other vegetable fixed oils, linseed-oil contains glycer- 

 ine and liquid acid fats. According to many authorities, 

 these fats in the presence of oxides, especially lead, produce 

 salts by the combination of the acid fats with the lead of the 

 oxide ; saponify, resulting in metallic soaps. Amongst 

 others, Prof. J. Spennrath combats this theory with many 

 valid arguments, amongst which he asserts that " if we should 

 treat any soap with diluted acid, which is capable of dissolv- 

 ing the metallic oxide contained therein, it is decomposed, 

 and the fatty acid separated. The latter then swims in the 

 liquid. A dried oil-paint can never be dissolved by diluted 

 acid in this way." Again, "a weak alkalized liquid, for in- 

 stance, a one per cent, soda solution, dissolves after a pro- 

 longed application any dried-up oil-paint coating. We then 

 obtain the coloring matter what was used in an unchanged 

 condition. A real soap cannot be decomposed by a soda 

 solution." 



Prof. Spennrath admits, however, that the rapid effects of 

 oxidation produce more or less effect upon any oxidizable 

 pigment, and several other recognized authorities assume, in 

 the case of at least one such pigment the red oxide of lead 

 that a chemical combination is produced, analogous to sapon- 



