The effect of oxidation upon linseed oil is to destroy all the glycerine, and to produce therefrom carbonic, formic, and acetic acids, together with some acrolein. When boiled at a high temperature without the addition of any metallic oxide, the glyceride is decomposed, acrolein is formed, and linoleic acid is set free. In fact, whether oil is oxidised by air or by metallic oxides, or whether it be simply heated, the action in each case first leads to the destruction of the glycerine and the liberation of linoleic acid. But linoleic acid very readily absorbs oxygen, and the oxidised substance becomes a tough elastic solid, which is essentially a varnish. In fact, the process which an oil undergoes in drying is not desiccation, or depriving it of moisture or of glycerine, but solidification, and the technical term “drying” is a misnomer. That, however, is of little consequence if we really know what is the chemical action of the “drying” process. When oxidised even at a low temperature, the glycerine is destroyed, and the oxidised products form a tough varnish. There are various methods of converting linseed oil into a drying oil or varnish:-- 1. Heating it to a high temperature with litharge. 2. Heating with red oxide of lead. 3. Heating with metallic lead. 4. Heating to a high temperature with manganic oxide. 5. Heating with manganese borate. 6. Heating with manganese oxalate. 7. By the joint action of air and heat upon the oil and manganous oxide, or a solution of manganese dioxide or manganous oxide in the oil. In the processes 1, 2, 3, there can be no doubt that a lead of linoleic acid is produced, and that this facilitates further oxidation in air, by forming salts with some of the acid products of such oxidation, while the oxidation of the linoleic acid continues. Heating with red lead favours oxidation, by the compound itself conveying oxygen to the oil. In the case of metallic lead, it must be noted that the metal is dissolved. Under certain circumstances, metals become dryers to oils; thus sheets of metallic lead are capable of acting as dryers to linseed oil. Linseed oil is pre-eminent in its capacity for absorbing oxygen. This action of metallic lead as a dryer is due to the metal becoming oxidised at the expense of the glycerine of the oil, and so passing into solution by combining with the linoleic acid, or with acetic or formic acid, caused by the oxidation of the glycerine. It is the destruction of the glycerine with concurrent oxidation of the fatty acid which causes the drying or hardening of the oil.