_Process 1._--The chemical action in the first process is doubtless one which takes place in three stages. It commences by depriving the oil of water; in the second stage, it destroys the mucilage, by charring it; in the third, it destroys, in part, the glycerine, and sets free the fatty acids. After the litharge or manganese compound is added, there is formed in the oil a solution of lead salts of the fatty acids, or a manganese salt of the fatty acids. The oil then, at the high temperature, loses glycerine by oxidation caused by the air, such oxidation being greatly facilitated by the presence of manganese compounds, which are repeatedly oxidised by the air and reduced by the oil, that is to say, they absorb oxygen and pass it over to the oil with great facility. It matters little, so long as the ultimate action is oxidation, what salt of manganese or what oxide is used, if it be capable of undergoing processes of an alternate character called oxidations and reductions. It is, however, certain that some manganese compounds are more suitable than others, owing to their more or less complete solubility in the oil, and their more readily undergoing the two different processes of oxidation and reduction in presence of air and of oil. _Process 2._--The credit of being the first to boil oil without resorting to the dangerous expedient of using an open fire and a high, temperature in the manufacture, is due to Vincent. He used manganese compounds, or both manganese salts and litharge. His method of boiling oil for the manufacture of printing inks is, with some modifications in technical details, carried out on a large scale at the present time in the preparation of ordinary boiled oil. The essential parts of the plant are a steam-jacketed close boiler with agitating gear, and a pipe for conducting a current of air into the oil by means of a blowing engine. From the head of the boiler there passes a funnel under the back of the furnace fire, by which the disagreeable products of the chemical action are conducted to a place where they are destroyed. These products, as already mentioned, are volatile fatty acids and acrolein. Oil boiling, as ordinarily carried out, is conducted by means of litharge along with compounds of manganese; in some processes these are mixed with salts of alumina and zinc. The oil so produced is brown and not clear, but it is clarified by keeping. Many samples of such boiled oil deposit insoluble matter when stored for some time, even although they may have become clear previously. This is not a desirable property. Sometimes rosin is added to hasten its drying. The defects to be noticed, even in the best samples of boiled oil, are the following:-- 1. The oil causes a brownish or yellow colour to be communicated to white lead or zinc white. 2. The oil darkens pigments containing brilliantly coloured metallic sulphides, such as vermilion, cadmium yellow, and ultramarine blue.