BOILED OIL.--Linseed oil absorbs oxygen; and, when the oil contains manganese, it absorbs oxygen much more greedily; and when a manganese oil--that is to say, a boiled oil containing manganese--is mixed with linseed oil, the substance absorbs oxygen, from a limited supply of air contained in a closed space, until no trace of any other gas but nitrogen remains. The power of absorbing oxygen possessed by 100 volumes of linseed oil, compared with that of 100 volumes of a mixture of linseed oil and so-called manganese oil, is as 9·4 to 100. This may be termed the measure of its drying power. A mixture of linseed oil, with a little more than one-fourth of its volume of manganese oil, has a power of absorbing oxygen four and a half times greater than either of the components of the mixture taken separately. In this case Chevreul argues that linseed oil may be considered as a “dryer” to manganese oil. Linseed oil, without any addition whatever, if boiled for three hours, becomes a better drying oil than it was previous to the action of heat. Oil boiled with 10 per cent, of litharge for three hours, is a much better dryer than when heated without this oxide. Oil boiled alone for five hours is an inferior drying oil to one heated for only three hours. Oil previously boiled alone for five hours, and boiled alone again for three hours, is scarcely altered in drying power, but it becomes a better drying oil if it is boiled the second time with litharge. It is inferior to a drying oil which has been boiled only three hours with litharge, without being submitted to a previous boiling. Oil boiled alone for five hours, boiled for a further period of three hours with manganese dioxide which has already been used for one operation, is very nearly as strong a dryer as that which has been boiled with litharge under the same conditions; but it is superior to an oil which has been boiled with manganese dioxide for eight hours. This no doubt arises from the longer boiling with manganese having caused a larger quantity of manganese to dissolve, and that the quantity dissolved is in excess of that which yields the best result. Finally, oil boiled for five hours, and then boiled alone once more for eight hours, becomes viscous, and the first coat requires a considerable time to dry. We thus see that the oxides of lead and of manganese in certain proportions concur with heat in increasing the drying power of linseed oil. This drying of oils is a process of slow oxidation. The following points of Chevreul’s appeared to be difficult of satisfactory explanation, and suggested to Prof. Hartley an examination _de novo_ of the facts, as well as an investigation of the chemistry of the subject generally:-- 1. Linseed oil not boiled acted as a dryer to the same oil boiled with manganese dioxide. 2. Linseed oil, boiled with either litharge or manganese dried more rapidly when mixed with turpentine.