The third [kind of change that oils of the kind proper for painting are liable to suffer in their nature, and which affect them as vehicles, that are confounded by painters under one term, viz fattening] is a change produced by artificial means, from exposing the oil a long time to the sun and air, (of the particular manner and use of which we shall speak more fully in its proper place; whereby it is freed from its grosser and more seculent parts, and rendered colourless and of a more thick and less fluid consistence, than can be produced by any other treatment: but at the same time made more reluctant to dry, particularly with vermilion, lake, Prussian blue, brown pink, and King's yellow; and indued with other properties that disqualify it for common use as a vehicle in painting. these qualities, nevertheless, may be rendered advantageously subservient to some particular purposes: though the nature, and even the preparation of fat oil is less understood at present than one could imagine it possible, with regard to a substance of su much consequence, both to some kinds of painting, and several other kindred arts. Oils in this state are called also fat oils; tho' it is a change that has not the least affinity with either of the others; but, on the contrary, differs oppositely from both of them in some very essential circumstances.