COLOUR.--The colour or tint of a pigment can only be estimated by comparing it with a standard sample; this is done as follows:--A sheet of black paper, with, a dead surface, is spread out on a table in front of a window, a small heap of the standard is placed on the paper, and next to it a similar heap of the sample to be compared; by means of a palette knife the surface of the two heaps is flattened out; on now carefully looking at the two heaps, the one which has the purest colour can readily be picked out. The heaps should be looked at from several points of view before a final judgment is arrived at. If the pigment is dark-coloured, it should be spread on white paper, taking care that the same kind of paper is used for the two samples. _Durability._--Durability is not a difficult point to test, but it takes some time to make a complete test. The best method is to mix a small quantity of the pigment in question with raw linseed oil, cover a piece of glass with the mixture, and expose it outside to the action of the sun and air for some time, noting at intervals how it behaves. It is well for the sake of comparison to coat a second piece of glass with the mixture, and keep this in a dark place. The difference between the two from time to time will show how the pigment behaves under the influence of light and air. As the durability of pigments is decidedly different according as they are used in oil or water-colour painting, the oil in the former case acting as a protective agent, it is well to use a similar test, using a little gum water as a vehicle to mix the pigment with. It will take from two to three months at least to properly test the durability of a pigment in summer, while in winter the time will be increased considerably. Glass is the best substance to use, as it is quite neutral, and does not of itself introduce into the test any injurious element, as wood or paper might do, although these bodies may be used if thought desirable. CHAPTER XII. VEHICLES AND DRYERS. Paint consists essentially of two parts, the pigment (see Chapters I. to VIII.), and the vehicle or medium. In the case of oil paints, a third substance termed a dryer becomes necessary, to facilitate the “drying,” or solidification of the vehicle. A perfect vehicle should mix readily with the pigment, forming a mass of about the consistency of treacle. It should itself be colourless, and have no chemical action upon the pigments with which it is mixed. When spread out in a thin layer upon a non-porous substance, it should solidify, and form a film not liable to subsequent disintegration or decay, and sufficiently elastic to resist a slight concussion.