60. Oil is one of the fluid mediums in which a pigment is suspended to enable the painter to spread it properly and cause it to adhere when dry. It is termed the vehicle, because it serves to carry the pigment evenly over the surface to which it is applied. The vehicle is usually combined with some other medium, or subjected to some special preparation, to give it the drying qualities it seldom in itself possesses. The medium so added is called a drier, as will be explained hereafter, and is varied in material and quantity, according to the character of the oil with which it is mixed.