"It goes without saying that the pigments of any one group contain particles of dimensions which fall into the other two groups, but no one pigment supplies the correct proportion of each of the three required dimensions, and each pigment has so large a percentage of approximate dimensions as to bar it from exclusive use in the other two groups. Given similar homogeneous coatings under identical conditions, we recognize the law that elasticity will vary directly with thickness. Direct deduction from this law teaches us that of two paint coatings equal in wear, in strength, opaqueness, and in all other qualities except thickness, we should choose the thinner coating. Therefore if we have two paint coatings fulfilling every requirement, the first compounded with pigments giving a thicker coating and the second with pigments yielding a thinner coating, we must choose the second formula and obtain the thinner coating. =Adhesive Power.= "The adhesion of the linoxin to the coarse group of particles and to the underlying material is vital to the life of the paint coating. If the coating parts from the surface beneath, we have scaling or peeling. It is universally admitted that this will result from use of zinc oxide as the sole pigment. We have only to conceive of our flat arch of reinforced linoxin and leave out our points of support, to realize that this is the inevitable result if the coating be subject to extreme exposure, although good results may be obtained from zinc oxide used alone, as, for instance, in interior house painting where extreme changes of temperature and exposure are avoided. "Three major lines of force hold our linoxin in place--adhesion toward the underneath surface, adhesion to the coarse particles, and cohesion within the linoxin itself. These lines must be represented by a flat arch of linoxin with a downward pointing magnet therefrom, to represent adhesion to the surface. Magnets on each side of the arch pointing toward the supporting coarse particles, and two magnets within the arch and pointing toward each other, or to the centre of the arch, these latter to represent the force of cohesion." CHAPTER VI THE SCOPE OF PRACTICAL PAINT TESTS