"A coating of linseed oil alone is not only weak, but the simplest and crudest experiments will show its porosity and this porosity increases rapidly with progressive oxidation, the porosity of course definitely hastening the over-oxidation or chalking. In proportion, therefore, to our success in filling the voids in the linseed oil film with proper pigment materials, we will in that degree succeed in excluding agencies of decay, not only from the mass of the paint coating itself, but also from the surface to be protected. These conditions are exactly parallel in the requirements and performance of the best-made concrete, and Taylor & Thompson in their work on concrete have clearly stated that to obtain imperviousness there must be freedom from voids, and that to obtain these conditions, the materials used must have at least three determining sizes. [Illustration: Equal Volume (One Cubic Centimetre) of Each Size of Shot Taken. Note that the Smaller Shot Cover more than Half as much again as the Larger Shot and the Voids are Smaller.] [Illustration: Diagram Illustrating Two Determining Sizes of Solid Particles in Concrete] [Illustration: Diagram Illustrating Three Determining Sizes of Solid Particles in Concrete] "'It is a fact that with particles of different sizes as against uniform size the densest mixture can be obtained. This is so evident as to require no proof.' It follows that the least density and hence the largest percentage of voids occur when the grains are all of the same size, and it is shown that the most voids occur in a mass of large particles. The least voids occur when the voids between the large particles are filled with smaller particles and when these smaller voids between the smaller particles are in turn filled with still finer particles. In other words--particles with three determining sizes will fill up a given space more completely than particles of two determining sizes and very much more completely than particles of one size. =Elasticity and Strength.= "The paint coating here again is governed by many of the laws which govern the similar material, i.e., concrete. We find, by again referring to Taylor & Thompson, on Concrete, page 275, that tests at the Watertown Arsenal on concrete convinced the investigators that the ultimate strength of a concrete is identical with the shearing strength of particles of stone making up the aggregate.