There has been considerable inquiry as to just what is meant by the statement that "a moderate percentage of inert pigments, combined with properly adjusted mixtures of white lead and zinc oxide, have given wonderful service in all the tests." The writer has been asked to define what "moderate" means. A "moderate percentage of inert pigments" should be defined as that amount of natural crystalline pigments that will, when mixed with white lead and zinc oxide, not materially detract from the hiding power of white lead and zinc oxide. It is possible to mix a certain percentage of these crystalline pigments with white lead and zinc oxide, and, by thorough grinding, incorporate them in such a manner that the mixture will show nearly as good a hiding power as the straight white lead and zinc oxide. When certain limits have been reached, however, and these limits must be determined by the manufacturer and painter in making practical tests, the further addition of inert pigments lowers the hiding power of the paint and therefore lowers the value of the paint. These remarks do not apply to artificial crystalline pigments, such as precipitated whiting, which possess greater hiding values than the natural pigments. =Perry's Principles of Paint Making.= Parts of the original paper[18] in which Perry so clearly set forth the principles from which the preceding laws were formed, follow: [18] Physical Characteristics of a Paint Coating. R. S. Perry. Michigan Chapter, Amer. Institute of Architects, 1907. =Sealing Quality or Imperviousness of the Coating.= "It has been emphasized that for durability and protection, the strength and imperviousness of a paint coating are vital factors. The protective value of the paint coating of course ceases with its chalking or disintegration, but, while it is true that the protecting or final life of the coating ceases with this disintegration, it is also true that a paint coating has always during its true life more or less porosity from the nature of the linoxin or oxidized linseed oil. Therefore during its protecting life the degree of its imperviousness influences its resistance to attack upon its own life and its protection of the underlying materials. The more impervious the paint coating without loss of strength, the slower the oxidation or disintegration of the paint coating itself and the greater protection to the underlying material.