It appears from an examination of the photomicrographs of the paint films that a paint coating closely resembles the surface of the earth, and is subject to the same basic laws that have caused the various geodetic changes in the earth's crust. Observation of a dried pond or lake bed will disclose types of fissuring and cracking similar to those shown by dried paint coatings in which the oil has been fully oxidized, and especially in the case of paints containing pigments which act upon the oil to produce compounds brittle in nature. At Atlantic City the panels were all clean and free from dirt, presenting continuous exposure of the films, and thus maintaining conditions for active checking. At Pittsburg, soon after the panels began to chalk, the large amount of dust and black soot in the atmosphere completely covered the panels with a very thick, resistant coating of carbon, which acted as a seal or protector, preventing disintegration to a great extent. This coating was extremely hard to remove, and photomicrographs, before and after removal of this coating by rubbing with a damp cloth, failed to reveal marked checking on any of the formulas except those made of strictly pure basic carbonate-white lead. The checking, even on these, was not as marked as at Atlantic City. It is presumed that after the chalking had taken place and the chalked pigment had been washed from the panels, the gradually increasing coat of carbon and lead sulphide had protected the panels from checking, or possibly the atmosphere of Pittsburg, which in other respects had deteriorated the panels to a greater extent than at Atlantic City, did not have the extreme action in causing checking that the Atlantic City atmosphere seemed to have effected. [Illustration: Combination Formula No. 1, Pittsburg BEFORE WASHING Mottled surface due to external coating of impurities.] [Illustration: AFTER WASHING] [Illustration: Formula No. 4, Pittsburg BEFORE WASHING] [Illustration: AFTER WASHING] [Illustration: Formula No. 38, Pittsburg Basic Carbonate--White Lead Panels on Fence BEFORE WASHING Checking evident even through the outer covering of foreign matter.] [Illustration: AFTER WASHING] [Illustration: Formula No. 36, Pittsburg Basic Carbonate--White Lead Panels on Fence BEFORE WASHING Peculiar network-like checking appearing through outer coat of impurities.] [Illustration: AFTER WASHING] [Illustration: Formula No. 40, Pittsburg] [Illustration: Formula No. 45, Pittsburg]