lack firm adherence. From micrometer measurements with the microscope, we find that single coats of dried paint vary in thickness from 1/500 to 1/1000 of an inch, the difference being due either to the manner of application, that is, whether under light or heavy pressure of the brush, or to the difference in the consistency of the paints tested. Few realize the thinness of coatings of paint or the strains to which its dried films are subjected.

Viaducts, tunnels, crossing bridges, and like structures, require different treatment from structures exposed chiefly to the action of rain-water and sunshine. Linseed oil, in drying, as already explained, undergoes a metamorphosis; and the result of this process is a solid, linoxyn. A  film  of  this  dried  lin- seed oil, or linoxyn, is not quickly formed without  dry air and  light,  but  once  formed,