from getting at it, as a combination of these will destroy steel, and that quickly, if carbonic or some other acid be also present. It is not our purpose to suggest or discuss methods of applying coverings to structural steel other than to insist that a mechanic, to do good work, must have good tools to work with, and fair conditions under which to do the work set before him. Many worthy paints are ruined because flowed on with a broad, thin, flat brush, not much better than a common whitewash brush, instead of being rubbed out and into the surface well with a good, full, round bristle brush; and then, again, painters are apt, if the pigment used is heavy, or the parts to be covered are difficult of access, to add an excess of thinners to save labor or cost of material. A layer of air exists on all surfaces; hence the importance of rubbing out