and potassium, is a conspicuous member of this group.

Inerts, or indeterminates, are pigments that have been observed to exert neither stimulative nor inhibitive influence sufficient to classify them in either of the foregoing groups.

In view of the very brief time that has elapsed since the investigation of the relation of pigments to corrosion began it would be unwise, if not unfair, to make a classification of pigments at all comprehensive upon the basis of stimulating and inhibiting qualities. It appears to be pretty well established, however, by numerous tests that some pigments that have heretofore been confidently used on steel are unsafe for protective covering of exposed iron work. As complete information as it is possible to secure  along  this  line  may  be  found in a