edly much better than they were twenty-five or thirty years ago, but there is still much room for improvement.

Pickling

Pickling is an effective way to clean steel; and it is much used in electro-plating and enameling, but not for bridgework. The reason is the trouble and expense which it involves. In applying the method, it must be remembered, that when metal is immersed in a hot bath of either sulphuric acid or muriatic acid, it will be necessary, immediately after removal therefrom, to neutralize any portion of the liquid which adheres to the steel.

Painting Newly-Erected Steelwork

As soon as practicable after a span is erected, provided the weather is propitious for painting, the metal should be thoroughly cleansed from any dirt, grease, rust, or other impurity which it has taken on since leaving the shop; and if, for any reason, any serious rusting has started, all traces of it should be removed. Next, all spots abraded either by accident or during cleaning should be touched up with some of the red-lead paint used in the shop (with, perhaps, the addition of a little Japan drier in order to hasten the job); and, after this spotting has dried sufficiently, the first field coat should be applied. This should be allowed ample time to dry before the next coat is put on.

Concrete Encasement

The encasing of bridge metal in concrete is an expedient which has come into vogue of late years. It is employed in most cases for protecting all metal below the level of the deck against locomotive gases, and is not often used above that elevation. The expedient, though, in one sense is quite old; because, for several decades, column feet of trestles and elevated railroads have been protected by enclosure in the concrete of pedestal tops.

In order to make such protection truly effective, the concrete should be water-proof. This result can be accomplished in a number of ways. A water-proofing membrane gives the most certain results; but in many instances it will be best to adopt a rich mixture and to add to the cement some five or ten per cent of its volume of hydrated lime. This matter of water-proofing is treated at length in Chapter XLIII.

As concrete is heavy, its use for a metal protector adds materially to the dead load of structure, thus necessitating the employment of more steel to sustain it; hence it behooves one to make the coating as thin as practicable and yet not too thin, because very thin concrete may not afford the requisite protection, unless it be placed by pneumatic gun, in which case it is termed gunite.