The chemical changes that may be effected in the structure of a stone by means of moisture may cause deterioration in the following ways The introduction of wet and subsequent frosty weather may tend to force off particles of the stone, due to the expansion of the water in its change to ice in the pores. This danger is greatly minimised by properly setting the stone on its natural bed, and by protecting the walls by undercutting the cornices, strings, etc., so as to throw collected moisture clear of the face of the building. It is therefore essential to know the absorbent qualities of the selected quarry before use. It may be safely asserted that in nearly all cases a non-absorbent stone, or one showing a very slight percentage of absorption in comparison to its weight, will be a good weathering material.

Many stones contain within themselves the elements of decomposition, only awaiting the introduction of some other chemical element to awaken the seeds of decay. In these cases the presence of soda, potash, oxides of irons, and many other salts and chemical compounds will, on the introduction of wet charged with carbonic acid gas, surphur, or ammonia (present in the air of most of our large towns), soon set up chemical reactions with the salts contained in the stone and cause consequent decay.

High winds are a source of destruction in the following manner: Air travelling at high velocities picks up in its passage particles of silica and other substances.

The impingement of these particles against the faces of the stone of a building, more especially the salient features, will exercise a grinding effect on the surface. This is especially noticeable in Tynemouth Priory and other buildings upon our seacoasts.

Lichens and other vegetable growths are also very destructive to stonework, especially in damp country districts.

There are other agents at work tending to the destruction of stone used in tidal work, but the more important in connection with ordinary buildings have been above named.