structure. All this, however, depends on the purity of the air and its freedom from smoke and acid fumes. Should these be present, the chemical com position becomes of first importance, and those prin ciples of selection must then be adopted which are stated below. Humid or ivet climates. Under this head may be included the climates of such countries as the British Islands, the North-West of Europe, and those portions of North America washed by the Ocean. Such climates are the most trying and de structive to buildings of which the materials are of a porous structure, or contain large proportions of calcareous matter; and here chemical composition, structure, and density all become elements for con sideration in the choice of a stone for buildings intended to be lasting. The presence in such countries of smoke, of sul phurous, hydrochloric, and other acids, powerfully aids in the destructive effect of rain or moisture ; for the rain itself takes up a considerable amount of the acid from the air, and spreads it over the exposed surfaces of the buildings. The reader will scarcely need to be reminded that limestones and dolomites are especially subject to disintegration from the influence of rain charged with acid ; and this country presents numerous un happy examples of its effects. Of these perhaps the