I. SOURCE OF AIR.—This cannot be too particularly examined. Many buildings are supplied with air from a low level, where the ground is too frequently left in a neglected condition, and exposed continuously to different sources of impurity. At night, in clear weather especially, when the cold produced by radiation from the earth is great, and more or less at all times, cold air is too often supplied from the surface of a street loaded with the offensive emanations which they evolve, and these become still more disagreeable when subjected to the action of heat in a warming apparatus. By taking air from the highest attainable point, avoiding the immediate vicinity of chimneys, a much better atmosphere is secured, and it may be conducted downwards to any required depth by a flue. In extensive public buildings, situated in localities noted for the impurity of their atmosphere, the erection of air-towers to draw down a comparatively wholesome atmosphere is an object of great importance. An elevation of five, ten, or twenty feet often gives a much better atmosphere. In all lofty buildings, fresh air may be supplied, when requisite, from above; and where local circumstances render it desirable, air is sometimes advantageously introduced by turrets from an altitude of one, two, or three hundred feet. When air is pure, and dry cool vaults are available, they are invaluable in cooling it during hot weather. In large towns, such as London and Manchester, where local impurities abound, and particularly when fog and frost are observed at the same time, special means may be adopted for removing those that are most offensive. Filters have been constructed for air on the same principle as for water. Few of the more simple arrangements of this kind, that have been sustained for any length of time, have been found more serviceable than filtration through any porous texture to exclude suspended blacks (soot). Washing the air also has, in some cases, been tried, by moistening the filter with a stream of water. Lime-water is preferred where sulphurous acid, carbonic acid, hydrochloric, or hydrosulphuric acid are present from manufacturing operations in any notable proportions. Perforated zinc, or porous gauze, steeped previously in a solution of hydrochlorate of zinc, have been used largely for the filtration of air.