I. SOURCE OF AIR.—This cannot be too particularly examined. Many of the public buildings in this country are supplied with air from the lowest levels, principally with the view of facilitating the movement of the air through the flues. Hence at night, in clear weather especially, when the cold produced by radiation from the ground is great, and more or less at all times, cold air is supplied from the surface of the ground, or the public streets, loaded with the offensive emanations which they evolve, and which become much more disagreeable when subjected to the action of heat in a warming apparatus. By taking air from the highest attainable point, avoiding merely the immediate vicinity of chimneys, and any ingress of air at the same level, a much purer atmosphere is secured; and it may be conducted downwards to any required depth with a flue. In extensive public buildings situated in localities noted for the inferiority of their atmosphere, the erection of air towers to draw down a comparatively wholesome atmosphere from a height of two or three hundred feet, would be an object of great importance. 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. Filter beds may be constructed for air on the same principle as for water; but few of the more simple arrangements of this kind that have been sustained for any length of time, have been found more desirable than filtration through any porous texture to exclude suspended blacks (soot), and in some cases also washing with water (particularly lime water), to condense sulphurous acid and other gases.