Keeping this standard in view, the physician may be called on for an opinion as to whether the ventilation of a given building is satisfactory or as to the merits of a proposed plan for ventilation. The first is a question of fact: What are the effects produced upon the inmates? Are there unpleasant odors in the building or not? What percentage of carbonic impurity is present? What is the number of cubic feet of air per head that is introduced and removed per hour? And what is the character of the fresh-air supply as to purity? Does it come from the cellar, or from other rooms, or from a foul area? Air-currents can usually be best investigated by the fumes of nascent muriate of ammonia produced by {179} exposing a cylinder of common blotting-paper, moistened with dilute hydrochloric acid, to the vapors coming from a crumpled fragment of the same paper moistened with common aqua ammonia and placed within the cylinder. The process for carbonic acid determination is simple, and can be learned in three hours in a laboratory under a skilful teacher. It does not seem worth while to describe it here. The determination of the amount of air passing through a given register, flue, or chimney in a given time is to be made by the use of an anemometer, an instrument which registers the velocity of the current of air passing through it. In judging of the merits of a plan of ventilation the following points should be remembered: The defect in most plans for ventilation is in the air-supply. Many people suppose that they have made all necessary provision for ventilation if they have put in tubes or openings for the escape of foul air, forgetting that these outlets will have no effect if corresponding inlets are not provided. Examine, first of all, therefore, the ducts, flues, and openings proposed for the fresh-air supply, with reference to their size and position and the amount of air to be furnished by them. These will almost invariably be found to be too small. The proper size of flues and registers for a given room is ascertained by dividing the number of cubic feet of air to be supplied per second by the velocity in feet per second which the air is to have in the flue or opening, bearing in mind that it is much better that these flues and registers shall be too large than too small, since it is easy to reduce their capacity, but, in most cases, impossible to increase it. When the fresh-air register is so situated that the current of air from it is liable to strike upon the person of an occupant of the room, the velocity of this current should not exceed 1-1/2 feet per second if unpleasant draughts are to be avoided; and it will usually be found best that the velocity of the air in the flue shall not exceed 6 feet per second, except in the case of very large flues, where the element of friction becomes of comparatively small importance. In the great majority of cases the amount of air to