When the question of ventilation first assumed a practical shape in this country, it was supposed that 600 cubic feet of air per hour were sufficient for a healthy adult, in a room where a number of people are congregated together. Subsequent experience however has shown that this is by no means enough. As much as 1,000 cubic feet have been found insufficient to keep the air free from closeness and smell; and it is highly probable that the actual quantity required will ultimately be found to be at least 1,500 cubic feet per hour per man. In sick wards we have more positive experience as to the quantity of air required to keep them sweet and healthy. It has been found in certain Parisian Hospitals, in which the ventilating arrangements were deficient, that pyœmia and Hospital gangrene had appeared among the sick in consequence. These diseases disappeared on the introduction of ventilating arrangements, whereby 2,000 cubic feet of fresh air per bed per hour were supplied to the wards. Notwithstanding this large quantity, however, the ward atmosphere was found not to possess sufficient freshness or purity: and the quantity of air had to be increased in subsequent ventilating arrangements. As much as 4,000 to 5,000 cubic feet per bed per hour have been supplied in certain Hospitals. At the rate of 4,000 cubic feet, the ward atmosphere is found perfectly fresh. [Sidenote: Night Ventilation.] At one of our largest London Hospitals it will be perceived that above the one door of each ward is a large ventilator, ordered to be open day and night--that beyond this ventilator, which opens into the landing-place, is a large window which opens into the external air; and thus admits fresh air into the ward at night, diminishing the foul night atmosphere. This night ventilation system is good; but it requires careful watching, as chilly patients, Nurses, and sometimes Sisters, are very apt to give, after they conclude the rounds are over for the night, a sly pull at the rope and to shut the window, and thus imbibe the foul air directly generated, in quiet. [Sidenote: Special Wards, whether desirable or not.]