The question of ventilation, and the amount of fresh air required to keep an inhabited air-space in a sweet and healthy condition, has been much discussed of late years, and very fully treated of by various writers; but there was a good deal of vagueness and want of precision in the manner of treatment previous to the Report of the Committee on Metropolitan Workhouse Infirmaries in 1867. In a paper in the 'Lancet’ in 1866 I attempted to show that a more scientific method might be employed, and suggested some formulæ, which were quoted by Dr. Parkes in a paper appended to the Report above mentioned. Professor Donkin also investigated the question there, and in a short but exhaustive paper showed that, general diffusion in an air-space being admitted, the same amount of air was required to ventilate it, whatever its size might be.