One breath of impure air shortens our life--Difficulty of getting pure air to breathe in houses and cars--Foul air in steam cars--Want of the proper knowledge regarding ventilation among all classes--Want of ventilation in this lecture room--Want of ventilation in the Cooper Institute, and in many other new and splendid buildings--Street cars very foul--My own chamber fully ventilated--I have no new patent idea, sufficient for all time without further thought--Constantly varying conditions require separate intelligent thought and action--The air moves horizontally in summer--Flues are then of no account--We must depend on open doors and windows--How to ventilate a sick room in the morning--The same in the evening--Windows should always lower from the top--To make air move in the summer is the great desideratum--When in motion the cold air falls and warm air rises; when at rest, it is arranged in horizontal layers, according to temperature--A flue is simply a passage for air of different temperatures--Experiments with flues of different temperatures--Expansion of air by heat--Weight required to keep it from expanding--Heating air weakens it instead of giving it power--Experiments showing draughts by lighted candles--Ventilation of churches--Illustrations not exaggerated--Examination of church in neighborhood--Fresh air taken from foul cellar--No fresh air supplied to churches used as hospitals in Washington--Depending on a sham ventilator painted on the solid wall--Foul air in Philadelphia schools--New York public schools--Many of the ventilators perfect shams--Covered up air-tight by the capping stones--Importance of the evaporation of water--A strong fire in basement will draw gas out of second story stove--A strong fire up stairs will draw foul gases from untrapped sewers--A very healthy location may thus be made very unhealthy--Drs. Palmer, Ford and Earle's report of epidemic at Maple Wood Institute--An arrangement for ventilation that ought to be in every house--Flues generally too small, especially in Philadelphia--Very large ones put in government hospitals, which proved thoroughly efficient--The leading points in regard to heating--The fresh air must be warmed before entering in winter--A hot water furnace requires additional moisture--Heating by steam--Steam-pipes ought to be laid through the street the same as gas and water--Two-thirds of heating surface should be for heating the fresh air and one third for direct radiation--Forty pounds of water required to be evaporated every minute for U. S. Senate Chamber--All stoves should have fresh-air boxes--Dampers in fresh air-boxes not good--Experience has fully demonstrated that careful attention to these things will be amply rewarded by increased health, strength, happiness and longevity.