The effect produced by heat upon the movements of air--Air a real substance--Exerts a pressure of fifteen tons on an ordinary sized man--It cannot be moved without the expenditure of power--The sun's rays the great moving power--They pass through the forty-five miles of atmosphere without heating it, and heat the solid substances of the earth's surface--Experiments showing the effect of radiant heat and reflected heat--The air of the room not pure and dry--The ordinary moisture absorbs from fifty to seventy times as much as the air--Many gases absorb much more--The moisture in the air the great regulator of heat--Air is heated by coming in immediate contact with hotter substances--Impossibility of any air remaining at rest--The practical application of these principles--The open fire acts like the sun, heating by radiation only--Probable electric or ozonic change in furnace-heated air--The stove heats both by radiation and circulation--The stove nor the open fire not suitable for large crowded rooms--Circulating warmed air best--Erroneous views in regard to ventilation--Experiments with liquids of different densities--When warming and ventilating by circulating air, the escape for the used air should be from the bottom of the room--But when ventilating with cooler air the escape should be from the top of the room--Windows should lower from the top and flues open at the bottom of the room--The fashionable system of heating by direct radiation, without any fresh air, very objectionable. PAGE 18 LECTURE III.