This subject of the direction taken by the breath upon leaving the body, has been warmly discussed within a few years. It has been a very difficult matter to prove conclusively and satisfactorily, but I think we have devised some very simple experiments that will prove to you very clearly what we have stated. I have here a simple glass tube two feet long and one and a half inch interior diameter; one end is closed with a rubber diaphragm, through which is passed a small rubber tube--the other end is all open. We will rest this about horizontal, and taking a little smoke in the mouth, it will be discharged with the breath into the glass tube; it is first thrown towards the top, but it soon falls, and now see it flowing along the bottom of the tube like water--watch it as it reaches the far end--there, see it fall almost like water. [Illustration] Now, by raising the closed end of the pipe, you see we can pour it all out, and by filling it again and raising the other end, it falls back. Thus you see that, notwithstanding the extra warmth in the breath, it is heavier than the atmosphere, and falls to the floor of an ordinary room like this, say, when the temperature is from 60° to 70°. This is owing to the carbonic acid and moisture contained in it. I have varied this experiment in a number of ways, by passing it through smaller tubes and discharging it into the air in one or two seconds after leaving the lungs, and by passing it through water of various temperatures, and discharging it into rooms of different temperatures, with the same general results. As the temperature of the air diminishes, the tendency of the discharged breath to rise increases. Much care is required in conducting these experiments, to avoid as much as possible, the local currents which are always present in a room. This is a very important fact to be borne in mind; yet notwithstanding this, there are times, under certain circumstances, in which the foul air will be found in excess at the top of the room. For the further examination of this subject, we have here a little glass-house with glass chimneys and fire-place in the first and second stories. [Illustration] As the flame of a candle is such a beautiful emblem of human life, we will remove the roof and part of the floor of the second story, and place four candles in our house. They are all of different heights, you see. We will call them a father, mother and two children. As carbonic acid is that much dreaded poison in our breath, and the heavy portion of it which causes it to fall to the floor, we will make a little by placing a few scraps of common marble in this glass vessel, and pouring over it some sulphuric acid.