237. Why does the bubble fall after it has been in the air some time?
Because the air contained in it has become cool, and, as it contains carbonic acid gas, it is heavier than the air.
238. What became of the warmth at first contained in the bubble?
It has been distributed in the air through which the bubble passed.
239. What does this simple illustration of the distribution of warmth explain?
It explains the law of convection, or heat distribution, over the surface of the globe.
240. Why does air ascend the chimney?
Because, being heated, it becomes lighter than the surrounding medium, and therefore flies upwards, through the outlet provided for it.
241. Why does air fly from the doors and windows towards the fire-place?