Next inform your friends that, without applying any extra heat, you will cause the water in the flask to boil vigorously again. This seems to them impossible, especially when you tell them that you are going to do it by means of cold water. Quickly turn the glass upside down, and squeeze a sponge soaked in cold water on its upturned under-surface. Immediately the liquid inside will begin to boil, as if extra heat had been applied ([Fig. 13]).
Fig. 13.—A curious boiling experiment.
But how are you to explain this apparently extraordinary phenomenon?
Well, directly the cold water comes in contact with the flask it causes the steam contained therein to condense, and, as no air can enter, thanks to the well-fitting cork, the pressure on the surface of the warm water is now considerably less than it was before.
Directly the pressure is lessened the vapor bubbles contained within the warm water are able to rise to the surface, and the water is seen to boil merrily.
Making Coal Gas
Here is a very simple way of obtaining coal gas.
Procure an ordinary long clay tobacco pipe, the bowl of which should be filled with very small pieces of coal. Carefully cover the top with soft clay, and put the bowl in the fire, with the long stem protruding through the bars. Now watch this end of the pipe very closely and see what happens.
