"Well," resumed Harry; "so the boiling point of water depends on the degree of force which the air, or what-not, is pressing on its surface with. The higher the spot on which you boil your water, the lower the point it boils at. Therefore, water boiling at the top of a mountain is not so hot as water boiling at the mountain's base. The boiling point of water on the summit of Mont Blanc, is as low as one hundred and eighty-four degrees. So, if water must be at two hundred and twelve degrees, to make good tea, don't choose too high a hill to build a temperance hall on. The heavier, also, the air is, from the quantity of moisture in it, the hotter water becomes before it boils. If the atmosphere were carbonic acid gas, water would get much hotter without boiling than it can under--" "Present arrangements," interposed Mr. Badges. "Consisting of a mixture of nitrogen and oxygen," continued Harry. "Water requires only a very low heat to make it boil in an exhausted receiver, out of which the air has been pumped, so as to leave none to press upon its surface. Owing to boiling depending upon pressure, you can actually make water boil by means of cold." "What next?" sighed Mrs. Wilkinson. "You can, indeed. Put a little boiling water in a salad-oil flask; so that the flask may be a quarter full, say. Cork the flask tightly. The boiling stops; and the upper three-fourths of the flask are full of vapor. Squirt a jet of ice-cold water upon the flask, above where the water is, and the water below will instantly begin to boil. The reason why, is this. The vapor in the flask presses on the surface of the hot water. The cold condenses the vapor--turns it back to water. That takes off the pressure for the time; and then the hot water directly flies into vapor, and boils, and so on, till it cools down too low to boil any longer. What reduces the boiling point of water on a hill or a mountain is, that the pressure of the atmosphere decreases as you ascend. A rise of five hundred and thirty feet in height above the level of the sea, makes a difference of one degree; so, give me a kettle of water and a thermometer, and I'll tell you exactly how near the moon we are." "I shouldn't think one could make good hot mixed punch up in a balloon, now," observed Mr. Bagges, reflectively.