The atmosphere, you know, diminishes in density, and consequently in weight, as it is more distant from the earth; the warm air, therefore, rises only till it meets with a stratum of air of its own density; and it will not ascend into the upper regions of the atmosphere until all the parts beneath have been previously heated. The length of summer even in warm climates does not heat the air sufficiently to melt the snow which has accumulated during the winter on very high mountains, although they are almost constantly exposed to the heat of the sun’s rays, being too much elevated to be often enveloped in clouds. EMILY. These explanations are very satisfactory; but allow me to ask you one more question respecting the increased levity of heated liquids. You said that when water was heated over the fire, the particles at the bottom of the vessel ascended as soon as heated, in consequence of their specific levity: why does not the same effect continue when the water boils, and is converted into steam? and why does the steam rise from the surface, instead of the bottom of the liquid? MRS. B. The steam or vapour does ascend from the bottom, though it seems to arise from the surface of the liquid. We shall boil some water in this Florence flask, (PLATE IV. Fig. 1.) in order that you may be well acquainted with the process of ebullition;--you will then see, through the glass, that the vapour rises in bubbles from the bottom. We shall make it boil by means of a lamp, which is more convenient for this purpose than the chimney fire. [Illustration: Plate IV. Vol. I. p. 84. Fig. 1. Pneumatic Pump. Ether evaporated & water frozen in the air pump. A Phial of Ether. B Glass vessel containing water. C.C Thermometers one in the Ether, the other in the water. Fig. 2. Boiling water in a flask over a Patent lamp.] EMILY. I see some small bubbles ascend, and a great many appear all over the inside of the flask; does the water begin to boil already? MRS. B. No; what you now see are bubbles of air, which were either dissolved in the water, or attached to the inner surface of the flask, and which, being rarefied by the heat, ascend in the water. EMILY. But the heat which rarefies the air inclosed in the water must rarefy the water at the same time; therefore, if it could remain stationary in the water when both were cold, I do not understand why it should not when both are equally heated? MRS. B. Air being much less dense than water, is more easily rarefied; the former, therefore, expands to a great extent, whilst the latter continues to occupy nearly the same space; for water dilates comparatively but very little without changing its state and becoming vapour. Now that the water in the flask begins to boil, observe what large bubbles rise from the bottom of it. EMILY. I see them perfectly; but I wonder that they have sufficient power to force themselves through the water. CAROLINE.