“Ah! my dear children,” murmured the vicar, with an air of pensive gravity, “‘_Tenues secessit in auras_,’ as the poet has it. Even thus is it with all the full-blown bubbles of our fancy, raised by the breath of hope; the moment they appear most vivid and promising to our imagination, they vanish ‘into air, into thin air,’ like the gaudy and unsubstantial soap-bubble you have just witnessed: but proceed to blow another.” “There is one!” exclaimed Louisa;--“see, it is of an oblong shape, like an egg!--there it goes!--but I declare it is now perfectly round!!--what can be the reason of its changing its figure?” “I am glad you have asked that question, because my answer will serve to illustrate an important property of air, and which, indeed, is common to all fluids. While the upper part of the bubble was attached to the bowl of the pipe, its gravity being resisted, drew it into an elliptical form; but the instant it was detached, the contained air pressed equally in all directions, and the bubble, in consequence, became a perfect sphere.”[38] “I do not exactly understand what you mean ‘by pressing equally in all directions.’” “The expression is surely sufficiently intelligible. Did you not learn in our conversation of yesterday, that air has weight, and exerts a pressure as much upwards as downwards and laterally? Were this not the case, how could the air in the interior of our bodies counteract the pressure of the atmosphere? The form of the bubble proves the same fact in a different way; for, had the air in its cavity pressed more in any one direction than in another, the bubble could not have been round, or, to speak more correctly, a sphere.” “What are you musing about?” cried the vicar, who had observed the attention of the boy riveted upon the bowl of the tobacco-pipe: “I am sure, from your countenance, that some circumstance is puzzling you.” “You are right, my dear sir; I was just then thinking how it can possibly happen, that the bubble should not have a hole in its upper part; for, while I am blowing it up, there must, of course, be a communication between my mouth and its interior, or else how could the air pass into it?” “True,” said his father; “but the act of throwing it off from the bowl of the pipe will unite this breach; for there exists a strong cohesive attraction between the attenuated particles of the lather; you will, therefore, perceive that, on this account, the bubble will be more readily and securely separated by a lateral than a perpendicular motion of the pipe.” “I wish,” said Tom, “that I could discover some method of preventing their bursting so soon, for there is scarcely time to examine them before they vanish. What can be the cause of their short duration?”