BUBBLE, in philosophy, small drops or vesicles of any fluid filled with air; and formed either on its surface by an addition of more of the fluid, as in raining, &c.; or in its substance, by an intestine motion of its component particles. Bubbles are dilatable or compressible, i. e. they take up more or less room as the included air is more or less heated, or more or less pressed from without; and are round, because the included air acts equally from within all around.