_Mrs. B._ Yes; fluids, generally speaking, are bodies of less density than solids. From the slight cohesion, of the particles of fluids, and the facility with which they slide over each other, it is inferred, that they have but a slight attraction for each other, and that this attraction is equal, in every position of their particles, and therefore produces no resistance to a perfect freedom of motion among themselves. _Caroline._ Pray what is the distinction between a fluid and a liquid? _Mrs. B._ Liquids comprehend only one class of fluids. There is another class, distinguished by the name of elastic fluids, or gases, which comprehends the air of the atmosphere, and all the various kinds of air with which you will become acquainted, when you study chemistry. Their mechanical properties we shall examine hereafter, and confine our attention this morning, to those of liquids, or non-elastic fluids. Water, and liquids in general, are scarcely susceptible of being compressed, or squeezed into a smaller space, than that which they naturally occupy. Such, however, is the extreme minuteness of their particles, that by strong compression, they sometimes force their way through the pores of the substance which confines them. This was shown by a celebrated experiment, made at Florence many years ago. A hollow globe of gold was filled with water, and on its being submitted to great pressure, the water was seen to exude through the pores of the gold, which it covered with a fine dew. Many philosophers, however, think that this experiment is too much relied upon, as it does not appear that it has ever been repeated; it is possible, therefore, that there may have been some source of error, which was not discovered by the experimenters. Fluids, appear to gravitate more freely, than solid bodies; for the strong cohesive attraction of the particles of the latter, in some measure counteracts the effect of gravity. In this table, for instance, the cohesion of the particles of wood, enables four slender legs to support a considerable weight. Were the cohesion destroyed, or, in other words, the wood converted into a fluid, no support could be afforded by the legs, for the particles no longer cohering together, each would press separately and independently, and would be brought to a level with the surface of the earth. _Emily._ This want of cohesion is then the reason why fluids can never be formed into figures, or maintained in heaps; for though it is true the wind raises water into waves, they are immediately afterwards destroyed by gravity, and water always finds its level. _Mrs. B._ Do you understand what is meant by the level, or equilibrium of fluids? _Emily._ I believe I do, though I feel rather at a loss to explain it. Is not a fluid level when its surface is smooth and flat, as is the case with all fluids, when in a state of rest?