_Emily._ The one endeavouring to rend a body to pieces, the other to keep its parts firmly united. _Mrs. B._ And it is this struggle between the contending forces of heat and attraction, which prevents the extreme degree of density which would result from the sole influence of the attraction of cohesion. _Emily._ The more a body is heated then, the more its particles will be separated. _Mrs. B._ Certainly: we find that bodies not only swell or dilate, but lose their cohesion, by heat: this effect is very sensible in butter, for instance, which expands by the application of heat, till at length the attraction of cohesion is so far diminished that the particles separate, and the butter becomes liquid. A similar effect is produced by heat on metals, and all bodies susceptible of being melted. Liquids, you know, are made to boil by the application of heat; the attraction of cohesion then yields entirely to the repulsive power; the particles are totally separated and converted into steam or vapour. But the agency of heat is in no body more sensible than in air, which dilates and contracts by its increase or diminution in a very remarkable degree. _Emily._ The effects of heat appear to be one of the most interesting parts of natural philosophy. _Mrs. B._ That is true; but heat is so intimately connected with chemistry, that you must allow me to defer the investigation of its properties till you become acquainted with that science. To return to its antagonist, the attraction of cohesion; it is this power which restores to vapour its liquid form, which unites it into drops when it falls to earth in a shower of rain, which gathers the dew into brilliant gems on the blades of grass. _Emily._ And I have often observed that after a shower, the water collects into large drops on the leaves of plants; but I cannot say that I perfectly understand how the attraction of cohesion produces this effect. _Mrs. B._ Rain, when it first leaves the clouds, is not in the form of drops, but in that of mist or vapour, which is composed of very small watery particles; these in their descent mutually attract each other, and those that are sufficiently near in consequence unite and form a drop, and thus the mist is transformed into a shower. The dew also was originally in a state of vapour, but is, by the mutual attraction of the particles, formed into small globules on the blades of grass: in a similar manner the rain upon the leaf collects into large drops, which when they become too heavy for the leaf to support, fall to the ground. _Emily._ All this is wonderfully curious! I am almost bewildered with surprise and admiration at the number of new ideas I have already acquired.