This is the case whenever a body is fused or melted; but if you add caloric to a liquid, can you tell me what is the consequence? CAROLINE. The caloric forces itself in greater abundance between the particles of the fluid, and drives them to such a distance from each other, that their attraction of aggregation is wholly destroyed: the liquid is then transformed into vapour. MRS. B. Very well; and this is precisely the case with boiling water, when it is converted into steam or vapour, and with all bodies that assume an aëriform state. EMILY. I do not well understand the word aëriform? MRS. B. Any elastic fluid whatever, whether it be merely vapour or permanent air, is called aëriform. But each of these various states, solid, liquid, and aëriform, admit of many different degrees of density, or consistence, still arising (chiefly at least) from the different quantities of caloric the bodies contain. Solids are of various degrees of density, from that of gold, to that of a thin jelly. Liquids, from the consistence of melted glue, or melted metals, to that of ether, which is the lightest of all liquids. The different elastic fluids (with which you are not yet acquainted) are susceptible of no less variety in their degrees of density. EMILY. But does not every individual body also admit of different degrees of consistence, without changing its state? MRS. B. Undoubtedly; and this I can immediately show you by a very simple experiment. This piece of iron now exactly fits the frame, or ring, made to receive it; but if heated red hot, it will no longer do so, for its dimensions will be so much increased by the caloric that has penetrated into it, that it will be much too large for the frame. The iron is now red hot; by applying it to the frame, we shall see how much it is dilated. EMILY. Considerably so indeed! I knew that heat had this effect on bodies, but I did not imagine that it could be made so conspicuous. MRS. B. By means of this instrument (called a Pyrometer) we may estimate, in the most exact manner, the various dilatations of any solid body by heat. The body we are now going to submit to trial is this small iron bar; I fix it to this apparatus, (PLATE I. Fig. 1.) and then heat it by lighting the three lamps beneath it: when the bar expands, it increases in length as well as thickness; and, as one end communicates with this wheel-work, whilst the other end is fixed and immoveable, no sooner does it begin to dilate than it presses against the wheel-work, and sets in motion the index, which points out the degrees of dilatation on the dial-plate. [Illustration: Plate I. Vol. I. p. 38. Fig. 1. Pyrometer. A.A Bar of Metal. 1.2.3 Lamps burning. B.B Wheel work. C Index. Fig. 2 A.A Glass tubes with bulbs. B.B Glasses of water in which they are immersed.] EMILY.