That explanation will not do, for if the lead be the least in bulk, it offers also the least surface to the caloric, the quantity of heat therefore which can enter into it in the same space of time is proportionally smaller. EMILY. Why, then, do not the three bodies attain the temperature of the oven at the same time? MRS. B. It is supposed to be on account of the different capacity of these bodies for caloric. CAROLINE. What do you mean by the capacity of a body for caloric? MRS. B. I mean a certain disposition of bodies to require more or less caloric for raising their temperature to any degree of heat. Perhaps the fact may be thus explained: Let us put as many marbles into this glass as it will contain, and pour some sand over them--observe how the sand penetrates and lodges between them. We shall now fill another glass with pebbles of various forms--you see that they arrange themselves in a more compact manner than the marbles, which, being globular, can touch each other by a single point only. The pebbles, therefore, will not admit so much sand between them; and consequently one of these glasses will necessarily contain more sand than the other, though both of them be equally full. CAROLINE. This I understand perfectly. The marbles and the pebbles represent two bodies of different kinds, and the sand the caloric contained in them; it appears very plain, from this comparison, that one body may admit of more caloric between its particles than another. MRS. B. You can no longer be surprised, therefore, that bodies of a different capacity for caloric should require different proportions of that fluid to raise their temperatures equally. EMILY. But I do not conceive why the body that contains the most caloric should not be of the highest temperature; that is to say, feel hot in proportion to the quantity of caloric it contains? MRS. B. The caloric that is employed in filling the capacity of a body, is not free caloric; but is imprisoned as it were in the body, and is therefore imperceptible: for we can feel only the caloric which the body parts with, and not that which it retains. CAROLINE. It appears to me very extraordinary that heat should be confined in a body in such a manner as to be imperceptible. MRS. B. If you lay your hand on a hot body, you feel only the caloric which leaves it, and enters your hand; for it is impossible that you should be sensible of that which remains in the body. The thermometer, in the same manner, is affected only by the free caloric which a body transmits to it, and not at all by that which it does not part with. CAROLINE. I begin to understand it: but I confess that the idea of insensible heat is so new and strange to me, that it requires some time to render it familiar. MRS. B.