All solid bodies, and most liquids, are susceptible of ignition, or, in other words, of being heated so as to become luminous; and it is remarkable that this takes place pretty nearly at the same temperature in all bodies, that is, at about 800 degrees of Fahrenheit’s scale. EMILY. But how can liquids attain so high a temperature, without being converted into vapour? MRS. B. By means of confinement and pressure. Water confined in a strong iron vessel (called Papin’s digester) can have its temperature raised to upwards of 400 degrees. Sir James Hall has made some very curious experiments on the effects of heat assisted by pressure; by means of strong gun-barrels, he succeeded in melting a variety of substances which were considered as infusible: and it is not unlikely that, by similar methods, water itself might be heated to redness. EMILY. I am surprised at that: for I thought that the force of steam was such as to destroy almost all mechanical resistance. MRS. B. The expansive force of steam is prodigious; but in order to subject water to such high temperatures, it is prevented by confinement from being converted into steam, and the expansion of heated water is comparatively trifling. --But we have dwelt so long on the subject of free caloric, that we must reserve the other modifications of that agent to our next meeting, when we shall endeavour to proceed more rapidly. CONVERSATION IV. ON COMBINED CALORIC, COMPREHENDING SPECIFIC AND LATENT HEAT. MRS. B. We are now to examine the other modifications of caloric. CAROLINE. I am very curious to know of what nature they can be; for I have no notion of any kind of heat that is not perceptible to the senses. MRS. B. In order to enable you to understand them, it will be necessary to enter into some previous explanations. It has been discovered by modern chemists, that bodies of a different nature, heated to the same temperature, do not contain the same quantity of caloric. CAROLINE. How could that be ascertained? Have you not told us that it is impossible to discover the absolute quantity of caloric which bodies contain? MRS. B. True; but at the same time I said that we were enabled to form a judgment of the proportions which bodies bore to each other in this respect. Thus it is found that, in order to raise the temperature of different bodies the same number of degrees, different quantities of caloric are required for each of them. If, for instance, you place a pound of lead, a pound of chalk, and a pound of milk, in a hot oven, they will be gradually heated to the temperature of the oven; but the lead will attain it first, the chalk next, and the milk last. CAROLINE. That is a natural consequence of their different bulks; the lead being the smallest body, will be heated soonest, and the milk, which is the largest, will require the longest time. MRS. B.