That seems to me the most reasonable division, and I cannot guess why the freezing point is called 32°, or what advantage is derived from it. MRS. B. There really is no advantage in it; and it originated in a mistaken opinion of the instrument-maker, Fahrenheit, who first constructed these thermometers. He mixed snow and salt together, and produced by that means a degree of cold which he concluded was the greatest possible, and therefore made his scale begin from that point. Between that and boiling water he made 212 degrees, and the freezing point was found to be at 32°. EMILY. Are spirit of wine, and mercury, the only liquids used in the construction of thermometers? MRS. B. I believe they are the only liquids now in use, though some others, such as linseed oil, would make tolerable thermometers: but for experiments in which a very quick and delicate test of the changes of temperature is required, air is the fluid sometimes employed. The bulb of air thermometers is filled with common air only, and its expansion and contraction are indicated by a small drop of any coloured liquor, which is suspended within the tube, and moves up and down, according as the air within the bulb and tube expands or contracts. But in general, air thermometers, however sensible to changes of temperature, are by no means accurate in their indications. I can, however, show you an air thermometer of a very peculiar construction, which is remarkably well adapted for some chemical experiments, as it is equally delicate and accurate in its indications. CAROLINE. It looks like a double thermometer reversed, the tube being bent, and having a large bulb at each of its extremities. (PLATE II. Fig. 2.) EMILY. Why do you call it an air thermometer; the tube contains a coloured liquid? MRS. B. But observe that the bulbs are filled with air, the liquid being confined to a portion of the tube, and answering only the purpose of showing, by its motion in the tube, the comparative dilatation or contraction of the air within the bulbs, which afford an indication of their relative temperature. Thus if you heat the bulb A, by the warmth of your hand, the fluid will rise towards the bulb B, and the contrary will happen if you reverse the experiment. But if, on the contrary, both tubes are of the same temperature, as is the case now, the coloured liquid, suffering an equal pressure on each side, no change of level takes place. CAROLINE. This instrument appears, indeed, uncommonly delicate. The fluid is set in motion by the mere approach of my hand. MRS. B. You must observe, however, that this thermometer cannot indicate the temperature of any particular body, or of the medium in which it is immersed; it serves only to point out the _difference_ of temperature between the two bulbs, when placed under different circumstances. For this reason it has been called _differential_ thermometer. You will see by-and-bye to what particular purposes this instrument applies. EMILY.