CAROLINE.
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.