The great sensibility of air to the influence of heat, has induced the chemist to employ it in substitution of mercury and spirit of wine in the construction of thermometers. The instrument is exhibited in Fig. 1 on the following page, and consists of a tube with a bulb at the upper end containing air. The cup beneath is to contain a little coloured fluid, and if a portion of the air in the bulb be expelled by heat, it will, on cooling, re-ascend the tube, and indicate changes in the temperature of the surrounding bodies, by the rise or fall of the coloured fluid.
The double air, or differential thermometer of Leslie, consists of a small glass tube, bent into the shape of the letter U, (Fig. 2,) and terminating at each extremity in a small hollow ball. The tube contains sulphuric acid, tinged red with carmine, sufficient to fill the greatest part of it.
The glass balls are full of air, and both communicate with the intermediate tube.
To one of the legs of the tube is fixed a small ivory scale, divided into 100 degrees; and the sulphuric acid is so disposed, that in the graduated leg its upper surface stands opposite to that part of the scale marked 0.
The glass ball attached to the graduated leg of the instrument is, by way of distinction, termed the focal ball. Supposing this instrument to be brought into a warm room, the heat will act upon both balls, and, expanding the included air in each equally, the liquid will remain stationary. But suppose the focal ball to be exposed to heat, while the other ball is not; the air included in the focal ball will expand, while that in the other remains unaffected. The air in the focal ball will therefore press more upon the liquid in the tube, which will, of course, advance towards the cold ball, and, therefore, the liquid will rise in the tube above 0, and the rise will be proportional to the degree of heat applied to the focal ball. This thermometer is, therefore, peculiarly adapted for ascertaining the degree of heat accumulated in a particular point, while the surrounding atmosphere is but little affected, as happens in the focus of a reflecting mirror. No change in the temperature of the room in which the instrument is kept is indicated by it, whilst the slightest alteration in the spot where the focal ball is placed is immediately shown by it.
This instrument, as arranged by Mr. Ronalds, is made more substantially than the chemical thermometer just described, and the bulb is larger in proportion to the other parts. It consists of a stand supporting the tube and bulb, the latter being furnished with two wires and rings, as shown in the cut. The tube contains a coloured fluid, and is provided with a graduated scale. If it be required to assign the amount of expansion by the electric shock when passed through the common air, a communication is made between the electrical machine and the rings by a chain, and the expansion is shown by the depression of the coloured fluid in the tube.
When thermometers are contrived to measure very great degrees of heat by the expansions they produce in substances, or, on the contrary, the expansion corresponding to different temperatures, they are characterized by the name of Pyrometers; for descriptions of several kinds of which see Pyrometer.