" Since dry air," observes Dr. Ure, " augments in volume three-eighths for 180 degrees, and since its progressive rate of expansion is probably uniform by uniform degrees of heat, a pyrometer might easily be constructed on this prin ciple :—form a bulb and tube of platinum, of exactly the same form as the thermometer, and connect with the extremity of the stem, at right angles, a glass tube of uniform calibre, filled with mercury, and terminating below in a recurred bulb, like that of the Italian barometer. Graduate the glass tube into a series of spaces equivalent to three-eighths of the total volume of the capa city of the platina bulb, with three fourths of its stem. The other fourth may be supposed to be little influenced by the source of heat. On plunging the bulb and two-thirds of the stem into a furnace, the depression of the mercury will indicate the degree of heat. As the movement of the column will be very considerable, it will be scarcely worth while to introduce any correction for the change of the initial volume by barometric variation ; or the instrument might be made with the recurved bulb sealed, as in Professor Leashes differential ther mometers. The glass tube may be joined by fusion to the platina tube. Care 'must be taken to let no mercury enter the platinum bulb. Should there be a mechanical difficulty in making a bulb of this metal, then a hollow cylinder, of half an inch diameter, with a platinum stem, like that of a tobacco pipe, screwed into it, will suit equally well.
A very convenient pyrometer for ascertaining the relative expansibility of the various metals that can be drawn into wire, was contrived by Mr. Gurney, which he employed in his chemical lectures. It is represented in the subjoined cut. a represents a wire of the metal to be examined, attached at the lower end to a peg fixed upon a piece of board; on this board is also a series of little pulley-wheels, turning freely on their axes, and around the peripheries of these wheels the wire is carried to the uppermost, whence it is conducted out of the vessel, and over a small central wheel b, of a circular graduated scale, and with a weight c tied to this end of the wire, which keeps it in a state of tension.
Thus prepared, the apparatus is immersed in a vessel:of water, or other fluid, heated to the desired temperature within their capability, which is determined by a thermometer placed therein. The exvinsion which then takes place is accurately denoted by the index e pointing it out upon the graduated circum ference, the index turning round as the elongation takes place. Upon abstract ing the heat, the wire contracts and draws back the wheel and index to its previous position. An instrument of this kind, carefully constructed, and with a smaller central wheel b, would, without doubt, show the expansibility of the ductile metals with great exactness, and very satisfactorily, as the wire may be of great length, he wound round a large number of pulleys, so as to cause an obvious elongation of an inch or more.
PYItOI'HORUS. An artificial product, which takes fire upon exposure to the air, and hence called air-tinder. It is prepared from alum by the addition of various inflammable substances. The simplest mode of preparing it is to mix three parts of alum with one of wheat flour, and calcine them in a phial or mattress. until the blue flame disappears; then keep it in the same phial till cold, well stopped with a good cork. In this powder be exposed to the atmo sphere, the sulphuret attracts moisture from the air, and generates taifficient heat to.kindle the carbonaceous matter mingled with it.