When the extremity a 6 of the instrument is put into the fire, the index 1 at first moves backwards 10 or 20 degrees, in consequence of the expansion of the black lead tube, before the heat has reached the platinum bar; but, in a short time, it moves rapidly forward, and indi cates on the scale the degree of expansion experienced by the bar. By various interesting experiments, for which we cannot find room, Mr. Daniell obtained the following results, which we give,, along with those of Mr. Wedge wood.
The differences between these results, are too striking to escape notice, and show how very imperfect is our knowledge of the effects of high temperatures. See the Quarterly Journal of Science, Literature, and the Arcs, vol. xi. p. 309-320. This instrument is made by Mr. Newman, Lisle-street.
4. Description of Dr. Urc's Pyrometer.
This instrument, which Dr. Ure does not mention as having been actually constructed, is described in his Dictonarzt of Chemistry, published in 1821. Art. 1'v :in:Jr:ma.
" Since dry air," says Dr. Ure, " augments 3-8ths for 180°, and, since its progressive rate of expansion is pro bably uniform by uniform increments of heat, a pyrometer might easily be constructed on this principle. Form a
bulb and tube of platinum, of exactly the same size as a 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 recurved bulb, like that of the Italian barometer. Graduate the glass tube into a series of spaces equivalent to 3-8ths of the total volume of the capacity of the platina bulb, with 3-4ths of its stem. The other parts may be supposed to be little influenced by the sources of heat. On plunging the bulb and 2-3ds of the stem into a furnace, the depression of the mercury will indicate the degree of heat. As the incre ment of the column will be very inconsiderable, it will be scarcely worth while to introduce any correction, for the change in the initial volume by barometric variation. Care must be taken to let no mercury into the platina bulb."