Datong and Petits Pyrometer was constructed for measuring the cubical expansions of various substances. By observing the difference of altitude at which mercury of different tempe ratures stood in the two arms of an inverted glass siphon, they determined the absolute expansion of the mercury; and by comparing this with the apparent expansion of mercury in a glass tube, they deduced the absolute ex pansion of the glass. A cylinder of the metal whose expansion was sought was then placed within a glass tube, closed at one extremity and terminating at the other in a capillary opening, and the rest of the tube occupied with mercury. Upon the whole being heated a portion of the mercury was expelled equal to the excess of the absolute expansions of the mercury and metal above that of the glass ; and as the expansions of the mercury and glass were previously known, the weight of the expelled mercury determined the ex pansion of the metal.
Prinsep's Pyrometer.—The unoxidable me tals (gold, silver, and platina) present marked temperatures, from the low melting point of silver to the high ignition of platina; and many intermediate links may be made by alloying the three metals together in different proportions. When such a series has been once prepared, the heat of any furnace may be expressed by the alloy of least fusibility which it is capable of melting. As the melt ing points of silver and gold are comparatively near to each other, Mr. Prinsep assumed only ten intermediate gradations of heat, the lowest of which corresponded to the fusing point of pure silver, and the others to the fusing points of silver alloyed with 10, 20, 30, per cent.
of gold. From the melting point of gold to that of platina, he assumed one hundred gra dations of heat, which were the melting points of pure gold and of gold alloyed with 1, 2, 3, &e., per cent. of platina. Among the advan tages of this mode of identifying temperatures are ;—the smallness of the requisite apparatus, nothing more being needed than a small eupol, containing in separate cells eight or ten PYremetrie alloys, each of the size of a pin's head ; the indestructibility of the specimens, since those melted in one experiment need only to be flattened under a hammer, when they will bo again ready for use; and the faci lity of notation, since two letters and the decimal of alloy will express the maximum heat.
Daniell's Pyrometer is a complex apparatus depending on the small expansibility of black lead compared with that of metals. A bar of black lead is bored nearly from end to end like a gun ; and into this bore is placed a rod of the metal to be experimented on. When both are exposed to a great heat, the metal expands more than the black lead, and causes a small cylinder of porcelain to protrude from the top of the tube or bore. This cylinder is connected with an index or hand which moves round a graduated arc ; and the degrees on this are are so calculated as to indicate the temperature to which the metal has been ex posed.