A pyrometer was constructed by Achard similar in form and principle to the common thermometer, but intended to indicate much higher degrees of heat. It consisted of a bulb and graduated tube of semitransparent porcelain highly baked, and containing a very fusible alloy, composed of bismuth, lead, and tin, which became liquid at about 212', and indicated higher temperatures by its expansion, which was visible through the semi-transparent tube.
Dugong and Petit employed a very direct mode of measuring the absolute, not linear, expansions of various substances. By observing the difference of altitude at which mercury of different temperatures 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 expansion of the glass. A cylinder of the metal whose expansion was sought was then placed within a glees tube, closed at one extremity and terminating at the other in a capillary open ing, 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 expansion of the metal.
Sir David Brewster has proposed to measure expansions by the number and intensity of the polarized tints produced by the inflexion of a plate of glees against which the expanding substance is made to press. (Brewster 's Cycloredia: artich. ' Pyrometer' and 'Optics.') Guyton's pyrometer, which was exhibited before the National Insti tute in 1803, and described in the ' Annales do Chimie; xlvi., p. 270, and in Nicholson s' Philosophical Journal,' vi., p. 89, consisted of a bar of platinum nearly 2 inches in length, placed in a groove of por celain. One extremity of the bar rested against the solid end of the groove, while the other pressed upon the short arm of a lever, the longer arm of which carried a vernier over a graduated circular arc. The whole was constructed of platinum, and a spring was made to press upon the vernier to prevent its displacement while in the act of withdrawing the instrument from the furnace. The indications of the vernier at the commencement and termination • of the experiment were the data from which the expansion was subsequently coin puted. " The defect of this instrument," observes Mr. Daniell, " arose
from the nature of platinum, which at a red heat becomes soft and ductile, so that the lever would be liable to bend, and thereby frustrate the experiment ; and this is supposed to have been the reason why the inventor never extended his experiments to tempera tures higher than that of the melting point of antimony." As early as 1821, Professor Daniell bad invented an instrument which, be states, "afforded correct determinations connected in an unexceptionable manner with the scale of the mercurial thermometer ;" but it was only suited to the experimental furnace of the chemist, so that, he continues, "the great still remained of a pyro meter, which might be universally applied to the higher degrees of heat, as the thermometer had long been to the lower, and which, in addition to its Use in delicate researches, might effect for the potter, the smelter, the enameller, and others, in the routine of their business, what the latter daily performs for the brewer, The distiller, the sugar refiner, and the chemist." The annexed diagram represents the second pyrometer invented by Mr. Daniell, for which the Rumford medal was awarded to him by the Royal Society. An account of it is given in the ' Phil. Trans.' for 1830-31.
of which, d, projects so that a point, near to it may be immediately opposite to the cavity in the black-lead bar when the latter is adjusted to the frame. About c as a centre, turns an arm dnn slightly bent at n, carrying at its extremity a graduated circular arc ee. The radius of this arc is five inches, and its moveable centre n is distant from the fixed centre c exactly half an inch. About n turns a straight and lighter arm, hg, five inches and a half in length, the distance from h to n being half an inch. The extremity, g, of this arm carries a vernier, by which the divisions of the graduated arc are subdivided into minutes, and also an eye-glass, i, to assist the reading. The other extremity terminates in a steel point, h, or, as the instrument is now constructed, a knife-edge, which, when the register is adjusted to the frame, is inserted in a small cavity, t, formed for its reception at the extremity of the porcelain index. A small steel spring let into the larger arm at as is made to press upon the lighter arm, whereby the latter has a constant tendency to move towards the commencement of the graduation.