A paper was communicated to the Royal Society by the late Mr. Prinsep, the assay-master of the Mint at Bewares, " On the Measure ment of High Temperatures,' and published in their ' Transactions' for 1323. " The fusing-peints of pure metals," observes that gentle man, "are determinate and unchangeable; they also comprehend nearly the whole range of temperature ; the unoxidable or noble metals alone embrace a range from the low melting-point of silver to the high ignition of platina. There are, it is true, only three fixed points in this scale, but as many intermediate links may be made as are required, by alloying the three metals together in different propor tions. When such a aeriea 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 melting-points of silver and gold are comparatively near to each other, Mr. l'rinsep assumed only ten inter mediate 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, &c. per cent. of gold. From the melting-point of gold to that of platina, he assumed one hundred gradations of heat, which were the melting-points of pure gold and of gold alloyed with 1, 2, 3, &c., per cent.. of platina. Among the advantages of this mode of identifying temperatures are ; — the smallness of the requisite apparatus, nothing more being needed than a small cupel, containing in separate cells eight or ten pyrometic alloys, each of the size of a pin's head ; the indestructibility of the specimens, since those melted in one experiment need only to bo flattened under a hammer, when they will be again ready for use ; and the facility of notation, since two letters and the decimal of alloy will express the maximum heat : thus a .3 0 expresses the temperature of the fusing-point of silver when alloyed with gold in the proportion of 7 to 3; and o .23 expresses the fusing-point of gold when alloyed with platina in the proportion of 77 to 23.
Several suggestions have been made for employing the expansion of air, on the principle of the differential thermometer, as a measure of high temperatures. It is proposed that one-half of the instrument be composed of platinum, so as to fit it for exposure to a great heat, and the other part of glass. The suggestion, we believe, is originally due to Mr. Schmidt (Nicholson's 'Journal,' xi.); but was brought forward under another form by Mr. Nicholas Mill, in the Monthly Medico Chirurgical Review and Chemico-Philosophical Magazine,' vol. i., Lend., 1524; again by Dr. Ure, in his ' Dictionary of Chemistry ; ' and lastly by Mr. Prinsep. The instrument, we believe, has been constructed upon each of the plans proposed. That of Mr. Prinsep appears the most complete (see a drawing of the apparatus in full operation at page 87 of his Memoir above referred to), and was employed by him to connect the fusing-points of his alloys with the thermometric scale; but the principle upon which they all rest involves the assumption that the increase of temperature I. proportional to the expansion of
the air.
A valuable table of the expansions of different substances collected from various sources by Mr. Francis Badly. is given in the first volume of the' Transactions of the Astronomical Society,' p. 416.
The above admirable article well nigh exhausts the subject of the pyrometer. Since the date of Its publication in the ' Penny Cyclo pedia,' very little has been done towards perfecting this instrument. We may, however, briefly notice a few points of interest in connection therewith.
Mr. Prinaep'a method of alloy. was tested some years ago In the labo ratory of the Sevres porcelain manufactory, with a view to detcrmino the temperatures of the kiln. used in baking porcelain. It was found oecessary to expel silver from the brief list, on account of its absorbing oxygen when in a state of fusion and spitting it out again when cooling. Alloys of platinum and gold in various proportions were tried, but it was found that similar alloys did not always melt at the same moment, although apparently placed under precisely the same circumstances. It was also found that the same bead of alloy could not be used more than once, on account of some molecular change produced by the heat which led to an alteration in the fusing point. It was however doter mined that the highest temperature attained in the kiln at Sevres was represented by the alloy 1100, when a well fused button was formed ; whereas when 54 parts of platinum were used, only a soften ing of the alloy was perceived. M. Brongniart, Traitd des Arts Cdmmiques ' remarks that if it be possible to determine exactly and promptly the moment when complete fusion takes place in these alloys, they will afford comparable measures of high temperature.; but not a method fit to be employed habitually in industrial operations.
So long since as the year 1805, M. Brongniart constructed a pyro meter for measuring the comparatively moderate heat of the glass oven, or furnace at Sevne, by means of the dilatation of a bar of dyer. The readiness with which this metal can be obtained pure renders the results of two instruments comparable, although it is-not adapted to measure very high temperatures. The bar of silver is two decimetres in length, and it is placed among the articles which' are being fired, resting in a groove of hard porcelain, one end of which is turned up to servo as a fixed point to the further end of the bar, while the near end presses against a porcelain rod, which gives motion to a needle over a graduated arc. For further particulars we must refer to Brongniart's work, above cited.
In the Great Exhibition of 1851, Mr. Ericsson exhibited in the United States department an instrument intended to supply what we think is still a desideratum, namely, a good pyrometer for the useful arts. In this instrument temperatures were indicated by the tension of a permanent volume of air, or of nitrogen gas, which was measured by the reading of a column of mercury under a vacuum. For the par ticulars of this instrument we must refer to the Jury Report.