PTBOKBTKB. 



I'YKOSIS. 



the doubt/ul asmimption u introduced Ut equal increment* of length 

 an the eflecU of equal increiueuU of temperature, and throve, having 

 determined the expansion between any two known poinU on the 

 thrnnometric acale, lay the temperature! of melting ioe and boiling 

 mercury, a mere proportion will of coune give the temperature at 

 which any othvr observed expansion took place. 



A paper was communicated to the Koyol Society by the late Mr. 

 Prinaep, the assay-master of the Hint at Benares, " On the Measure- 

 ment of High Temperatures,' and published in their ' Transactions ' 

 for 1S28. " The fuaing-puintH 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 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 melting-points of silver and gold are 

 comparatively near to each other, Mr. Prinaep 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, tc., 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 ton 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 be Battened 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 8 '3 o expresses the temperature of the fusing-point of silver 

 when alloyed with gold in the proportion of 7 to 3; and o '23 i- 

 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, ag 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 Keview and Chemico- Philosophical Magazine,' vol. i., Loud., 

 1 - . I ; again by Dr. Ure, in his ' Dictionary of Chemistry ; ' and lastly 

 by Mr. Prinwp. The instrument, we believe, has been constructed 

 upon each of the plans proposed. That of Mr. Prinsep appears the 

 mc*t complete (see a drawing of the apparatus in full operation at 

 page 87 of hi* Memoir above referred to), and was employed by him 

 to connect the f uaing-poiuts of his alloys with the thermometric scale ; 

 but the principle upon which they all rest involves the assumption 

 that the increase of temperature U proportional to the expansion of 

 the air. 



A valuable table of the expansions of different substances collected 

 from various sources by Mr. Francis Baily. is given in the first volume 

 of the ' Transactions of the Astronomical Society, 1 p. 416. 



The above admirable article well nigh exhausts the subject of the 

 pyrometer. Since the date of it* 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. Prinsep's method of alloys wan tested some yean ago in the labo- 

 ratory of the Sevres porcelain manufactory, with a view to determine 



the temperatures of the kilns used in baking porcelain. It was found 

 naoMstry to exjiel 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 Minilar 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 fuaing point. It was however deter- 

 mined that the highest temperature attained in the kiln at Sevres was 



represented by the alloy ^ ^ j-100, 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, ' Traito dee Arts 

 Ci'ramiques, 1 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 temperatures ; but iiot 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 

 M-.rn. or niuIHe furnace at Sevres, by means of the dilatation of a bar 

 of silver. The readiness with which this metal can be obtained pure 

 renders the results of two instruments comparable, although it is nut 

 adapted to measure very high temiwratures. 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 serve 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 

 M. lirongniart'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 Keport. 



In the ' Transactions of the Itoyal Society of Edinburgh,' vol. xiii. 

 will be found an account of an ingenious pyrometer by Mr. Adie, for 

 measuring the expansion of different kinds of stone. We have not 

 space to describe the instrument, but we may mention one curious 

 effect, namely, that there was an increase in length iu white Sicilian 

 marble, every time its temperature was raised. 



We may also refer to M. Pouillet's method of measuring directly 

 the linear expansion of solids applicable to very high temperatures. 

 It consists of a solid plate of metal on which is placed a radius turning 

 on a centre, and traversing a graduated arc, the divisions of which are 

 read off by a microscope. The radius carries a telescope of short 

 focus, fixed at right angles to its direction, while a similar telescope is 

 fixed to the plate itself, allowing the radius to traverse under it. The 

 bar which is the subject of experiment is placed in a copper trough, 

 furnished with parallel plates of glass, through which its ends can bo 

 seen. Now if one extremity of the bar be kept opposite the fixed 

 telescope, while the moveable telescope is directed to the other 

 extremity at the commencement of tne observation, any expansion 

 of the bar caused by raising its temperature may be estimated by the 

 arc through which the radius must be turned in order to bring the 

 moveable telescope to bear on the other extremity in its new position, 

 the distance of the radius from the bar beiug accurately known. For 

 very high temperatures the bar may be placed in a furnace, and when 

 raised to the required temperature, apertures may be opened in the 

 furnace walls, so as to give a view of the ends of the bar, and allow its 

 expansion to be measured as before. By means of this apparatus an 

 expansion in the bar of jj.uth millimetres or about j^Jojjth inch can be 

 appreciated. 



We may also refer to an ingenious contrivance adopted by General 

 Colby in measuring the base line of Loch Foyle, for the Ordnance 

 Survey of Ireland. It consisted of a compound bar of iron and brass 

 so arranged that their different powers of expansion and contraction 

 should preserve exactly the same distance between two points at the 

 extremities of the bars, instead of allowing for the change in length 

 according to the observed temperature. The two bars, one of iron the 

 other of brass, each 10 feet long, were placed parallel to each other 

 and riveted together in the centre. It was ascertained by experiment 

 that the iron and brass bars expanded and contracted by varying 

 temperatures in the proportion of three to five. Across each extremity 

 therefore of these combined bars was fixed a tongue of iron, with a 

 minute dot of platinum so placed, that under every degree of expan- 

 sion and contraction of the rods the dots at each end always remain at 

 the constant distance of 10 feet. 



PYROMOKINTANNIC ACID. [OxYPHENtc ACID.] 



PYROMUCIC ACID. [Mucio ACID.] 



I'Y in M'KCTIC ACID. [Puerto ACID.] 



PYRol'HORUS. [LEAD: Tartrate o/.J 



P VI to 1 ' 1 1 OSPUOR1C ACID. [PHOSPHORUS.] 



I'YKOPIN. A red albuminous substance found in elephants' teeth 



PYROSIS, or WATER BRASH, is a disorder of the stomach', 

 attended with the vomiting or eructation of a thin watery liquid! 



