PYli 



PYR 



The utility of this instrument, it was 

 obvious, would be much increased by 

 connecting it with the mercurial ther- 

 mometer, and by ascertaining- the pro- 

 portion between the degrees of each ; 

 and this was don'.- by Mr. Wedgwood. 

 The scale of his pyrometer commences 

 at red-heat fully visible in day-light. The 

 mercurial thermometer cannot easily 

 measure any temperature above 500 or 

 S5'J ; aiul hence, between the termina- 

 tion of the scale in the one, and its com- 

 mencement in the other, there is a range 

 of temperature requiring to be measur- 

 ed. This Mr Wedgwood did, by the' ex- 

 pansions of a square piece of silver, mea- 

 sured in a gauge of earthen-ware, con- 

 structed in tiu j same way as his pyrome- 

 ter; and by the same method, he found 

 out the proportion between each degree 

 of his scale, and that of any of the usual 

 thermometrieal scales. Each degree of 

 his pyrometer he found to be equal to 

 13u of Fahrenheit. The commencement 

 of his scale, or the point marked 0, cor- 

 responds with 1077-2 of Fahrenheit's 

 scale. From these data, it is easy to re- 

 duce either to the other, through their 

 whole range. The scale of Wedgwood 

 includes an extent of temperature equal 

 to about 32,000 of Fahrenheit, or 54 

 times as much as that between the 

 freezing and boiling points of mercury. 

 Its commencement, as has been stated, is 

 at 1077} of Fahrenheit, or red-heat fully 

 visible in flay-light; its extremity is 240 ; 

 but the highest heat that he measured 

 with it is 160, or 21,877 of Fahrenheit ; 

 being the temperature of a small air- 

 furnace, and 30 degrees of his scale 

 above the point at which cast-iron melts. 



Guyton has proposed a pyrometer for 

 measuring high temperatures, in which 

 platina, a metal not fusible even at very 

 intense heats, is employed as the mea- 

 sure of expansion. A rod or plate of 

 this metal is placed horizontally in a 

 groove framed in a mass of hardened 

 white clay ; one extremity of the rod is 

 supported on the mass which terminates 

 the groove ; the other presses against a 

 bended lever of platina, the longest arm 



of which forms an index to a graduated 

 arc. The expansion, which the rod of 

 metal suffers from exposure to heat, is 

 indicated by the change of position in 

 this index. The mass of clay, being high- 

 ly baked, will not introduce any import- 

 ant error from its contraction ; and the 

 expansion, which it may suffer during 

 the exposure to heat, will affect only the 

 small distance between the axis of mo- 

 tion of the index, and the point of con- 

 tact of the plate, so as rather to diminish 

 the effect than to increase it. Platina, hav- 

 ing the important advantage of not melt- 

 ing by any heat we have to measure, and 

 of not suffering any chemical change 

 from it, is well adapted to the construc- 

 tion of a pyrometer. 



Besides these, various metallic pyro- 

 meters have been invented, capable of 

 measuring low temperatures, by the ex- 

 pansion being multiplied by the aid of 

 wheels, levers, or other mechanical con- 

 trivances, or being magnified by micro- 

 scopes. Such are the pyrometers of 

 Muschenbroeck; that described by Fer- 

 guson ; one invented by Mr. Ellicot, with 

 which he measured the expansions of 

 various metals ; one by Mr. Smeaton, and 

 applied to the same purpose ; Mr. Rams- 

 den's, superior to the preceding ones in 

 delicacy and accuracy ; Mr. Crichton's, 

 in which advantage is taken of the dif- 

 ference of expansion between a rod of 

 zinc and a rod of iron, to give a curva- 

 ture to the bar composed of the united 

 rods, proportioned to the temperature to 

 which they are raised ; by which bend- 

 ing motion is given to an index, that, at 

 its other extrimity, where the scale is 

 marked, describes a considerable space ; 

 and, lastly, one by Ilegnier, on a princi- 

 ple somewhat similar, of which a report 

 is presented to the French National In- 

 stitute. The strict accuracy of these in- 

 struments may, from the nature of their 

 construction, be regarded as doubtful. It 

 has been found, by Ellicot's pyrometer, 

 that the expansion of bars of different, 

 metals, by the same degree of heat, is as 

 follows : 



Gold. 

 73 



Silver. 

 1J3 



Brass. 



95 



Copper. 



89 



Iron. 

 60 



Steel. 



56 



Lead. 



149 



PYROMUCOUS add. When sugar and 

 other sweet tasted substances are dis- 

 tilled, among other products there is al- 

 ways a notable quantity of an acid liquid. 

 This acid, when rectified, obtained the 

 names of syrupus acid, and afterwards 



pyromucous acid. It is now known from 

 the recent experiments of Fourcroy and 

 Vauquelin, that this acid is nothing else 

 than the acetic, holding in solution a 

 portion of empyreumatic oil. 

 PYROPE, in mineralogy, a species of 



