i :,> 



FTP. MIS TRY. 



iv 



Api 1 



n- 



hnt' thtr. 

 MBCtcr. 



Letlies 

 ttiflrremial 

 llicrmoii'.c- 

 icr. 



1'L.Tl 



CXLI. 



Mortimer's 



r 



PtATt 



1 ,;. .'. 



pptioil. I'.ir nn account nstrticlion of differ- 



, 



:s the one com- 



monlv g poir.t on it is 



m:irk'.d 32, t! :s marked 



so that in this thcrtmmeu r tl-.f interval between fr- 



anJ boiling w-tt r U diki.-d into 180 degrees. The 



pract: .-t ought to be provided with a variety of 



ihcm.o:.-. tcrs ; tome graduated as high as the boiling 



point of mercury, others as low it the freezing point of 



that metal. He should be even provided with a spirit of 



wine tlifrmomrter, graduated as low as 100'. If he is 



i .-J in maki. ' cxpi rimtnts on the capacity of bodies 



-it to nave thermometers so delicate as to 



point out very minute changes of temperature. Dr 



Crawford had a tin rmometer in which every one of the 



usual degrees was divided into 50 parts, so that he could 



measure, with tolerable precision, the hundredtii part of a 



r.heii's thermometer. It was with this 



'.L-licate instrument that he determined the specific 



heat of the gaseous bodies. 



The differentia] thermometer, invented by Mr Leslie, is 

 an instrument of considerable value in ascertaining minute 

 chan^, jjcrature confined to a particular spot, 



our object is not to determine the variation of the 

 temperature of the room in which we are, but the pecu- 

 liar vnriatiun in any point, indcpcndt nt of the general 

 variation. It consists of two equal glass balls joined to- 

 gether by a glass tube bent somewhat in the form of a 

 I'. The tube is filled with a coloured liquid. If one of 

 the glass balls be placed in a position hotter than that in 

 which the other ball is placed, the air within it expands 

 and presses down the liquid in the end of the tube near- 

 A scale of degrees is attached to this leg of the 

 tube, by which the heat that the ball experiences is indi- 

 cated. Seethe thermometer represented in Plate CXLI. 



\ ariou* thermometers, or pyrometers as they have been 

 ca'i-ed, have been contrived to measure high degrees of 

 heat, as the fusing points of the metals, to most of which 

 a common thermometer cannot be applied. One of the 

 first of these was contrived by Dr Cromwell Mortimer. 

 It consisted of a rod of steel or brass, which, by its ex- 

 pansion, moved a lever turning upon an axis, and pointing 

 out the various degrees upon a plate. Though this in- 

 strument is not susceptible of much accuracy, yet as Dr 

 Mortimer employed it to determine the fusing point of 

 tin, lead, zinc, and antimony, and as the degrees which 

 he pointed out are still to be found in modern chemical 

 books, though seldom any reference is made to the ori- 

 ginal experimenter, we shall give a description and figure 

 of his pyrometer in this place. In Figure 2. AB is a 

 round rod of steel or brass, a quarter of an inch thick 

 and thr.c feet one inch lung. When the rod is of brass 

 three feet Lui/, the point A must be of steel one inch 

 long, to prevent its wearing away or losing its point, 

 which conical point is made to s-.'rew on and off. CD, 

 (l are two iron supporters, joined by a flat cross bar at 

 bottom D t/ two inches long, in the iirddlc of which is a 

 point |th of an inch high under D, winch goes into a hole 

 at the bottom of the rod B, and serves to keep the rod 

 in its place at bottom ; as the cross bar * % , having a hole 

 in it through which the rod passes, doea in the middle, 

 or about two thirds up the supporters, and the point A 

 got* into a small hole in die under side of the lever, ail 

 which keep the upright rod linn and ateady in us place. 

 The iron supporters are fl-t, or parallel to the front of 

 the machii.e from C to X, and c t.> i, wh. ! ;' < \ are 

 twisted half round, to that the tower parts XI), xd 



stand at ' >s with the upper partf. This con. f 



nee gives the freer access to thr rod for the sand or >ppartv. 

 fluid i:it:> which the machine is set to measure its heat, *~~~Y~*' 

 the supporter! standing two inches asunder at D</; and 

 that the degrees of htat may be compared uniformly in 

 different experiments, the botiom of the rod must always 

 be immersed to the same height in the matter to be ex- 

 amined. On that account there is a mark made quite 

 round the rod, l.J inch from the bottom B. For the 

 deeper the rod is immersed into any matter, it will be 

 lengthened the more by the same degree of heat. EF it 

 tin- lever which turns, upon the axis G. At F is fasten- 

 ed a string, which, passing twice round the pulley H, has 

 lit I hanging to the other end ot it, of about jib. 

 being enough to keep the string always wretched. At 

 the other end E of the lever is hung another weight L, 

 which must be heavy enough, n,>t only to counterbalance 

 the larger arm GF, but press down >>n the point A with 

 a weight -.uffici. nt to keep it steady. MNO i< the back p LATt 

 part of the plate, like the dial plate of a clock made of cxi i 

 brass, the front of it being as in I'l^ure :!. The jjulicv I . ;; . :;. 

 H turns on an axis C, in Figure :',, wiucti goes through 

 the plate, and on the othtr side or front of the dial-plate 

 carries a hand or index AB. 



Iron being a metal very ill adapted for frequent ex- 

 posure to heat, in consequence of its great incombusti- 

 bility, Guyton Morvcau has lately proposed to substi- 

 tute a rod of platinum instead of iron. We do not con- 

 sider it as worth while to give a particular account of 

 his instrument ; but the reader v ho u inquisitive upon 

 the subject, will find it described in the Annalct tie Chimic, 

 vol. xlvi. p. 276. 



But the most ingenious, and the best and most con- Wedjre- 

 venient method hitherto thought of for measuring high wood'* 

 degrees of heat, was contrived by Mr Wcdgewood, and P)' rome - 

 the apparatus is well known by the name of llcdgcirito.l's 

 pyrometer. It consists of small cylindrical pieces of por- 

 celain clay, carefully made of a determinate size, and 

 baked in a low red heat. He employed for the purpose 

 a particular bed of Corniah clay. This chy was first 

 diffused through water, ar.d the liq.iid parsed through 

 fine lawn. The clay was tlu-n suffered to subside and 

 to dry. It is mixed with about fths of its weight of 

 water, kneaded into a paste, and formed into small 

 pieces, in little moulds of metal, V^-ths of an inch in 

 breadth, with the sides pretty exactly parallel ; v^-ths of 

 an inch in depth, and 1 inch long. These piece*, when 

 perfectly dry, are put into another iron mould or gage, 

 consisting only of a bottom with two sides, half an inch 

 deep, to the dimensions of which sides the breadth of 

 the pieces is to be paired down. Tlie constituents of 

 the clay employed, according to Mr Wedgewo-jd's ana- 

 lysis, are fths alumina, and |ths silica. 



It is the property of clay pieces, after having been 

 baked in a red h--at, to shrink permanently in their di- 

 mensions, according to the dt-gree of iieat to which they 

 have been exposed. This diminution of bulk !> the 

 principle of Wedgcwood's pyrometer. Far measuring 

 the diminution which these pieces undergo when exposed 

 to heal, a gage is made of two pieces of br.is>, '21 mchei 

 long, with the sides exactly straight, divided into nu-hcl 

 and tenths, fixed lialf an inch asunder at one end, and 

 T ' ff ths at the other, on a brass piato ; so that one f the 

 clay pieces, when pared down in the iron gage, will just 

 lit the wide end. This gage is div;di d int.. 2 W degrees, 

 each of which, of course, it one tenth oi an men dis- 

 tant from the succe'-ding one. The wide rxtienjty of 

 the gagv is mark, d 0, and the degree at the narrow .-x- 

 tremity is marked 24O. Let us suppose a clay piece to 



