CHEMISTRY. 



153 



Chemical be exposed foe a few minutes to the heat of a furnace, 



Apparatus. an( j to j, ave been diminished in bulk by the heat one- 



""V ' fifth part. It is obvious, that when applied to the gage, it 



will slide up just one half of the whole, and will stop 



over against the degree marked 120, indicating the heat 



of the furnace to have been 120. 



Wedgewood, by a very ingenious set of experiments, 

 contrived to connect his pyrometer scale with that of 

 Fahrenheit. He found that 0, on his scale, correspond- 

 ed to 1077 of Fahrenheit, and that every degree of 

 his pyrometer is equal to 130 of Fahrenheit. Hence, 

 2i0 3 Wedgewood, or the extremity of the scale, corre- 

 sponds with 32277 of Fahrenheit. For the descrip- 

 tion of his pyrometer, and the experimental investi- 

 gationc by Wedgewood himself, the reader is referred 

 to the Philosophical Transactions, 1782, vol. Ixxii. 

 p. 305; and 1784, vol. Ixxiv. p. 358. Objections have 

 been made to the accuracy of Wedgewood's pyrome- 

 ter, from various quarters ; but none of the objections 

 which we have seen are warranted by accurate experi- 

 ments. 



2. Some very accurate instruments for ascertaining the 

 expansion of solid bodies by heat, have been invented 

 by Ellicott, Smeaton, and Ramsden. For an account 

 of th.-ne, we refer the reader to the Philosophical Trans- 

 actions, 1736, vol. xxxix. p. 297 ; and 1751, vol. xlvii. 

 p. 479; and 175-t, vol. xlviii. p. 598. 



3. For containing freezing mixtures, it is best to em- 

 ploy a double vessel of tin-plate, which, in consequence 

 of the confim-d stratum of air, prevents the external at- 

 mosphere from acting so speedily on the mixture, and 

 destroying its efficacy. Several convenient enough ves- 

 sels for such purposes have been figured and described by 

 Mr Walker, in the Philosophical Transactions for 1795, 

 vol. Ixxxv. p. 270. 



4. For ascertaining the specific heat of bodies, or the 

 capacity of bodies for heat, a very simple method was 

 employed by Meyer. And from the subsequent expe- 

 riments of Leslie and Dalton, there is reason to believe 

 that the method is accurate. He took equal bulks of 

 the bodies whose specific heats he 'wanted to know, heat- 

 ed each to a given temperature, and then allowed them 

 to cool a given number of degrees. The time of cool- 

 ing in seconds multiplied into the specific gravity of each 

 body gave its specific heat. When liquids are the sub- 

 jects of experiment, they must of course be confined in 

 a vrctrl. The specific heat of this vessel must be pre- 

 viously determined, and the tff ct which it has upon the 

 rate of cooling of the liquid must be attended to, other- 

 wise the results obtained will be inaccurate. 



The apparatus employed to ascertain the specific heat 

 of bodies by mixture, by Black, Wilke, Crawford, &c. 

 requires no description. But there are two pieces of ap- 

 paratus connected with this subject, of so complicated 

 a nature, that they will require to be particularly no- 

 ticed. The first of these is the apparatus contrived by 

 Crawforr!, for determiniug the specific heat of gaseous 

 bodies : The second is the instrument invented by La- 

 voisier aiid La Place, for ascertaining the specific heat 

 of bodies, by the quantity of ice which they are capable 

 of melting. To this instrument they gave the name of 

 calorimeter. 



Dr Crawford'* very delicate apparatus for determin- 

 ing the specific heat of the gases, was the following : 

 AB, CD, Fig. 4-. represent two slight brass cyhiidri- 

 cal vessels, of equal weights and capacities ; the weight 



VOL. VI. PART I. 



PlATK 

 ( XI. I 



Fig. +. 



of each was 7 ounces, 3 drachms, and 38 grains, troy ; 

 the length, 8-j- inches ; the breadth, 2| inches near the 

 upper extremity, and 2|- inches near the bottom. They 

 were each capable of containing 20 ounces troy of wa 

 ter ; and, consequently, the solid contents of each is 

 37.91 cubic inches. These cylinders terminate in the 

 small tubes Is, rk, at the extremities of which are fixed 

 the brass cocks J and k. These cocks are joined to the 

 cross bar MN, which consists of two elastic brass pl.itcs 

 that are screwed together at the middle, but are capa- 

 ble of being separated from each other to a little dis 

 tance at their extremities ; in consequence of which, the 

 necks of- the cocks that are fixed to the stems of the 

 brass vessels, can be slipped in between them, where they 

 are lodged in grooves formed for receiving them, and arc 

 made fast by small screws that pass through the extre- 

 mities of the plates. To the middle of the cro-,s bar is 

 fixed a square socket, the inner surface of which is finely 

 polished, and which is of such a size as to be accurately 

 adapted to the polished upright bar EF, and at the same 

 time easily moveable upon it. At equal distances from 

 the upright bar, are placed the tinned vessels, GH, IK, 

 that have a little more than double the capacity of the 

 brass cylinders. The distance of these vessels from the 

 upright bar are so adjusted, that when the cylinders are 

 made to despend, they shall be directed by the bar and 

 socket precisely into the centres of the tinned vessels ; 

 and when, in descending, they have arrived at one-eighth 

 of an inch from the bottoms of those vessels, they are 

 stopped by means of a pm fixed in the upright bar at 

 the point d. The cylinders being made for the purpose 

 of receiving the gases whose specific heats were to be 

 ascertained, were made as slight as was consistent with 

 standing the external pressure of the atmosphere when 

 exhausted. With a view to diminish the inaccuracy un- 

 avoidably occasioned by the heats communicated by the 

 C'-cks s and k, and by the cross bar MN, the tube;, /.v, 

 rk, were made very slight, and of a considerable length ; 

 the distance from the cocks to the brass cylinders being 

 4^ inches. From the length and slightncss of the tubes, 

 it was found that the brass cylinders could not be kept 

 sufficiently steady in the centres of the tinned vessels by 

 the cross bar MN. For this reason, another bar, OP, 

 was added, a lateral view of which ts given in Fig. ~5. 

 In this figure, ab represents a narrow brass plate, of a 

 considerable thickness, to which is fixed the socket, np, 

 moveable upon the upright bar formerly described. Thi; 

 two thin plati s en, dk, are rivett-d to ab, at the points 

 oandyj; and thc-ir extremities, / and m, are formed into 

 grooves, which are adapted to one half of the circumfe- 

 rence of the stems of the brass cylinders, to the other 

 half f which are closely applied two brass pieces, which 

 are also grooved, and which are fastened to the plates 

 by the screws r and *. The two male scicw, g and h, 

 are passed through the extremities of the thick plate 

 ab. These are introduced into two corresponding fe- 

 male screws, in the thin plates en, dk, by means of which 

 it is evident, that the latter may be made to approach 

 nearer to the former, or to recede to a greater distance 

 from it. This motion is communicated to the stems of 

 the brass cylinders, by means of which they are ad- 

 justed in such a manner, as that when they are introdu- 

 ced into the water, they shall be accurately in the cen- 

 tres of the tinned vessels, where they are kept steadily 

 in their proper positions. The superior and inferior cross 

 bars are connected together by a thin plate of brass, 

 v 



Chemical 

 Apparatus. 



Crawford's 

 apparatus 

 for deter- 

 mining the 

 specific 

 heat of the 

 gases. 



Fig.. 



