,'J52 Mr. Daniell on a Nnv Register-Pyrometer, 



terior. The metal to be tried was placed in a small black- 

 lead receptacle, of the same thickness as the pyrometer tube, 

 in the middle of the muffle. Now it is evident that the pyro- 

 meter so situated would indicate the mean heat of the whole 

 of the muffle; which heat might, and did, vary in different 

 parts. Of two pieces of silver of the same size placed within 

 an inch of each other, one fused some time before the other." 

 I also sufjaested that " means mifrht be contrived to surround 

 the instrument with the metal in a stale of fusion; but that it 

 required particular opportunities, which it was to be hoped 

 that those would avail themselves of who had them in their 

 power." 



That the latter method is the only one which can admit of 

 accuracy will be evident from a ievf reflections. Setting aside 

 the inequality of the heat of different parts of the same 

 heated muffle, which however is a consideration of the utmost 

 importance, it is obvious that its temperature must considei- 

 ably exceed the true melting point of the metal exposed to its 

 influence. Just as a piece ot ice would never melt in a cham- 

 ber of the temperature of 32°, but would require a consider- 

 ably higher heat in proportion to its mass to supply the ca- 

 loric which becomes latent during the process, — a mass of iron 

 would exhibit but little signs of liquidity till subjected to a 

 heat much above its true point of fusion. When once in a 

 liquid state, both would rapidly rise to the temperature of the 

 medium to which they were exposed. When metals are melted 

 for the purposes of the arts, they of course require to be heated 

 very far beyond their fusing points, that they may flow into 

 the minutest fissures [hollows] of the moulds in which they 

 are cast, notwithstanding the cooling influences to which they 

 are suddenly exposed. In some of the finer castings of brass, 

 the perfection of the work depends upon the intensity to which 

 the metal is heated, which in some cases is urged even beyond 

 the melting point of iron. With a fire whose power in all 

 cases must so greatly exceed the temperature required, it is 

 necessary to bestow great care in supplying the metal gra- 

 dually, as we have before described ; as it is inconceivable 

 with what rapidity it rises after the solid pieces are completely 

 dissolved. Evidence of the same fact may be derived from the 

 experiments of MM. Clement and Desormes, which I have 

 before quoted. They calculated the heat of melted iron at 

 3988°, and of iron just on the point of melting at 3161°, — a 

 difference of 800°. And it is clear from the circumstances of 

 the experiment, that the former must have considerably ex- 

 ceeded the true melting point, or it never could have been 

 transported in a liquicl state from the crucible to the appa- 

 ratus 



